One of the main requisites of growing high-quality seed is to maintain the varietal integrity of the seed crop you are producing. To do this you must make sure that the genetic makeup of the crop isn’t compromised with any genetic mixing via pollen from other plants of the same species. This requires producing a seed crop at an adequate spatial isolation from any cultivated, wild, or weedy plants of the same species that are capable of intermating with your crop via their pollen. You must consider a number of biological and environmental factors that can affect pollen movement by insects and wind when determining the minimum isolation distance for each crop and cropping system.
The Species Boundary
First, it’s important to know which crops are capable of sexually intermating or “crossing” with the seed crop that you are planning to produce. The word species means “kind” in Latin. In general, each species represents a distinct kind of organism that is able to intermate and produce fertile offspring only with other individuals of its own kind or species. Of course, there are exceptions. For example, a number of species of deciduous trees will sometimes cross with related tree species and produce interspecific hybrids. Thankfully, though, there are very few exceptions among vegetable species.
In the vegetable crops the species designation almost always defines a boundary to potential crossing when determining placement of seed crops for isolation. The two notable exceptions among the vegetables are the several species of peppers (Capsicum spp.) that are sexually compatible and easily cross and at least two wild species of tomato that will readily cross with the cultivated tomato (Solanum lycopersicum). (see chapter 12, Tomato, “Isolation Distances”.) Another exception is crossing among squash species in the Cucurbita genus, though it does not occur nearly as readily as with tomatoes and peppers (see Chapter 9, Cucurbitaceae, “Isolation Distances,”).
Knowing the species designation of each crop you work with also allows you to learn a great deal about the reproductive habits of the crop. When questions arise as to identifying the species of a given crop, it is recommended that you contact researchers or seed professionals who are well versed in the crop before planting.
Increased Isolation Considerations for the Organic Seed Grower
The importance of maintaining adequate spatial isolation has increased significance in the modern era for the organic seed grower. Pollen contamination from genetically modified (GM) crops will disqualify the seed crop from consideration as a certified organically grown product. Many of the opponents to GM crops see the resultant genetic mixes as fraught with potentially damaging effects on the ecological balance in organic farming systems and having potentially harmful effects on human health. Certainly, everyone can agree that the introgression of modified genetic traits into seedstocks intended for organic agriculture may be very hard to get out, resulting in long-lasting effects on the integrity of the germplasm. Most professionals involved in the production of organic seed advocate for at least a two- or threefold increase in the minimum isolation distance when GM crops of the same species are in the vicinity over the normal minimum isolation distances stated for each particular crop species in this text.
The Myth of Standardized
Isolation Distances
The first thing that most farmers learn when growing a seed crop is that their crop must be isolated from any other crops of the same kind in order to produce seed that is genetically pure and hasn’t crossed with a neighbor’s crop. For most farmers, the best information that they can get on how much isolation they’ll need for their seed crop is often vague and not very specific to their environment or the crop species that they are growing. The recommendations that are most often repeated is that seed crops require either 0.5 mi (0.8 km) or 1 mi (1.6 km) between cross-pollinated crops of the same species, and that self-pollinated crops need little or no isolation (although a barrier crop is sometimes recommended) from other crops of the same species. This information is often just hearsay from farmers or local agricultural officials, or it is found in much of the published literature from older seed saving publications and on the Internet. It often leads to poor planning of seed crops, which in turn can lead to pollen contamination and genetic mixing and outcrossing.
If you’re located in a region where there is a concentration of production for the particular seed crop of interest, isolation guidelines have often been worked out between regional growers’ cooperatives and local governmental organizations, like the state Extension Service of the Land Grant Universities in the United States. Unfortunately, much of the specialized information generated by these groups is difficult to access, is unknown outside of these regions, and is not easily found in the literature.
The other common misunderstanding among seed growers has to do with the idea of varietal or genetic purity in crop varieties. In most of the seed-growing literature there seems to be the implication that if growers follow the recommendations for isolating their seed crop, then they have eliminated any chance of an outcross and can expect to harvest a seed crop that is genetically pure. In reality there is no such thing as perfect isolation, unless perhaps truly extraordinary steps are taken. Even when the strictest isolation distances are observed by the seed companies for stockseed and foundation seed, cases of genetic crossing have occurred that can only be explained by pollen mixing. While pollen of any given crop has an average range of movement based on the types of insects or direction and strength of wind that carries it, there are certainly the unusual events where the pollen is carried several times farther than its usual trajectory due to an extraordinary insect pollinator or wind gust that may come along. Many seed workers know that pollen is easily carried on clothes, on the fur of animals, and in water. Ultimately, having a pollen-proof space to produce truly pure seed would require high-tech equipment comparable to some of the gadgetry of NASA and would be prohibitively expensive. All experienced seed growers learn how to deal with the occasional crosses that occur in their stocks; in fact, they expect it to happen and select or “rogue” against it in every generation. Indeed, sometimes these chance crosses result in new combinations that the savvy grower can isolate and develop into a new variety. Remember, nature is always creating new genetic combinations to be tested in the great experiment of life on Earth.
What Is the Intended Use of Your Seed Crop?
The first question that seed growers must ask themselves when determining the placement of a crop is, “What is the intended use of the seed that I am producing?” The relative genetic purity of seedstocks necessary for a seed company’s commercial sales is quