The Cucurbitaceae family includes several of the most economically important vegetable crops worldwide. It is alternatively called the cucumber family, squash family, or gourd family. This family has close to 120 genera with more than 800 species, including three economically important Old World vegetables: cucumber (Cucumis sativus), watermelon (Citrullus lanatus), and melon (Cucumis melo). The important New World vegetable contribution to this family is a wide array of squash types from four species of the Cucurbita genus. All of the cultivated genera of the Cucurbitaceae family have trailing, climbing “vines,” with fleshy, tender stems that almost always have tendrils. Cucumbers, watermelons, melons, and winter squash are often called vine crops in North America, while most summer squash varieties have fruit that are borne on plants with a bush habit. The other important crops of the Cucurbitaceae include several species of gourds, a number of ornamental vines, several minor melon species, and chayote, an herbaceous perennial vine that yields both an edible root and a rich textured fruit similar to summer squash. All of these crops are frost-sensitive.
Botanically speaking, all of the cucurbit crops covered in this manual are classified as fruit. Certainly, melons and watermelons are treated as fruit in culinary terms, as they are generally eaten as dessert. Yet all of the important cucurbits are considered vegetable crops, starting with the production and distribution of their seed, the cultivation methods that are employed to grow them, and the marketing and storage used in their distribution. Most of the cucurbits have a monoecious flowering habit, with unisexual flowers that are morphologically similar from species to species. The main differences are the size and the shades of yellow of the flowers, which is distinct between these crops.
The crops of the Cucurbitaceae are very important in almost all agricultural cultures worldwide. While there are limits on which cucurbit crops can be grown successfully in many temperate climates, there is ever-increasing fresh market production of cucumbers, summer squash, and some types of melons under cover in regions and seasons where it isn’t normally possible to get commercial production. In tropical and subtropical climates circling the globe cucumbers remain the number one consumed vegetable on a fresh weight basis. Melons and watermelons are also widely grown and held in high regard in these hot climates.
Seed production for the various cucurbit crops is done across a fairly large geographic swath of the Earth’s surface. In general, these crops are heat lovers and require warm springs followed by appreciable heat and warm nighttime temperatures during the summer months. Most of the cucurbit seed crops require summer temperatures of at least 77 to 86°F (25 to 30°C) during the day and produce the best seed crops when temperatures don’t dip below 68°F (20°C) during the night. The exception to this would be most summer squash types and cucumbers, which can thrive with temperatures between 68 and 77°F (20 to 25°C) during the day and temperatures that don’t usually drop below 59 to 62°F (15 to 17°C) at night. Cucurbit seed is produced commercially in many parts of the Middle East, South Asia, Southern Europe, North Africa, Israel, Indonesia, Taiwan, Korea, China, and Japan. In North America most commercial cucurbit seed is currently grown in the Arkansas River Valley of Colorado, the Central Valley of California, and in central and west Texas. Cucurbit seed is also still produced regionally in many agricultural societies that still hold value in culturally significant varieties of these crops.
Family Characteristics
Reproductive Biology
All of the cultivated vegetables of the Cucurbitaceae family are annuals and must produce a substantial vine, flower, and then produce some of the largest fruit on Earth, all in one growing season. Indeed, the giant show pumpkins of the Cucurbita maxima species are the largest fruit produced by any angiosperm species grown worldwide.
Most of the cucurbit vegetables have monoecious flowers, though there are some notable cases of andromonoecy across several species of this family. While the pistillate and staminate flowers of these various species can vary greatly in their size and color, from the various shades of yellow of the Cucurbita spp. to the greenish yellow of the watermelons, they are anatomically very similar across species. Cucurbit flowers are sensitive to temperature and daylength, with the ratio of staminate to pistillate flowers going up under the longer days near the summer solstice and more pistillate flowers appearing as the daylengths shorten.
The monoecious flowering habit is an effective biological way to ensure a relatively high degree of cross-pollination. The cross-pollination in all of the vining types of cucurbits is probably enhanced by the fact that the vines of one plant are often physically interwoven with the vines of adjacent plants as they reach the reproductive phase of their life cycle. This means that bees that are methodically working the flowers of one area of the crop are repeatedly moving from one plant to another. Cucurbits seem to be largely pollinated by honeybees and wild bee species in North America, as other pollinators do not seem to be as interested in the cucurbit flowers. Farmers growing cucurbits (which need pollinators for both fresh market and seed crops) are often concerned that they will not get adequate pollination from native populations of bees. This concern is heightened for many diversified organic growers, who often have flowering crops that they believe are more desirable, both in the quality and density of flowers, to the bees than their cucurbit crops. Though the number of flowers per unit area of land for the cucurbit crops is relatively low in comparison with many of the other flowering crops like clover or buckwheat, the fact is that if bees are present, then the cucurbits always seems to have good visitation from a number of bee species.
Life Cycle
All of the vegetable crops of the Cucurbitaceae are tender, warm-season annuals. They are all sensitive to frost and require a reasonably hot climate to mature their fruit. As annuals that have to put on an appreciable amount of vegetative growth early in the growing season to support a good fruit set, these crops need to be planted under favorably warm, settled weather conditions that will result in vigorous, unchecked growth. Traditionally these crops, when grown in regions most favorable to seed production, have been direct-seeded at the time of planting. Direct-seeding the cucurbits has long been regarded as superior to transplanting by many growers, as it seems to produce a more productive plant. But increasingly, organic growers are now transplanting their cucurbit crops for several sound reasons:
1. Less predictable spring weather in recent years means that “settled conditions” aren’t always as likely at the time of planting.
2. Transplanting enables the farmers to avoid 10 to 14 days of weed growth that would normally be growing along with the crop seedlings.
3. In areas where either the striped or spotted cucumber beetle is a problem, transplanting is a way to grow plants past the most susceptible stage (the cotyledon stage) before transplanting them into the field.
The cucurbit crops must become established, producing flowers relatively early in summer to produce a good fruit that will mature safely ahead of the first killing frost. Always choose the crop and the particular variety of it based on the knowledge that it will produce a healthy, mature seed crop in most years at your location. This is especially important when you’re growing a variety or a crop species that you have not grown before in your area. The crop-to-crop differences and even the variety-to-variety differences in adaptation and relative maturation across regions can be significant and can mean the difference between a successful seed crop or none at all. For this reason I strongly urge all seed growers considering seed crops of these species to grow a seed suitability trial, to evaluate the seed maturity and seed yield of any varieties that you may be considering growing commercially.
Climatic Adaptation
The cucurbits all produce their best seed crops with warm nighttime temperatures, which support pollen tube growth and help ensure a high percentage of fertilization of the embryos in each fruit that is set. Adequate heat units during the day are also important to stimulate rapid fruit growth and maturity. In summary, the best regions to produce seed of these crops need to have: (1) a relatively long, frost-free growing season; (2) enough hot weather for good fruit production; and (3) warm nighttime temperatures for maximum seed set.
Seed Harvest
The most important aspect of seed harvest for the cucurbit crops is making sure that the fruit that is harvested for seed is fully ripe, even overripe, but making sure as well that it isn’t rotting and moldy to the point where the seed is damaged. In fact, allowing the fruit of all of these crops to go a bit beyond full ripeness may ensure that the seed is fully mature. As with other wet-seeded crops, like the Solanaceae, the overripe fruit also usually release their seed from the flesh or placental tissue much more easily than is the case with slightly underripe or even perfectly ripe fruit.
Fruit of the Cucurbitaceae crops is usually harvested by hand for seed production in order to have human eyes make a final assessment of whether: (1) the fruit is fully ripe; (2) there are marked levels of disease or rot on a particular fruit or plant; and (3) the fruit from a particular plant is true to type. Harvesters can reject individual fruit if they are either not yet ripe or rotting, and they can make a final act of selection if the plant has excessive disease or off-type fruit. At harvest, fruit are sometimes windrowed in the field or hauled out of the field before seed is extracted. Depending on the crop and climate, growers are sometimes able to make multiple harvests of ripe fruit from the same field. (See the “Seed Harvest” section of each Cucurbitaceae crop for information on post-harvest fruit handling.)
Seed Extraction,
Fermentation, and Cleaning
Seed Extraction: Mechanical extraction of larger quantities of seed from all of the cucurbit crops is done using what is alternately called a vine harvester, cucurbit machine, or cucurbit seed extractor. This machine is pulled through the field while workers toss the ripe fruit into it, or the ripe fruit is gathered and run through the extractor while it is running in a stationary position. All of the cucurbits are run through these machines in much the same fashion, though each crop requires different settings. You must gain experience extracting the seed of each particular type of crop, as it can be damaged easily if the seed extractor is not set properly.
Seed Fermentation: Cucumber seed is borne in a gelatinous sac that adheres tightly to the seed. This sac needs to be removed from the seed either through physical abrasion or through fermentation (see Cucumber, “Seed Extraction, Fermentation, and Cleaning”). While cucumbers are the only cucurbit crop where the seed is routinely fermented, there are cases in which all of the other cucurbits are fermented, either to either loosen the seed from the placental tissue or to make it easier to clean. Importantly, the fermentation used for the cucurbits other than cucumbers is usually done for less time than the 3 to 4 days needed to adequately ferment cucumbers. (See the “Seed Extraction, Fermentation, and Cleaning” section for each individual cucurbit crop for specific recommendations.)
Seed Cleaning: The final cleaning of all cucurbit seed, through the use of a sluicebox or with a simpler decanting method, is essentially identical to that described for tomato (see chapter 12, Tomato, “Washing Seed”). The main difference is that for each crop with its variable seed size and weight, the amount and force of the water that is released down the flue will need to be adjusted to clean seed of the different species and different crop types. The drying of the seed needs immediate attention after this wet seed processing.
Fermentation and Seedborne Diseases: Fermentation of cucumber seed is advantageous in helping to control at least one of the most important diseases of cucumber, gummy stem blight (Didymella bryoniae). As the environment that exists during the fermentation process can vary considerably from one location to another, it is important to remember that any disease that is affected may not be completely controlled by this process. Gummy stem blight has become one of the most devastating diseases of cucumber in North America in the past 20 years, and organic research needs to be conducted to determine the optimum fermentation conditions to reliably control the transmission of this disease via cucumber seed.
All of the cucurbit crops have plant characteristics such as leaf size, shape, and color, as well as vine length, that can be evaluated before flowering, thus affording growers an opportunity to eliminate off-type plants before they flower and spread their genes throughout the population. When the cucurbit plants start to flower it is possible to evaluate their time of flowering and the proportion of male to female flowers, which can be quite variable in these mostly monoecious species.
At the time of flowering and within the first couple of weeks of fruit development it is possible in many of the cucurbits to evaluate the basic shape and color of the fruit, which enables you to eliminate plants at an early stage in the flowering of the crop. The earlier that you can eliminate any off-type plants, the less time an off-type individual has to spread its pollen (and thus its genes) throughout the population.
As cucurbit fruit reach a marketable size it becomes easier to select them for their trueness to type for size, shape, and the color of their skin. Occasionally, there will be a single off-type fruit on a plant that otherwise has fruit typical of the variety. When this occurs it is undoubtedly due to environmental conditions during the formation or early growth of that particular fruit, and, if the other fruit are normal, then the plant should not be eliminated. As with all seed crops, routine elimination of the plants that are most susceptible to any endemic diseases is always worthwhile.
Isolating a crop of the Cucurbitaceae to avoid cross-pollination is relatively straightforward as long as you are willing to learn the species name of the crop you intend to grow for seed in this large diverse family. Cross-pollination across species boundaries that has resulted in viable offspring has occurred between varieties from two different squash species under controlled conditions, but is very rare in the field and most long time squash seed growers have not observed it.
In general, all of the members of the Cucurbitaceae family generally need 1 mi (1.6 km) of isolation distance when you’re growing seed crops of two different varieties of the same species in open terrain. When barriers on the landscape are present it is possible to separate two crops of the same species by as little as 0.5 mi (0.8 km), especially when these adjoining crops are relatively small (see chapter 13, Isolation Distances for Maintaining Varietal Integrity).
As with all of the cross-pollinated crops, you need to distinguish between the distances that are necessary when the two adjacent cucurbit crops are of the same horticultural type versus when you’re growing two different types of the same species. Perhaps the best example of this among the Cucurbitaceae are the widely varied crops within the species Cucurbita pepo, which includes everything from zucchinis to patty pans, acorns, delicatas, jack-o’-lantern (true) pumpkins, and even a subspecies of bottle gourds—all of which will cross easily. Hence, the minimum distances needed to isolate varieties from two of these disparate groups needs to be doubled over the basic isolation distances stated above, with 2 mi (3.2 km) between two crops when grown in open terrain and 1 mi (1.6 km) when significant barriers exist on the landscape between the two crops. For examples of the different horticultural types within each crop species, see the “Isolation Distances” section for each specific crop.
Cucumber
Cucumber (Cucumis sativus L.) probably originated in Africa but was likely carried to South Asia and the Middle East, its primary center of diversity, at an early stage of its domestication more than 3,000 years ago. Cucumbers quickly spread across equatorial regions of the Eastern Hemisphere. Cucumber cultivation became so prominent in the agriculture of southern China in the early stages of domestication that this region became a secondary center of diversity for this important vegetable crop. The Romans introduced cucumbers to Europe. Columbus carried cucumber seed to the Western Hemisphere on his first voyage in 1492, and by 1539 the conquistador Hernando de Soto found “cucumbers better than those of Spain” being cultivated in southern Florida.
Cucumbers occupy two basic market classes: the fresh market types usually sold as whole fruit, and processed types that are pickled using fermentation or short-brine methods. The major fresh market types include the Middle Eastern Beit Alpha type, the Asian trellis type, the North American slicer, and the European or Dutch greenhouse type (still called English cucumbers in some markets). The major processing types are the North American and the European pickling types, which vary in the extent of their spines. It should be remembered that that these are only the major types in commerce; many regional types and varieties outside of these classes are grown for local fresh market and pickles across many agricultural regions where cucumbers have been grown for thousands of years.
The cucumber is one of the most widely grown vegetable crops across much of the tropical and temperate regions of the world. It is especially valued in tropical climates, where it is often the number one vegetable eaten by volume. In traditional Chinese medicine cucumbers are noted for their “cooling effect” on the body, which is recognized as more than the simple effect gained from eating a fruit largely made up of water. The expression “cool as a cucumber” may have more to it than just describing a person who remains calm under pressure.
Much of the best commercial cucumber seed production is done in relatively hot, arid climates. However, all of the climates that are best suited to cucumber seed aren’t excessively hot during the early parts of the season. This includes parts of the Middle East, Southern Europe, North Africa, Israel, Indonesia, Taiwan, and southern China. In North America most of the commercial cucumber seed is currently grown in the Sacramento Valley of California. Before the advent of hybrids, however, significant seed was produced in southern Ontario, southern Quebec, and Michigan.
Crop Characteristics
Cucumbers are typically monoecious, with separate staminate (male) and pistillate (female) flowers on the same plant. The staminate flowers usually form first during the longer days near the summer solstice, with a higher concentration of pistillate flowers emerging as the days get shorter. The proportion of male and female flowers is roughly equal during the peak season for fruit production. Cucumbers are usually highly cross-pollinated due to the monoecious condition, though they are self-compatible and will frequently self-pollinate.
There are also cucumber types that are hermaphrodites, with all perfect flowers, and andromonoecious varieties with a combination of staminate and perfect flowers, much like most melons (C. melo). The popular North American round-shaped specialty type ‘Lemon’ is an example of an andromonoecious variety. There is also a gynoecious flowering type that has predominantly female flowers and is used to produce hybrid cucumbers. This gynoecious type was grown in Korea as an open-pollinated variety with a low incidence of monoecious plants when an American plant breeder, Elwyn Meader, first collected it in the late 1940s while working as an agronomist under the Marshall Plan for the US Army. He recognized its potential value as a female parent in producing hybrids and brought it back to the United States, where it was selected for all-female flowering (see “Growing the Seed Crop”).
The male flowers of cucumber are often formed in clusters of three to five flowers, with each attached to the plant on a thin peduncle at a leaf axil. Each of these staminate flowers has five petals that are fused at their base and five stamens tightly packed within each staminal collar. The female flowers are easily identified, as they have the prominent ovary at the base of the flower. These pistillate flowers are usually borne as either a solitary flower or as two or three flowers per node. Any spines that will appear on the fruit will be apparent upon close inspection of the ovary. There are three broad stigmatic lobes on top of a short, thick style. Both the stigmatic surface of the pistillate flower and the anthers of the staminate flower turn a deep golden yellow when sexually mature in most cucumber varieties.
A female cucumber flower (Cucumis sativus).
A developing cucumber fruit after fertilization of the ovary.
Climatic and
Geographic Suitability
Large-scale seed production of cucumbers is done in warm to hot seasonal conditions. Optimum growth and fruit set occur between 75 and 90°F (24 to 32°C), with nighttime temperatures that don’t usually dip below 68°F (20°C). Nighttime temperatures below this can cause erratic pollen tube growth, resulting in lower fertilization rates and ultimately lower seed yields. As with all crops, temperatures that are too high at the time of the pollination will also retard pollen tube growth and prevent fertilization and subsequent seed formation. In cucumbers this can happen at temperatures above 95°F (35°C). As with many of the cucurbits, the best seed production areas are regions with a reasonable amount of heat during the day, warm temperatures at night, and relatively low humidity. Too high a level of humidity during the growing season should be avoided due to the potential for increased levels of foliar disease.
Seed Production Practices
Soil and Fertility Requirements
Fertile loam soils that are well drained are desirable for cucumber seed production. Lighter, sandy loams that are able to warm up easily are especially useful in shorter-season areas with cool springs. Silt and clay loams with good tilth are desirable in longer-season areas for their increased ability to deliver a steady supply of water and nutrients. Heavier soils that are well drained will usually translate to greater seed yields over the course of the season.
Cucumbers are heavy feeders that thrive when ample amounts of well-decayed organic matter or compost are incorporated into the soil. There is a long tradition of using well-decayed horse or cow manure for growing cucumbers, which favors the kind of unchecked growth that produces high yields of fruit and seed. If using the traditional method of planting the crop into hills, then two to three shovelfuls of stable manure are worked into soil at the spot of each hill. A soil pH of between 6.5 and 7.0 is desirable.
The cucumber crop is well adapted to producing a full complement of fruit even in many shorter-season temperate climates. In fact, it is often noted that cucumbers require the shortest span of time from planting to production of fruit of any of the common fruit-bearing vegetables. This means that the crop can be direct-seeded in most temperate regions and still mature a reasonable seed crop. Seed can be planted when spring weather has settled and the danger of frost has past. Soil temperatures should be at least 68°F (20°C) at the time of planting, and temperatures between 75 and 95°F (24 to 35°C) will ensure emergence in 3 to 5 days. If planted in rows, seed should be sown liberally, six to eight seeds per foot (30 cm), and then thinned to anywhere from 8 to 16 in (20 to 41 cm) apart, depending on the growth habit of the plants.
Alternately, a more traditional method of growing the crop is to plant the seed into “hills” that are spaced equidistant from each other in the field. The spacing used for cucumber hills is often 5 × 5 ft (1.5 × 1.5 m) or 6 × 6 ft (1.8 × 1.8 m) apart. Placing the hills equidistant from one another allows for “check row” cultivation, where it is possible to run cultivation equipment through the field in both directions. This is a very good option for more thorough mechanical weed control right up until the point where the cucumber vines start to trail out into the open space between the hills. Seed is often planted into the hills at a rate of 6 to 12 seeds per hill, with seed sown in an area that is at least 6 in (15 cm) in diameter. Each hill is then thinned to three or four plants once it is clear which are the most vigorous and healthiest individuals. The idea of a “hill” is also often misunderstood, as this area where the seed is sown may not be a noticeable hill, although sometimes there is an actual raised mound when manure is worked into the spot before planting.
Cucumber seed is fully mature when the fruit turns either to a pale ghostly yellow (for white-spined varieties) or to a deep golden yellow (for black-spined types). In order to have seed with fully developed endosperm, good vigor, and the highest possible germination rate, it is very important to allow the fruit to achieve these respective colors and not rush the harvest. In drier temperate climates it isn’t unusual for cucumber seed growers to leave the fruit on the plants until the first frost. The mature fruit can then be located, harvested, and tossed directly into a mobile seed extractor, if one is available for larger commercial plants. If the fruit is harvested for hand extraction after frost has killed the vines, it is usually best to get it out of the field in a timely fashion, as repeated frosts and saprophytic organisms can quickly cause excessive rot that may damage some portion of the seed crop. In general, it is never a good idea to allow the fruit to get moldy in the field.
A cucumber fruit that has become too mature for eating, but that is perfect for leaving on the vine to fully ripen its seed.
When the fully ripened fruit that is harvested is sound, it is sometimes held for a number of days in a warm, well-aerated place before extracting the seed. A number of growers believe that such an after-ripening period produces an even higher-quality seed than that extracted immediately after harvest. This, of course, is only practical when the seed crop is relatively small.
Seed Extraction,
Fermentation, and Cleaning
Mechanical extraction is used for larger commercial seed lots and is usually done with what is often called a cucurbit machine or cucurbit seed extractor. At harvest the fruit is put into a hopper that leads to a series of rollers or rotating blades; these macerate the fruit to free the seed as much as possible from the pulp. The pieces of rind, the pulp, and the seed then drop into a large rotating cylinder made of screen material. Much of the small pieces of the rind, pulp, and liquid go through the screen material with the seed into a trough at the bottom of the extractor. Seed that adheres to the pulp is loosened through the spinning motion of the cylinder, passing through the screen. Because the cylinder is at a slight angle, the larger pieces of fruit and pulp slowly work their way to the end of the cylinder and into another trough. Examine the contents of this waste trough before composting them for appreciable amounts of seed that may not have been captured.
Cucumber seed shares an unusual morphological trait with tomato. Both crops have seed that is enclosed within a placental sac that is difficult to remove mechanically. As with tomato, there are basically two options for separating the seed from this gelatinous, tightly adhering sac. The traditional method to rid the seed of this sac is to ferment the freshly extracted seed, thereby loosening the sac by way of the yeast consuming the sugars within this gelatinous membrane. Alternatively, chemical extraction is commonly used in conventional production of cucumber seed. It uses diluted hydrochloric acid and is still very controversial with a number of organic certification agencies. The concentration of hydrochloric acid that is necessary to strip away this membrane make this practice environmentally unacceptable to most proponents of organic agriculture. Therefore, the fermentation seed extraction method will be the only method covered here.
A
B
Cucumber seed can be extracted from the fruit by hand (A) and placed in a container prior to fermentation (B).
Seed Fermentation: The seed that successfully passes through the screen with smaller pieces of pulp and the peel of the cucumber fruit is then put into barrels or other suitable containers to ferment in a process similar to that used for tomatoes (see chapter 12, Tomato, “Seed Fermentation”). The fermentation is activated from native, naturally occurring yeasts that are present on the exterior of the cucumbers. The fermentation in cucumber starts quickly and in my experience may even be stronger than what occurs in tomatoes. Certainly, there are very few instances when it is necessary to add baking yeast and sugar to boost the process so long as the seed mixture is kept within an appropriate temperature range. As with tomato fermentation, there is some debate as to whether adding water to the seed mash hinders the fermentation process. Sometimes it is necessary so that all of the seed is submersed in liquid; however there is often enough liquid present, especially from overripe cucumbers, to easily ferment the batch. Also, like tomatoes, there is some debate as to whether the addition of water encourages sprouting. While there isn’t any published research on this question, there is speculation that naturally occurring sprout inhibitors may be diluted when water is added to the mix.
The temperature range, duration of fermentation time, and separation of the cucumber seed from the pulp is essentially the same as for tomato.
Seed Cleaning: The final cleaning of cucumber seed through the use of a sluiceway or with the simpler decanting method is identical to that described for tomato. The main difference may be when using the sluice to clean cucumber seed, where the amount and force of the water may need to be increased to clean the heavier seed (see chapter 12,
Tomato, “Washing Seed”). The drying of the seed requires immediate attention, just as with tomato seed, after this wet seed processing.
Fermentation and Seedborne Diseases: Fermentation of cucumber seed is advantageous in helping to control at least one of the most important diseases of cucumber, gummy stem blight (Didymella bryoniae). As the environment that exists during the fermentation process can vary considerably from one location to another, it is important to remember that any disease affected by this process may not be completely controlled by it. Gummy stem blight has become one of the most devastating diseases of cucumber in North America over the past 20 years, and organic research needs to be conducted to determine the optimum fermentation conditions to reliably control the transmission of this disease via cucumber seed.
Genetic Maintenance
Several important traits can be checked before the plant begins flowering. As with all crops, seedling vigor and early robust growth are always important traits to select for in crops under organic production systems. Evaluate the cucumber plants for the length of the internodes before flowering; determinate varieties have shorter internodes than the indeterminate types. There are also substantial differences in leaf characteristics like size, shape, and color that can often be detected.
When the plants start to flower it is possible to evaluate and select those of a given variety for uniformity in the timing of flowering and the production of their first female flowers. There is also variation for the number of male or female flowers per node. In most cases it is easy to distinguish the spine color of the fruit, before the ovary has been fertilized and starts to expand. As the fruit develops, the basic shape and the relative length-to-diameter ratio of the fruit are sometimes evident even before the cucumbers have attained a marketable size. The earlier that you can eliminate any off-type plants, the less time that those off-type individuals have to spread their pollen (and hence their genes) around.
As the fruit reach a marketable size it becomes possible to select them for their trueness to type for skin color, the color of mottling on the skin (if present), and the intensity of mottling. Shape and size of the fruit and spine color should also be rechecked at vegetable maturity. Some cucumber varieties may also have raised ribs, which may be distinct for their width and color and are usually distinct for a particular variety if present. Then at fruit maturity it is possible to check for the skin color, which does relate to spine color (see “Seed Harvest”). As with all seed crops, routine elimination of the plants that are most susceptible to any endemic diseases is always worthwhile.
Isolation Distances
For cucumbers the basic isolation distance between different cucumber seed crops without barriers is 1 mi (1.6 km), especially when these adjoining crops are relatively small (see chapter 13, Isolation Distances for Maintaining Varietal Integrity).
As with all of the cross-pollinated crops, you should distinguish the distances that are necessary when the two adjacent cucumber crops are of the same horticultural type versus different types of cucumbers. When growing two varieties of the same type of cucumber—for instance, two Beit Alpha types—the minimum isolation distance between them should be 1 mi (1.6 km) in open terrain. When there are larger commercial seed crops of two different types of varieties, if one is a Beit Alpha and the other is an Asian trellis type, then the minimum isolation distance separating the two crops in open terrain should be at least 2 mi (3.2 km). If the seed crops are being grown in terrain where there are natural barriers on the landscape that hinder the movement of insect pollinators, then you can lessen the isolation distance between two cucumber varieties of the same type to 0.5 mi (0.8 km); 1 mi (1.6 km) between two different types of varieties.
Please note that the Armenian cucumber, snake cucumber, and serpent cucumber are actually melons (Cucumis melo, Flexuosus group) and not true cucumbers, and therefore they will not cross with cucumbers
(C. sativus).
Melon
The center of origin for melon (Cucumis melo L.) is Africa, but the plant was likely carried to South Asia, its primary center of diversity, at an early stage of its domestication more than 3,000 years ago. Melons quickly spread across equatorial regions of the Eastern Hemisphere, reaching China and East Asia and undergoing further diversification, to a degree that the melons originating from this part of the world are generally considered a different subspecies, C. melo subsp. agrestis, that is distinct from the melons most frequently grown in the West, C. melo subsp. melo.
A further division of the prevalent types of melons based on their ancestry and horticultural characteristics is found in the botanical groups that were originally created by Naudin in 1859. While his original groups have been added to and changed often depending on the author, melons are usually divided into between 7 and 15 botanical groups. The five botanical groups of melon that are the most important on a worldwide basis, both commercially and in the scope of their geographic use, are these:
1. Reticulatus group includes muskmelons and cantaloupes of North America and Israeli ‘Galia’ types; their fruit has a rind with a fine reticulated mesh, is aromatic as it matures, and it easily slips from the stalk at full maturity.
2. Cantalupensis group includes the true European cantaloupes and Charentais types; their fruit has a thick, scaly, or warted rind; slips; and is aromatic.
3. Inodorus group includes winter melons, casaba, canary, and honeydew types; their fruit has a fine, smooth rind; does not slip; and has no odor at maturity.
4. Conomon group, widely grown across Eastern Asia, includes Asian pickling melons that are not sweet and are used like cucumbers; there also a few types that become sweet at maturity, with a smooth, thin rind and crisp flesh; their fruit does not slip; and it is not aromatic.
5. Flexuosus group from the Middle East includes the snake melon, serpent cucumber, and Armenian cucumber, with long curved fruit that is harvested immature and eaten raw or cooked; fruit is ribbed with a wrinkled rind, does not slip; and is not sweet or aromatic.
All of these types are fully sexually compatible and will cross easily when grown too close to one another (see the “Isolation Distances” section).
‘Sharlyn,’ is a ‘Galia’-type melon (Cucumis melo, Reticulatus group) with firm flesh and flavor that is somewhere between a muskmelon and a honeydew.
While these groups give us a cultural context in which to categorize the most prevalent melon types grown worldwide, it is important to know that much of the melon breeding that has been done since 1960 is based on crosses between varieties from different botanical groups in order to increase storability and disease resistance. This means that in the modern era there are many varieties that are intermediate between these older groups. At present, many seed companies and seed growers are more interested in classifying melons by the practical characters that are most frequently seen as differentiating the most prevalent types and varieties. These groups are distinguished by: (1) fruit color, which can be orange, green, or white; (2) aromatic versus non-aromatic; (3) fruit that slips versus fruit that doesn’t slip; (4) fruit that has a netted rind versus a smooth rind; and (5) fruit that softens at maturity versus fruit that remains firm at maturity.
Melons are heat lovers that are best adapted to tropical, subtropical, and some long-season temperate zones. Heat and sun are important in developing the sugars that make many of the melon types so popular across the globe. Much of the best commercial melon seed production is done in hot, arid climates. This includes parts of the Middle East, Southern Europe, North Africa, Israel, Indonesia, Taiwan, Korea, China, and Japan. In North America most of the commercial melon seed is currently grown in the Arkansas River Valley of Colorado, the Central Valley of California, and west Texas.
Crop Characteristics
Reproductive Biology
Most melons are andromonoecious, with a combination of hermaphrodite flowers and male flowers on the same plant. The hermaphrodite or perfect flowers have both stamens (male flower parts) and pistils (female flower parts). The hermaphrodite flower is borne singularly in leaf axils and has a short, thick style and a broad three-lobed stigma with nectaries at its base. The anthers in these hermaphroditic flowers are small and do not produce much pollen; thus they contribute only minimally to the pollination of these flowers. The staminate flowers are formed in the leaf axils in clusters of three to five. The form and structure of the staminate flowers is almost identical to that of cucumbers (see Cucumber, “Reproductive Biology”).
There are also some melon varieties that are monoecious with separate staminate and pistillate (female) flowers on the same plant, much like true cucumbers, C. sativus. These monoecious-flowering types are especially found in the C. melo, Flexuosus group, where this trait is nearly universal. Under both monoecy and andromonoecy, flowering melons are usually highly cross-pollinated, though they are self-compatible and will frequently self-pollinate. As with some of the other cucurbit species, the first or crown set fruit often does not produce as well in terms of quality or yield as the subsequent two or three fruit to set. Therefore, growers will sometimes pull off the crown set fruit and pinch off the apical growth of the primary leader after they are assured that there are two or three fruit set. In the language of the seed growers, the plants have then been “stopped” to produce higher quality and yield in the fruit that are favored.
Climatic and
Geographic Suitability
Large-scale seed production of melons is done in hot seasonal conditions. Optimum growth and fruit set occur between 80 and 95°F (27 to 35°C), with nighttime temperatures that don’t usually dip below 70°F (21°C). Nighttime temperatures below this can cause erratic pollen tube growth, resulting in lower fertilization rates and ultimately lower seed yields. As with all crops, temperatures that are too high at the time of the pollination will also retard pollen tube growth and prevent fertilization and subsequent seed formation. In melons this can happen at temperatures above 100°F (38°C). For melons the best seed production areas are regions with a significant amount of heat during the day, warm temperatures at night, and relatively low humidity. Too high a level of humidity during the growing season should be avoided due to the potential for increased levels of foliar disease.
Seed Production Practices
Soil and Fertility Requirements
Fertile loam soils that are well drained are desirable for melon seed production. Melons form a large, deep taproot that is at least 3 ft (0.9 m) deep. Heavier clayey loam soils are desirable for their nutrient- and water-holding capacity but must be free of waterlogging. Lighter, sandy loams that are able to warm up easily are especially useful in establishing early crops to avoid the hottest seasonal temperatures during the period of fruit set (see “Climatic and Geographic Suitability,” above).
Melons, like all cucurbits, are heavy feeders, and they thrive when ample amounts of well-decayed organic matter or compost are incorporated into the soil. There is a long tradition of using well-decayed horse or cow manure for growing all the cucurbits, which favors the kind of unchecked growth that produces high yields of fruit and seed. If you’re using the traditional method of planting the crop into hills, then work two to three shovelfuls of stable manure into soil at the spot of each hill. Turning in leguminous cover crops well in advance of planting has also long been used to grow robust melon crops. Melons and other cucurbits seem to perform best when at least half of their nitrogen is derived from organic sources. This gives good organic farmers a high likelihood of producing good yields if the climate is well suited to melon production. A soil pH of between 6.5 and 7.5 is desirable. Melons grown in arid regions are often grown successfully on slightly alkaline soils.
Growing the Seed Crop
Melons are traditionally direct-seeded in climates that are well suited to seed production. Seed can be planted when warm weather has truly arrived, all danger of frost has passed, and soil temperatures are at least 75°F (24°C); it is even better when the soil temperature is well above 80°F (27°C), with 90°F (32°C) being the optimum. This enables the unchecked growth that will produce healthy, vigorous plants if the soil, nutrition, and soil moisture are adequate.
If planted in rows melon seed crops need to be given more spacing than the 18 in (46 cm) that is often used when growing for fresh harvest. Melon plants for seed should be planted at least 24 to 48 in (61 to 122 cm) apart within the row, depending on the vigor and size of the variety. Several seeds can be planted at the appropriate interval for the variety and then thinned once you are assured that the selected plants are well established and healthy. The rows should be at least 6 to 7 ft (1.8 to 2 m) apart for optimum production.
Alternatively, a more traditional method of growing the crop is to plant the seed into hills within the rows in the field. The spacing used for melon hills can be anywhere between 4 ft (1.2 m) apart within the row by 6 ft (1.8 m) between the rows to the comparable 6 × 8 ft (1.8 × 2.4 m) apart. Placing the hills equidistant from one another at 6 × 6 ft (1.8 × 1.8 m) spacing allows for “check row” cultivation, where it is possible to run cultivation equipment through the field in the same fashion in both directions. This is a very good option for more thorough mechanical weed control right up to the point where the melon vines start to trail out into the open space between the hills. Seed is often planted into the hills at a rate of six to eight per hill, with seed sown in an area that is at least 6 in (15 cm) in diameter. Plants are then thinned to two or three once it is clear which are the most vigorous and healthiest individuals in each hill; make sure, however, that the plants aren’t crowded during this pre-thinning period. The idea of a “hill” is also often misunderstood, as this area where the seed is sown may not be a noticeable hill, although sometimes there is an actual raised mound when manure is worked into the spot before planting.
Seed Harvest
For optimum seed quality and ease of harvest it is best to allow the melon fruit to remain on the plant for 1 to 2 weeks after the majority of favorable plants have fully matured. This after-ripening period serves multiple purposes:
1. It allows the ripe fruit to fully mature, as cucurbit fruit often produces a somewhat better seed quality if fruit is allowed to “after-ripen” after the fruit has reached the ripe edible stage.
2. It allows any laggard high-quality fruit to finish ripening.
3. It softens the placental tissue in the seed cavity to the point where it easily separates from the seed upon cleaning.
Fruit maturity can be detected in multiple ways depending on the type of melon. In many cases the “ground color” or major color of the rind will have either a major or minor change in the color, from some shade of green to a warmer shade of color with a hue of yellow or gold. For many varieties from the North American types, most ‘Galia’ types, and many of the ‘Ananas’ and true cantaloupes the fruit will slip from the plant, whereas the honeydew/casaba varieties (Inodorous group), the Charentais varieties, and the Armenian cucumber (Flexuosus group) do not slip from the plants and need an experienced grower to know when the plants are ripe.
If the seed crop is relatively small and the fully ripened fruit is sound, the fruit can be gathered and held for a number of days in a warm, well-aerated place to complete the after-ripening, particularly if outdoor conditions are unfavorable. Otherwise, the seed extraction is done in the field, either by hand or through the use of a vine seed extractor as specified below.
Seed Extraction,
Fermentation, and Cleaning
Seed cavity of a ripe North American cantaloupe. Unlike cucumber seed, melon seed is not enclosed within a placental sac. Despite this fact, most seed growers still put the seed through a 1- to 2-day fermentation step during prior to final cleaning and drying.
Mechanical extraction is used for larger commercial seed lots and is usually done with what is often called a cucurbit machine or vine seed extractor. At harvest the ripe fruit is put into a hopper that leads to a series of rollers or rotating blades, which macerate the fruit to free the seed as much as possible from the pulp. The pieces of rind, the pulp, and the seed then drop into a large rotating cylinder made of screen material. Much of the small pieces of the rind, pulp, and liquid go through the screen material with the seed into a trough at the bottom of the extractor. Seed that adheres to the pulp is loosened through the spinning motion of the cylinder, passing through the screen. Because the cylinder is at a slight angle, the larger pieces of fruit and pulp slowly work their way to the end of the cylinder and into another trough. Examine the contents of this waste trough before composting them for any appreciable amounts of seed that may have not been captured.
Melon seed, despite its resemblance to cucumber seed, does not have a placental sac and technically does not require the same kind of fermentation step that frees the cucumber seed from the sac. After the extraction described above, the seed is often cleaned under a high-pressure spray of water that removes any of the orange or green placental material from the seed. While this is often enough to clean the seed, some growers give the melon seed a 1- to 2-day fermentation period, which helps free it from all placental material and also helps avoid any staining that can occur in orange-fleshed types from the beta-carotene in the placenta. The problem with fermentation in all of the cucurbits other than cucumber is that it can easily damage the seed and lower the germination percentage if it is done for too long. If the conditions are right, only about 24 hours of fermentation is necessary. It is the same process described for tomato (see chapter 12, Tomato, “Seed Fermentation”). Watch the process closely, and separate the seed from the fermentation bath as soon as the desired results are reached.
Seed Cleaning: The final cleaning of melon seed through the use of a sluiceway or with the simpler decanting method is identical to that described for tomato. The main difference may be that when using the sluice, the amount and force of the water may need to be increased to clean the heavier melon seed (see chapter 12, Tomato, “Washing Seed”). The drying of the seed needs immediate attention, just as with tomato and cucumber seed, after this wet seed processing.
Genetic Maintenance
For melon, as with all of the cucurbit crops, there are several important traits that can be evaluated before the plant begins flowering. Seedling vigor and early robust growth will lead to the establishment of healthy seedlings. Because melons are more exacting than many other crops in their need for favorable environmental conditions to produce a healthy seedling, it is definitely important to select and maintain a level of vigor and health in all of your seedstocks. This is especially true for seed produced for organic production systems. It is also important to evaluate melon plants for the length of the internodes before flowering: Determinate varieties have shorter internodes than the indeterminate types. There are also substantial differences in leaf characteristics such as size, shape, and color that can be detected and rogued out if any off-types appear before flowering.
Once the melon crop starts to flower, it is possible to evaluate and select the plants of a given variety for uniformity in the timing of flowering and the production of their first hermaphrodite flowers, which can be easily recognized by the presence of the attached ovary. Evaluation and selection for the number of pistillate flowers per node and the proportion of pistillate to hermaphroditic flowers can be considered, as with other cucurbit crops.
As the fruit develop, their basic shapes are sometimes evident long before they have attained a marketable size. Melon fruit shape can vary greatly from type to type, from the globe or almost round shape of many cantaloupes to the long serpent shapes of Armenian cucumbers. If you are monitoring the crop from an early stage and watching both the flowering habit and growth of the first hermaphroditic flowers and the setting of the fruit, then you can monitor the shape and type of fruit right from the formation of the ovaries. Indeed, as with all cucurbits, you can select for fruit shape even before the ovary has been fertilized and the seed begins to form. (See the “Genetic Maintenance” section in the Cucurbitaceae family introduction.) As with all seed crops where the fruit is the main agricultural crop of interest, the earlier you determine that a fruit is not true to type, the earlier you can rogue out the plant and eliminate it as a source of pollen that might contaminate the rest of the seed crop.
With the melons, as with other cucurbits, it is often possible to observe and select for traits other than just fruit shape, including the major or ground color of the fruit, the depth of the color, and whether the fruit is ribbed, has warts, or has prominent sutures. You can also see the extent of the mesh or netting as the fruit matures to get a final idea of the true ground color and secondary color of the mesh, ribs, and sutures. As stated for all of the cucurbits, these traits change over time, and a good grower familiar with a particular variety can often recognize the presence or absence of an important trait at the earliest juncture and quickly eliminate off-type plants from the population. At maturity, it is possible to see all of the external characters already mentioned in their full glory for evaluation. You can also then check individual plants for internal traits such as color, intensity of color, and seed cavity size as well as sensory characteristics like flavor, sweetness, texture, and juiciness.
For melons the basic isolation distance that should be used between different melon seed crops without barriers is 1 mi (1.6 km), especially when these adjoining crops are relatively small (see chapter 13, Isolation Distances for Maintaining Varietal Integrity).
As with all of the cross-pollinated crops, you should distinguish between the distances necessary when the two adjacent melon crops are of the same horticultural type versus when there are two different types of melons being grown. When growing two melon varieties that are of the same type—for instance, two honeydews with pale green flesh—then the minimum isolation distance between them needs to be 1 mi (1.6 km) in open terrain. When there are seed crops of two different types of varieties—for instance, if one is a North American muskmelon with orange flesh and the second is a honeydew type with pale green flesh—then the minimum isolation distance separating the two crops in open terrain should be 2 mi (3.2 km). Indeed, even if both varieties were honeydew types, but they were distinctly different in their days to maturity, size, disease resistance profile, or flesh color (there are white, pale green, and orange-fleshed honeydews), then they would also require this increased isolation distance in open terrain.
If two melon seed crops are being grown in terrain where there are significant natural barriers on the landscape that hinder the movement of insect pollinators, then you can lessen the isolation distance between two varieties of the same type to 0.5 mi (0.8 km) and to 1 mi (1.6 km) between two varieties of distinctly different types.
Note: It is important to remember that melon (C. melo) plants will not cross with true cucumber plants (C. sativus). But all growers of any seed crop in the Cucumis genus should be thoroughly certain whether the crop they are growing is one of the many variants of a melon (C. melo) or a true cucumber (C. sativus). There are a number of C. melo types within either the Flexuosus or Conomon botanical groups that are either called cucumber in English or have a form more akin to a cucumber, and that are often eaten before full maturity like a cucumber. As an example, there have been seed growers who have contaminated their seedstocks when growing a melon crop near an Armenian cucumber crop, thinking that the latter was a true cucumber and that the two would not cross. All members of the C. melo genus, no matter how diverse in phenotype, will cross readily if proper minimum isolation distances are not observed.
Squash
Squash is the general term used to refer to the five species of Cucurbita that bear large edible fruit. These species are indigenous to the New World and have long been important cultivated crops in the Western Hemisphere. All five of these species produce fruit that are sometimes eaten when immature (known as summer squash) as well as in the mature stage of growth (referred to as winter squash). All of the squash types bear edible seed that is an important part of the diet in some cultures. Though less common, flowers and leaves of squash are also eaten in some instances. The fruit of the wild progenitors of all of the modern squash types are extremely bitter, and it is believed that the first agricultural societies to domesticate these species did so primarily for their edible seed. As genetic variants with non-bitter fruit flesh were discovered, farmers selected and bred these non-bitter types for culinary use. These selected plants with their edible fruit became the ancestors of our modern-day squash types.
The five domesticated species of the Cucurbita genus produce a wide array of squash types that are used around the world as fresh vegetables, storage vegetables, seed crops, and ornamental and ceremonial icons in some cultures. Any list of important squash types will leave out many significant regional types and sub-types, but an overview of these five species may at least catch a glimpse of the major types and their uses.
Cucurbita pepo L. is by far the most widely grown and economically important species worldwide. There were two centers of origin for this species that occurred independently of each other. The first (C. pepo subsp. pepo) occurred in southern Mexico about 10,000 years ago and the second (C. pepo subsp. ovifera) in what is now the southeastern United States at least 4,000 years ago. C. pepo can be grown across many temperate zones and is somewhat tolerant of cool weather in maturing fruit. This species is extremely polymorphic and includes most of the commercial varieties that are grown and eaten as immature fruit or summer squash. The major C. pepo summer squash cultivar groups include zucchini, cocozelle, crookneck, straightneck, vegetable marrow, and scallop. Also in this species are most of the pumpkins used as Halloween jack-o’-lanterns and varieties grown for edible seed, including variants known as the hull-less seeded types. Winter squash types include the acorn cultivar group and unusual varieties such as ‘Spaghetti’ and ‘Delicata’. Winter squash of C. pepo usually do not store as well as those of C. maxima or C. moschata, and as a consequence, they were often known as autumn squash by squash devotees of an earlier era in North America. Also in this species are small gourds used as ornaments and that occur in a wide range of colors (often striped) and shapes. The wild forms of C. pepo (known variously as var. ozarkana, var. texana, and subsp. fraterna) produce bitter gourds. These grow in the southeastern US and northeastern Mexico and cross readily with their cultivated relatives, thus contributing genes to crop populations of C. pepo from time to time.
Acorn (top) and Yellow Crookneck (below) are two of the most popular commercial types of C. pepo.
C. maxima Duchesne is probably the second most widely grown squash species. It is primarily used as a winter squash, although there are some types that are used as a fresh vegetable when harvested at an immature stage. Like C. pepo, some varieties are well adapted to cooler climatic zones—notably some of the giant-fruited C. maxima varieties grown high in the Andes Mountains in Bolivia and Peru and neighboring countries. Cucurbita maxima is typically cultivated in cooler areas than C. moschata or C. argyrosperma, although there are C. maxima types adapted to hot areas ranging from the humid Amazon River basin to the arid southwestern United States. The center of diversity for C. maxima is the temperate zone of the Andean region of northern South America. Its wild progenitor, C. maxima subsp. andreana, is still found in much of this region and readily crosses with the cultivated types, imparting the bitter fruit flesh trait found in wild Cucurbita species and reinvigorating the domesticated types with genetic variability. Unlike C. pepo and C. moschata, which both spread widely through trade before the era of European conquest, it appears that C. maxima remained close to its center of diversity until Spanish and Portuguese sailors carried seed of it along their trading routes. From that point onward it spread quickly, and selection for the varied types and sizes of fruit has resulted in a wide diversity across the many cultures that grow them. Many of these squash have very high-quality flesh, both in terms of the sugars and dry matter that confer good eating quality and in terms of the carotenoid pigments that give the flesh beautiful shades of orange and deep yellow color and are nutritionally significant for their vitamin A and nutraceutical content. In fact, of all the squash species, C. maxima fruit are the most nutritionally significant based on our present knowledge of nutrition.
The maturity of Cucurbita maxima squash, like this ‘Red Kuri’ (a.k.a. ‘Uchiki Kuri’), can be judged by the color and texture of the fruit’s stem, which will turn tan and corky as it matures.
A number of C. maxima varieties hold the distinction of having the largest fruit of any plants on Earth. Some of the show pumpkin varieties bred in the recent past and their counterparts in the Andes can weigh more than 1,000 lbs (454 k) when grown by a skilled grower. Other C. maxima types that have been important over the past century are in the buttercup, banana, Hubbard, marrow, delicious, turban, and Kabocha classes. The Kabocha type, derived from crosses of ‘Buttercup’ with various Japanese long-storage varieties, has become commercially important across a number of temperate regions around the world. Most C. maxima squash are harvested and eaten at full maturity when they have developed their maximum sweetness, solids, and carotenoid content. However, people also harvest immature fruit to eat as summer squash.
C. moschata Duchesne is an important species of squash in many tropical, subtropical, and warm temperate zones, as this species needs more heat than C. pepo and C. maxima to mature fruit and produce a commercially acceptable seed crop. There is some evidence that either there are two independent locations that served as centers of origin for this species or that one followed the other after trade moved the seed from one place to another. One of the problems in identifying a likely region of domestication is that the progenitor wild species for C. moschata is unknown and may be extinct. One center of diversity for C. moschata is the coastal lowlands of Peru, where there is evidence that C. moschata was first cultivated at least 5,000 years ago. A second putative area of domestication was in the border region between southern Mexico and Guatemala, at least 4,000 years ago.
When Columbus arrived in the Caribbean he found C. moschata widely cultivated by indigenous peoples. It was also found to be cultivated in parts of what is now the southeastern United States long before the arrival of Columbus in the New World. A variety now called ‘Seminole Pumpkin’ was grown by the Seminole people of Florida. And though C. moschata is especially well adapted to hot, humid climates, farmers eventually developed varieties through selection that could be grown as far north as the Great Lakes region of North America.
The ‘Tahitian’ squash (C. moschata), seen here in the foreground, is quite similar in appearance to many C. argyrosperma varieties. Determining the species of the seed crop is always important to know if isolation is necessary.
The fruit are usually either smooth and tan-colored with a short bell shape or a longer crookneck, or they are the short, often furrowed, oblate-shaped pumpkin types that have a tan- to buff-colored rind and flesh that is yellow to pale orange, or sometimes blackish green. The short bell types include the popular, high-quality butternut varieties, while the traditional crookneck varieties and the short, furrowed Cheese pumpkins do not have such a high-quality flesh and are used both for table stock and as fodder for livestock. The carotenoid content of most of the varieties in this species does not approach the levels of the best C. maxima varieties. Varieties of C. moschata are sometimes grown for production and sale of their nutritious seed. Widely grown throughout the Americas, C. moschata also has centers of diversity in Asia and Africa.
C. argyrosperma Huber (formerly C. mixta Pang.) varieties were long thought to be part of the C. moschata species, as they share key morphological features. One distinctive trait of certain varieties of this species is their distinct silver seed, hence the Latin species name argyrosperma. The center of diversity for this species is southern Mexico, probably around 8,000 years ago. Its wild progenitor, C. argyrosperma subsp. sororia, produces bitter gourds (as do all other wild Cucurbita species) and grows as a dominant weed throughout much of lowland Mexico, where it often hybridizes with its cultivated counterpart in farmers’ fields. Historically, the geographic spread of C. argyrosperma was slow compared with C. pepo, C. maxima, and C. moschata, though it has spread to other tropical climatic regions, including Africa.
The cultivated varieties of C. argyrosperma have smooth rinds that are striped or solid with pale greens or earth tones and either are shaped as oblate spheres or have long necks attached to a small bell that contains the seed cavity. The amount of phenotypic diversity in this species is limited, as are the number of commercial varieties; two examples of the latter include the ‘Silver Seeded Gourd’ and the ‘White Cushaw’ or ‘Green Striped Cushaw’. Many varieties of C. argyrosperma were selected almost exclusively for their large nutritious seed, rather than for the fruit flesh; the culinary quality of the flesh is generally poor, pale, and rather low in carotenoid content. There are a number of regions throughout Mexico, Central America, and the Caribbean where landraces or farmer varieties are still grown and continue to adapt to the needs of the farmers.
Cucurbita ficifolia Bouché is the fifth domesticated species of Cucurbita, but the least well known because of its limited geographic distribution. It is sometimes called the Malabar gourd because it was incorrectly thought to have originated in India. Like the other cultivated Cucurbita, C. ficifolia originated in the Americas—probably at high altitude in southern or central Mexico, although its wild progenitor is still unknown. C. ficifolia fruit look somewhat like watermelons in shape, size, and color pattern and typically have white fruit flesh (not found in any other squash species!) and flat black (less commonly tan) seed. It is cold-tolerant relative to other cultivated Cucurbita, and because of this trait it has at times been used as a rootstock for grafting cucumbers. Perhaps due to its capacity for long, sprawling, vigorous growth, Cucurbita ficifolia is erroneously said to be a perennial, although like all of the other four cultivated squash species it is an annual. One of the most popular ways to consume C. ficifolia in Latin America is prepared with raw sugar as a candied fruit, or at times it is used as a food for livestock.
Attempts have been made in some arid climates (Arizona in the United States and in Australia, in particular) to domesticate one of the wild, xerophytic wild species, C. foetidissima HBK (called the buffalo gourd), as either a starch crop or an oilseed crop. The very large storage root produced by this species is a good source of carbohydrates, and the large quantity of seed per fruit is rich in oils and proteins. The commercial ventures targeting development of these new crops, however, were not widely successful and have been largely abandoned.
Crop Characteristics
Reproductive Biology
All squash are monoecious, meaning that there are separate staminate (male) and pistillate (female) flowers on the same plant. The staminate flowers usually form first during the longer days near the summer solstice with a higher concentration of pistillate flowers that emerge, as the days get shorter. The proportion of male and female flowers is typically 4:1 to 5:1 but may be roughly equal during the peak season for fruit production. Squash are usually highly cross-pollinated due to the monoecious condition, though they are self-compatible and will frequently self-pollinate.
A male squash flower (left) and a female flower (right), with the developing fruit at its base.
The flowers of squash are among the largest of all vegetable crops. The petal color ranges from pale yellow to a deep orange-yellow, depending on the species, type, and variety. The male flowers are formed on a stout peduncle at a leaf axil. Each of these staminate flowers has fused petals with staminal collars that contain three stamens flanked by ample nectaries. The female flowers also produce ample amounts of nectar and are easily identified as in all cucurbits by the prominent ovary at the base of the flower petals. These pistillate flowers have a broad stigma with two lobes on top of a short thick style. The ovaries may have either three or five carpel chambers. The bountiful nectar produced in both the staminate and pistillate flowers is the primary attractant to most pollinating species that visit squash flowers. The sticky, large pollen grains produced in staminate flowers are easily picked up by pollinating insects entering the flowers to get nectar. Squash are typically pollinated by bees—either the common honeybee, Apis mellifera, or a range of native bees, including most notably the so-called squash or gourd bees of the genera Peponapis and Xenoglossa. The latter are solitary, ground-nesting native bees thought to have co-evolved with Cucurbita throughout much of the Americas. In places where they are numerous, they are often active well before dawn, beating out the later-rising honeybees to the floral resources in the wild or cultivated squash flowers.
Climatic and
Geographic Suitability
Large-scale seed production of squash is done in warm to hot seasonal conditions. Optimum growth and fruit set occur between 72 and 90°F (22 to 32°C), with nighttime temperatures that don’t usually dip below 68°F (20°C). Nighttime temperatures below this can cause erratic pollen tube growth, resulting in lower fertilization rates and ultimately lower seed yields. As with all crops, temperatures that are too high at the time of the pollination will also retard pollen tube growth and prevent fertilization and subsequent seed formation. In most squash varieties this can happen at temperatures above 95°F (35°C). As with many of the cucurbits, the best seed production areas feature a reasonable amount of heat during the day and warm nighttime temperatures.
Seed Production Practices
Soil and Fertility Requirements
Fertile loam soils that are well drained are desirable for squash seed production. Lighter, sandy loams that are able to warm up easily are especially useful in shorter-season areas with cool springs. Silt and clay loams with good soil tilth are desirable in longer-season areas for their increased ability to deliver a steady supply of water and nutrients. Heavier soils that are well drained will usually translate to greater seed yields over the course of the season for this reason.
Squash plants are heavy nitrogen feeders, and they thrive when ample amounts of well-decayed organic matter or compost are incorporated into the soil. There is a long tradition of using well-decayed horse or cow manure for growing squash and other cucurbits such as cucumbers, which favors the kind of unchecked growth that produces high yields of fruit and seed. If you are using the traditional method of planting the crop into hills, then work two to three shovelfuls of weed-free stable manure into the soil at the location of each hill. A soil pH between 6.5 and 7.0 is desirable.
Growing the Seed Crop
The diverse types of squash have a wide range of climates to which they are adapted for producing a full complement of mature fruit. In fact, it is often noted that C. pepo squash require a relatively short span of time from planting to fruit production for what is one of the largest of the common fruit-bearing vegetables. This feature means that the crop can be direct-seeded in most temperate regions and still mature a reasonable seed crop. In contrast, C. moschata and C. argyrosperma generally need relatively long seasons to produce good yields of a high-germinating seed crop. Seed for all of the species can be planted when spring weather has settled and the danger of frost has passed. Soil temperatures should be at least 68°F (20°C) at the time of planting, and temperatures between 75 and 95°F (24 to 35°C) will ensure emergence in 3 to 5 days.
The modern spacing for seed production on bush-type squashes (that is, varieties selected genetically to have short vine internode lengths) can be as little as 12 in (30 cm) between plants within the row and 18 in (46 cm) between plants for vining types. The modern spacing used between rows for bush types is sometimes as close as 3 ft (0.9 m) to as much as 6 ft (1.8 m) for vining types. This tight spacing, especially on the within-row spacing for bush-type cultivars, is generally much closer than the spacing that was traditionally used in squash seed production before the late 20th century. This shift in cultural practice is attributable, in large part, to the following: First, much of the conventional squash seed production is now done using high external inputs, essentially delivering higher nutrients to the smaller root volume per plant that results when the plants are crowded. Second, much modern squash seed that is produced is F1 hybrid seed. Since the plants of the inbred lines that are used as parents in hybrids are sometimes less vigorous than the plants of the OP varieties, this tight spacing in hybrid seed production is adequate for these less vigorous inbred lines.
The more traditional method of growing the crop, as a vegetable or as a seed crop, is to plant the seed into hills placed equidistant from one another in the field. The spacing used for hills in bush types is usually 3 to 4 ft (0.9 to 1.2 m) apart within the rows and 4 to 5 ft (1.2 to 1.5 m) between rows. This spacing can vary based on the size and bushiness of the typical plant of the variety. (See Cucumber, “Growing the Seed Crop” for a more general discussion on seeding the hills.) Throughout much of Latin America and elsewhere in the tropics, squash is commonly planted (in hills) as an intercrop with corn, even in large-scale commercial plantings.
The spacing for the vining types of squash when planted into hills can vary even more than the bush types due to the wide range in the size of the vines across varieties. Some shorter-vining semi-bush types only trail up to 3 ft (0.9 m), while some of the most vigorous C. maxima varieties have vines that trail up to and sometimes beyond 12 ft (3.7 m). The within-row spacing between hills for the semi-bush or short-vined types can be 4 to 6 ft (1.2 to 1.8 m), with 6 to 8 ft (1.8 to 2.4 m) between rows. Larger-vining types have traditionally been planted into hills that are 6 to 10 ft (1.8 to 3 m) apart within the row and 8 to 12 ft (2.4 to 3.7 m) apart between rows. This between-row spacing depends upon the vigor of the vines.
A number of organic squash seed growers are also using an equidistant spacing between hills for “check row” cultivation as is used in a number of cucurbit crops. This is most advantageous with the smaller vining types in squash where a spacing of 5 × 5 ft (1.5 × 1.5 m), 6 × 6 ft (1.8 × 1.8 m), and up to 8 × 8 ft (2.4 × 2.4 m) is used efficiently. Placing the hills equidistant from each other allows for “check row” cultivation, where it is possible to run cultivation equipment through the field in both directions. This method is a very good option for more thorough mechanical weed control right up to the point where the squash vines start to trail out into the open space between the hills.
Seed Harvest
Squash seed is mature when the fruit’s color is fully developed and when the stem has completely dried to its mature color. In order to have seed with fully developed endosperm, good vigor, and the highest possible germination rate, it is very important to allow the fruit to achieve these respective colors and not rush the harvest. In drier temperate climates it isn’t unusual for squash seed growers to leave the fruit on the plants until light frost occurs. The mature fruit can then be located, harvested, and tossed directly into a mobile seed extractor if one is available for larger commercial seed lots. If the fruit is harvested for hand extraction after frost has killed the vines, it is usually best to get it out of the field in a timely fashion, as repeated frosts and saprophytic organisms can quickly cause excessive rot that may damage some portion of the seed crop. Prolonged exposure to temperatures below 50°F (10°C) generally results in chilling injury. In general, it is never a good idea to allow the fruit to get moldy in the field. In tropical climates, mature squashes are harvested when the rind loses luster and vine tendrils close to the fruit dry and shrivel.
When the fully ripened fruit that is harvested is sound, it is sometimes held for a number of days in a warm, well-aerated place before extracting the seed crop. Unlike many other crops, there is good evidence that for Cucurbita an after-ripening period produces an even higher-quality seed than when the seed is extracted immediately after harvest. This post-harvest storage is, of course, only practical when the seed crop is relatively small. If necessary for prolonged storage, winter squash can be kept anywhere from at least 5 weeks (C. pepo acorn types) to perhaps 6 months (C. maxima Kabocha types), depending on the variety, and on whether there is a suitable rodent-free storage area that is cool and dry (temperatures optimally between 50 and 59°F/10 and 15°C, with a relative humidity of 50 to 75%). Rodents can pose a significant problem in squash seed production if they are allowed to consume seed by burrowing into the fruit wall during storage.
A
B
C
D
E
F
SQUASH SEED HARVEST AND EXTRACTION: (A) Hard-shelled squash are split open in the field before extraction; (B) Squash fruit are thrown into the hopper of the mechanical seed extractor, leading to a series of rotating blades; (C) A screened cylinder separates the seed from the main mass of flesh and rind; (D) Seeds along with small pieces of pulp and rind are diverted to a side trough, with larger pieces of squash ending up in the waste trough at the end of the extractor cylinder; (E) The seed and any adhering pulp are collected; (F) Seed and pulp are poured into containers for the fermentation step.
Seed Extraction,
Fermentation, and Cleaning
Mechanical extraction is used for larger commercial seed lots and is usually done with what is often called a cucurbit seed extractor. At harvest the fruit is put into a hopper that leads to a series of rollers or rotating blades; these macerate the fruit to free the seed as much as possible from the pulp. The pieces of rind, the pulp, and the seed then drop into a large rotating cylinder made of screen material. Much of the small pieces of the rind, pulp, and liquid go through the screen material with the seed into a trough at the bottom of the extractor. Seed that adheres to the pulp is loosened through the spinning motion of the cylinder, passing through the screen. Because the cylinder is at a slight angle, the larger pieces of fruit and pulp slowly work their way to the end of the cylinder and into another trough. Examine the contents of this waste trough before composting them for any appreciable amounts of seed that may have not been captured.
Seed Fermentation: The seed that successfully passes through the screen with smaller pieces of pulp and the peel of the squash fruit is then put into barrels or other suitable containers to ferment in a process similar to that used for tomatoes (see chapter 12, Tomato, “Seed Fermentation”). The fermentation is activated from native, naturally occurring yeasts that are present on the exterior of the squash. The fermentation in cucurbits starts quickly and in my experience may even be stronger than the fermentation that occurs in tomatoes, but such fermentation is not common for squash seed processing. Certainly, there are very few instances where it is necessary to add baking yeast and sugar to boost the process, so long as the seed mixture is kept within an appropriate temperature range. As with tomato fermentation, there is some debate as to whether adding water to the seed mash hinders the fermentation process. Sometimes it is necessary to add water so that all of the seed is submersed in liquid; however, there is often enough liquid present, especially from overripe squash fruit, to easily ferment the batch. Also, like tomatoes, there is some debate as to whether the addition of water encourages sprouting. While there isn’t any published research on this question, there is speculation that naturally occurring sprout inhibitors may be diluted when water is added to the mix.
The temperature range, duration of fermentation time, and separation of the squash seed from the pulp are essentially the same as for tomato.
Seed Cleaning: The final cleaning of squash seed through the use of a sluiceway or with the simpler decanting method is identical to that described for tomato. The main difference may be that when using the sluice to clean squash seed you may need to increase the amount and force of the water to move the heavier squash seed through the sluice (see chapter 12, Tomato, “Washing Seed”). The drying of the seed needs immediate attention, just as with tomato seed, after this wet seed processing.
Genetic Maintenance
As with all vining crops where the vines of individual plants tend to intermingle, it is important to consider increasing the spacing between plants in order to clearly identify the phenotype of individual plants if selection to type is necessary. Plants of the largest vining types can quickly become indistinguishable when you’re trying to discern the differences between individual plants in the field by the time the crop is flowering.
Several important traits can be checked before the plant begins flowering. As with all crops, seedling vigor and early robust growth are always important traits to select for in all crops under organic production systems. You can evaluate squash plants for the length of the internodes before flowering: Determinate varieties (typically called bush types in squash) have shorter internodes than the indeterminate types. There are also subtle to substantial differences in leaf characteristics such as size, shape, degree of lobing, and color that can often be detected.
Once the plant starts to flower, it is possible to evaluate and select the plants of a given variety for uniformity in the timing of flowering and the production of their first female flowers. There is also variation for the number of male or female flowers per node. In most cases it is easy to distinguish the spine color of the fruit before the ovary has been fertilized and starts to expand. As the fruit develops, the basic shape and the relative length-to-diameter ratio of the fruit are sometimes evident even before the squash have attained a marketable size. The earlier you can eliminate any off-type plants, the less time any off-type individual has to spread its pollen (and hence its genes) around.
As the fruit reach a marketable size it becomes possible to select them for their trueness to type for color and the intensity of color. Shape and size of the fruit should also be rechecked at vegetable maturity. Some squash varieties may have raised ribs, which may be distinct for their width and color and are usually distinct for a particular variety if present. As with all seed crops, routine elimination of the plants that are most susceptible to any endemic diseases is always worthwhile.
Isolation Distances
The basic isolation distance that should be used between different squash seed crops of the same species when there are no barriers is 1 mi (1.6 km), especially when these adjoining crops are relatively small (see chapter 13, Isolation Distances for Maintaining Varietal Integrity).
As with all of the cross-pollinated crops it’s important to distinguish between the isolation distances necessary when there are two adjacent squash crops of the same species that belong to the same horticultural type and the distance required when growing two different types of squash of the same species. For example, when growing two similar varieties of C. pepo jack-o’-lantern pumpkins, the minimum isolation distance between them should be 1 mi (1.6 km) in open terrain. When there are two different types of C. pepo squash, if one is a jack-o’-lantern pumpkin and the other is a zucchini, then the minimum isolation distance separating the two crops in open terrain should be at least 2 mi (3.2 km). If the seed crops are being grown in terrain where there are natural barriers on the landscape that hinder the movement of insect pollinators, then you can lessen the isolation distance between two C. pepo varieties of the same type to 0.5 mi (0.8 km) and 1 mi (1.6 km) between two different types of varieties.
As mentioned in the descriptions above of C. pepo and C. argyrosperma, populations of wild or weedy conspecific relatives can grow near crop fields in certain regions and thus can potentially outcross with the crop populations. Such outcrosses can result in contamination due to the transfer of the genes responsible for fruit flesh bitterness into the resultant wild/cultivated hybrid seed. Bitterness is caused by chemical compounds known as cucurbitacins. These are present in fruit and other plant parts of wild Cucurbita species but also are present in high amounts in cultivated forms of C. pepo ornamental gourds. Thus, make sure that seed production fields of squash are isolated from known populations of wild Cucurbita or ornamental C. pepo gourds to guard against contamination from such hybridization.
Watermelon
The center of origin of watermelon (Citrullus lanatus [Thunb.] Matsum & Nakai) is the Kalahari Desert region of southern Africa, though its early history is shrouded in mystery. It is one of the oldest cultivated crops of the African continent and has been cultivated in Egypt for at least 4,000 years. The probable ancestor of watermelon, C. colocynthis, is a wild perennial with bitter fruit and small seed. It is found in archaeological sites across southern Africa and still grows wild in West Africa. David Livingston, the Scottish medical missionary who worked in Africa in the mid-19th century, found copious wild watermelon populations growing in the Kalahari after heavy rains in 1857. India and China are secondary centers of diversity, as watermelon cultivation spread to both of these regions via trade routes long before the existence of any written accounts of their cultivation. Watermelons have long been coveted by desert cultures as a way of transporting potable water for travel across wide expanses of desert. This is especially important during certain seasons, when the quality and quantity of water at certain points along the route is unknown or questionable.
Watermelon seeds are mature at “eating maturity” of the fruit, but the ripe fruit are often left in the field for an extra 1 to 2 weeks to ensure good seed quality.
Watermelon is one of the crops that thrives in hot weather and is best adapted to tropical, subtropical, and many long-season temperate zones. Heat and sun are important in developing the sugars that make watermelon so popular across the globe. Watermelon is especially well suited to growing in hot, arid climates, both as a vegetable and as a seed crop. This includes parts of the Middle East, Southern Europe, North Africa, Israel, Indonesia, Taiwan, Korea, China, and Japan. In North America most of the commercial melon seed is currently grown in the Arkansas River Valley of Colorado, in the Central Valley of California, and in west Texas.
Crop Characteristics
Reproductive Biology
Most commercial watermelons are monoecious, though some andromonoecious varieties, with a combination of hermaphrodite flowers and male flowers on the same plant, do exist. Monoecy in watermelon ensures a large degree of cross-pollination in populations of any appreciable size. Flowers are solitary, borne in the leaf axils. They are pale yellow to light green in color, with the pistillate or hermaphroditic flowers formed in every sixth or seventh axil and staminate flowers occupying the other axils.
Each of these staminate flowers has five petals that are fused at their base, like cucumber, and three stamens tightly packed within each staminal collar. The pistillate flowers are easily identified as they have the prominent ovary at the base of the flower. They have three broad stigmatic lobes on top of a short, thick style. The hermaphroditic flowers that occur in some varieties have three stamens tightly positioned around a thick, short style with a three-lobed stigma. The flowers open shortly after sunrise, and the stigma is receptive for most of the day, depending on the environmental conditions. Insect pollinators are attracted to both the pollen and the nectar, which is produced at the base of the corolla. Each flower only opens for one day.
The hermaphrodite or perfect flowers have both stamens (male flower parts) and pistils (female flower parts). The hermaphrodite flowers are borne singularly in leaf axils and are essentially much like the pistillate flowers in monoecious types, except there are also small anthers borne close to the stigma. The anthers of these hermaphroditic flowers do not produce much pollen and therefore only contribute minimally to the pollination of these flowers. The form and structure of the staminate flowers is similar to that of cucumbers (see Cucumber, “Reproductive Biology”).
Climatic and
Geographic Suitability
Successful commercial production of watermelon seed requires hot days and warm nights—temperatures that are consistently warmer than is needed for other cucurbits, even melons, to produce the best-quality seed. Optimum growth and fruit set occur between 80 and 95°F (27 to 35°C) with nighttime temperatures that don’t usually dip below 70°F (21°C). Temperatures below 70°F (21°C) during anthesis and early seed growth can hinder the development of vigorous, high-quality seed. As with all crops, temperatures that are too high at the time of the pollination will also retard pollen tube growth and prevent fertilization and subsequent seed formation. In watermelon this can happen at temperatures above 100°F (38°C). For watermelons the best seed production areas are regions with a significant amount of heat during the day, warm temperatures at night, and low humidity. The ancestors of watermelon evolved under very low humidity in the Kalahari Desert; the modern cultivated crop also produces well as desert crop in areas that hold the heat at night. Too high a level of humidity during the growing season should be avoided due to the potential for increased levels of foliar disease.
Seed Production Practices
Soil and Fertility Requirements
Watermelons require deep, rich soils with good tilth and superior nutrient- and water-holding capacity to produce a superior crop. Many growers prefer growing watermelons in sandy soils, especially in shorter-season temperate zones where early vigorous growth is essential in maturing a crop. However, heavier soils with good drainage can deliver bountiful fruit and seed crops by supplying the crop with adequate moisture and nutrients, which helps to ensure unchecked growth through the season. As with all cucurbits, watermelons are heavy feeders that thrive when ample amounts of well-decayed organic matter or compost are incorporated into the soil. It has also been reported that ample potassium is important in watermelon for strong rinds that are resistant to cracking at maturity. Watermelons are more tolerant of acidic soils than most other crops and can be grown at a soil pH as low as 5.0. This being said, a soil with a pH of between 6.0 and 6.8 is more desirable. Watermelons grown in arid regions are often grown successfully on slightly alkaline soils.
Watermelons form a large taproot that extends 3 to 4 ft (0.9 to 1.2 m) into the soil. Their secondary root system is well branched and more extensive than that of melons, so they require a wider plant spacing than melons in order to thrive (see “Growing the Seed Crop,” below). This extensive root system is concentrated in the top 18 in (46 cm) of the soil. For this reason it is very important to avoid compacted soils and to only cultivate close to the surface of the soil when growing the crop. It goes without saying that the use of any heavy equipment in a watermelon field during the growing season should be avoided to realize the best yields.
Watermelons are traditionally direct-seeded in climates that are well suited to seed production. Seed can be planted when warm weather has truly arrived, all danger of frost has passed, and soil temperatures are at least 75°F (24°C); even better is when the soil temperature rises above 80°F (27°C), with 90 to 95°F (32 to 35°C) being the optimum for strong germination. This allows young watermelon plants the unchecked growth that will produce a healthy, vigorous plant if the soil, nutrition, and soil moisture are adequate.
If planted in rows, watermelon seed crops need to be given more space than the 18 to 24 in (46 to 61 cm) between plants often used when growing watermelon crops for fresh market. Watermelon plants for seed should be planted at least 36 to 48 in (91 to 122 cm) apart within the row, depending on the vigor and size of the variety. Plant several seeds at the appropriate interval for the variety, then thin them once you are assured that the selected plants are well established and healthy. The space between rows should be increased to at least 10 to 12 ft (3 to 3.7 m) to allow vines enough room to thrive in production of watermelon seed crops. This wider spacing between rows is very important for watermelon growers who are concerned with diseases—it increases the airflow through the developing crop and slows the progress of any foliar diseases that may develop. This is especially important in growing regions where diseases may be more prevalent.
Alternatively, watermelons can be planted into traditional hills within the rows in the field. The spacing used for watermelon hills can be anywhere between 4 ft (1.2 m) apart within the row by 10 ft (3 m) between the rows to the comparable at 6 × 10 ft (1.8 × 3 m) apart or as much 8 × 12 ft (2.4 × 3.7 m). Seed is often planted into the hills at a rate of six to eight per hill, with seed sown in an area that is at least 6 in (15 cm) in diameter. Plants are then thinned to two or three per hill once it is clear which are the most vigorous and healthiest individuals in each hill; however, make sure that the plants aren’t crowded during this pre-thinning period. The idea of a “hill” is also often misunderstood; this area where the seed is sown may not be a noticeable hill, although sometimes there is an actual raised mound when manure is worked into the spot before planting.
Many organic growers transplant watermelons and melons to: (1) get an early start if their spring weather isn’t hot; (2) plant well-developed seedlings into fields that are weed-free; and (3) avoid early infestations of cucumber beetles (Acalymma vittatum and Diabrotica spp.). When transplanting, many growers will plant three or four seeds per cell, thinning to two plants for placement in the hill when transplanting. The direct-seeded row or hill plant spacing can also be used for transplanted crops. In some situations, under heavy cucumber beetle pressure, some growers will cover the seedlings with a floating row cover at planting time to minimize the damage from these pests.
Seed Harvest
For optimum seed quality and ease of harvest, it is best to allow the fruit of all cucurbit seed crops to remain on the plant for at least 1 to 2 weeks after the majority of favorable plants have fully matured. This after-ripening period serves multiple purposes:
1. It allows the ripe fruit to fully mature, as cucurbit fruit often produce a somewhat better seed quality with after-ripening.
2. It allows any laggard high-quality fruit to finish ripening.
3. It softens the placental tissue in the seed cavity to the point where it easily separates from the seed upon cleaning.
Determining when watermelon fruit are mature is accomplished in multiple ways, depending on the variety of watermelon. The most commonly used signs of fruit maturity are to check if the tendril on the shoot bearing the fruit has dried and withered, or to see if the “ground spot” (the spot on the underside of the fruit where the fruit rests on the ground) has turned a warm yellowish cream color. Another important sign of watermelon fruit maturity is when the dominant rind color turns from a bright to a duller, flatter shade of the same color. This duller color of the rind is often described as a “waxiness” that covers the rind as the fruit becomes fully mature. Lastly, many people thump or rap on the fruit to judge ripe fruit. There is a certain resonance of a lower tone that will develop as the fruit ripens, but you must train yourself to hear this with each different variety you grow. None of these signs of maturity is foolproof, especially when considered across different varieties and environments; you must determine which are accurate indicators for your crop (or combination of traits). Don’t hesitate to cut open a number of fruit to practice determining full maturity.
Larger watermelon seed crops are almost always extracted using a vine harvester or vine seed extractor directly in the field (see “Seed Extraction, Fermentation, and Cleaning” below). If the seed crop is relatively small and the fully ripened fruit is sound, the fruit can be gathered and held for a number of days in a warm, dry, well-aerated place to complete the after-ripening if the weather is turning unfavorable.
Seed Extraction,
Fermentation, and Cleaning
A
B
C
D
E
WATERMELON SEED EXTRACTION, FERMENTATION, AND CLEANING: (A) Extracting watermelon seed and pulp by hand; (B) The collected mass of seed and pulp; (C) Tom Stearns of High Mowing Seeds with containers of fermenting watermelon seed; (D) Spray-washing watermelon seed over a wire mesh screen; (E) The washed seed drying on the wire screen.
Mechanical extraction is used for larger commercial seed lots in the cucurbit crops and is usually done with what is often called a vine harvester. At harvest, the ripe fruit is put into a hopper that leads to a series of rollers or rotating blades, which macerate the fruit to free the seed as much as possible from the pulp. The pieces of rind, the pulp, and the seed then drop into a large rotating cylinder made of screen material. Most of the small pieces of the rind, pulp, and liquid go through the screen material with the seed into a trough at the bottom of the extractor. Seed that adheres to the pulp is loosened through the spinning motion of the cylinder, passing through the screen. Because the cylinder is at a slight angle, the larger pieces of fruit and pulp slowly work their way to the end of the cylinder and into another trough. Examine the contents of this waste trough before composting them for appreciable amounts of seed that may have not been captured.
Watermelon seed is unique among the commonly cultivated cucurbits, as the seed is distributed somewhat evenly in the flesh or endocarp of the fruit. Because of this, hand extraction of the seed is different from scooping of the seed out of a central cavity, as is done with the fruit of other cucurbits. Seed extraction by hand requires that all of the endocarp be macerated and pressed against a screen with a mesh that will allow the seed to pass through, but will catch most of the rind and some of the fiber of the fruit’s flesh. Much pulp and all of the juice will pass through as well, which is fine if you choose to use the fermentation method. If you’re not using the fermentation method, it is possible to clean out much of the fiber from the flesh by then passing this stew of watermelon juice, flesh, small pieces of rind, and seed across a screen with a fine mesh, one that will not allow the seed to pass through but that will allow much of the other material to pass through when sprayed with a high-pressure stream of water. This process can be repeated a couple of times, with a cleaning of the screen between the spraying events to remove any fiber that gets caught in the screen. At this point the seed can then go to the final seed-cleaning steps using a sluiceway as described in “Seed Cleaning,” below.
Seed Fermentation: Watermelon seed does not have a placental sac, so it does not require the fermentation step used with cucumber that frees the seed from this sac. After the extraction described above, most watermelon seed is cleaned under a high-pressure spray of water that removes any fleshy material. While this is often enough to clean the seed, a number of seed growers give melon seed a 1- to 2-day fermentation period, which helps free the seed from all endocarp material. Fermentation of watermelon seed can also help eliminate bacterial fruit blotch of watermelon (Acidovorax avenae subsp. citrulla), which is the most serious seedborne disease of watermelon and can devastate entire crops if conditions are favorable. This disease grew to epidemic proportions in North America during the 1990s. The result was that most seed companies now require any commercial growers purchasing watermelon seed to sign a waiver stating that they won’t sue the seed company if they incur losses from this disease.
Research done in the 1990s demonstrated that a 1- to 2-day fermentation of watermelon seed coupled with 15 minutes of treatment with 1% hydrochloric acid or 1% calcium hydrochlorite, followed immediately with the washing and drying of the seed, is an effective treatment for bacterial fruit blotch of watermelon. There are two questions for organic seed growers and organic seed companies: (1) Are concentrations of 1% hydrochloric acid or 1% calcium hydrochlorite acceptable for organic certification? (2) How effective is the fermentation process by itself?
The problem with fermentation in watermelon, as in all of the cucurbits other than cucumber, is that it can easily damage the seed and lower the germination percentage if it continues for too long. There are always specific varieties that are more easily damaged by fermentation than others in the same crop species. In watermelon, the triploid seedless types are much more susceptible to damage during fermentation than the traditional seeded, diploid types.
If the conditions are right, only about 24 hours of fermentation is necessary for watermelon or melons. It is the same process described for tomato (see chapter 12, Tomato, “Seed Fermentation”). The main difference is that, once you get the watermelon seed mass fermenting, it only requires a few hours to separate and clean the seed from the placental tissue. Thus you must watch the process closely and separate the seed from the fermentation bath as soon as the desired results are achieved.
Seed Cleaning: The final cleaning of watermelon seed through the use of a sluiceway or with the simpler decanting method is identical to that described for tomato. The main difference may be with the amount and force of the water, which may need to be increased to clean the heavier watermelon seed (see chapter 12, Tomato, “Washing Seed”). The drying of the seed needs immediate attention, just as with tomato and cucumber seed, after this wet seed processing.
Genetic Maintenance
An initial selection for seedling vigor and early robust growth is always important to maintain good watermelon stocks, as it will always lead to the establishment of a healthy seedling. This is especially important for varieties selected for organic production methods. It is often easier to identify superior vigor when conditions are less than ideal upon planting or transplanting the crop. The variation in watermelon leaf type is not as pronounced as it is in many of the other cucurbits, but there may be some variation in the shape and size of the lobes or in their color that could indicate an off-type outcross. Different watermelon varieties may also vary in the length of the internodes; shorter semi-determinate varieties have shorter internodes than the indeterminate types. Variants for all of these traits can usually be detected and rogued out of the field before flowering.
For watermelon, as with all of the cucurbit crops, it is often possible to detect off-type fruit shapes and fruit color at or very near the time of flowering and early fruit set, especially if the off-types are somewhat different from the variety you are growing. As the fruit approaches maturity it becomes easier to discern differences in both shape and color. Deviations from the round, globe, oval, or elongated oval of the larger watermelon types can vary to some degree within a variety, but all seed growers must set a standard range within the fruit shape to which they adhere for varietal integrity. Color variation from dark black-green to very light gray-green can distinguish a variety, and fruit may exhibit anything from solid color to many variations of stripes. The coloration of the fruit should be checked as the crop matures; this is sometimes easier to judge before the waxy coating that accompanies ripeness develops. As with all seed crops where the fruit is the main agricultural crop of interest, the earlier you determine that the fruit is not true to type, the earlier you can rogue out the plant and eliminate it as a source of pollen that might otherwise contaminate the rest of the seed crop.
At fruit maturity it is possible to test a number of fruit quality traits. An important characteristic of watermelon fruit is the thickness and toughness of the rind. There is a standard way to determine this with a handheld punch device that measures the amount of pressure needed to pierce the rind. This is a destructive test, but when harvesting for seed rather than for market sale, it doesn’t matter. The simpler, faster, and perhaps easier way to judge the thickness of the watermelon rind is to drop the fruit from approximately knee height and see if it cracks on impact with the ground. This method can be very effective but will vary with the soil type and moisture content of the particular soil where the crop is being grown. Keep in mind that there is probably variation for this rind toughness trait in any open-pollinated watermelon variety; such selection can benefit the commercial value of the variety.
The maintenance of quality traits associated with the flesh of watermelons should also be considered. It is possible to check individual plants for internal traits such as color and intensity of color, as well as sensory characteristics like flavor, sweetness, and texture. Fortunately, selection for these traits at full fruit maturity is a non-destructive test as far as seed harvest is concerned. Fruit evaluated for these internal characteristics can be tossed right into the vine harvester at the time of harvest or into containers for subsequent extraction. Once you find an objectionable fruit, however, you must be sure that its seed, and that from other fruit from the same plant, won’t be harvested with the crop.
Watermelons have at least six major classes of flesh color: scarlet red, coral red, orange, canary yellow, golden yellow, and white. The first five colors are all due to the presence of carotenoid pigments, which can vary in concentration, thereby resulting in various shades. Selection for the maintenance of a particular color, and the specific shade and intensity of that color, is important over the long run.
A very important quality trait is the flavor of the watermelon. As with other sweet vegetable crops, there is much more to the flavor of a watermelon variety than just the sweetness. In fact, some of the best breeders do not rely purely on a Brix reading of the sugar content when performing selection for flavor. Watermelon flavor is complex and is a combination of the sweetness and various volatile flavor compounds that constitute the flavor underlying the sugar content of a variety. The taste of these volatiles can vary from what is sometimes called a rich caramel flavor, which some people find syrupy and objectionable, to a range of floral aromatics that are usually desirable but can be overwhelming when present in excess. The only way to distinguish the differences is with the human tongue, experience, and knowledge of what is desirable in the marketplace. A taste test may be necessary, as there may be plant-to-plant variation in flavor or texture in any open-pollinated variety, especially when the variety is not stable or exhibits variation in some other trait.
The texture of the watermelon flesh can also vary quite considerably. The eating texture can range from soft to firm and from crisp to fibrous, even stringy. As with flavor, the best way to judge the texture is with a bite test, especially after you gain experience in sensory evaluation of watermelon. Another trait to be aware of when evaluating ripe watermelons is the tendency of some varieties (or individual plants within varieties) to produce fruit with hollow heart, where the flesh has fractured during the maturing process, leaving a cavity in the flesh at edible maturity. Lastly, the mature color of the seed can vary greatly in color, from white to tan, brown, red, black, speckled, or some combination of these colors. Watermelon seed can also vary considerably in size. Growers have sometimes first detected outcrosses in the previous generation by noticing the unusual variation in seed size at seed harvest.
Isolation Distances
For watermelons the basic isolation distance that should be used between different watermelon seed crops without barriers is 1 mi (1.6 km), especially when these adjoining crops are relatively small (see chapter 13, Isolation Distances for Maintaining Varietal Integrity).
As with all of the cross-pollinated crops, it’s important to distinguish between the distances that are necessary when the two adjacent watermelon crops are of the same horticultural type versus when there are two different types of watermelons being grown. When you’re growing two watermelon varieties of the same type—for instance, two small, round icebox types with red flesh—then the minimum isolation distance between them needs to be 1 mi (1.6 km) in open terrain.
When there are seed crops of two different types of varieties—for instance, if one is a blocky oval ‘Crimson Sweet’ type with red flesh and the second is an oblong orange-fleshed Texas type—then the minimum isolation distance in open terrain should be 2 mi (3.2 km). Indeed, even if both varieties to be grown were blocky, oval, red-fleshed shipping varieties, but were distinctly different in their days to maturity, size, or disease-resistance profile, they would still require this increased isolation distance if grown in open terrain.
If two watermelon seed crops are being grown in terrain where there are significant natural barriers on the landscape that hinder the movement of insect pollinators, then you can lessen the isolation distance between two varieties of the same type to 0.5 mi (0.8 km); 1 mi (1.6 km) between two varieties of distinctly different types.
Note: It is important to note that there are feral citrons and feral watermelons growing in various areas of the world where watermelons are grown for seed. All watermelon seed growers need to determine if any of these feral forms grow in the regions where they plan on producing watermelon seed.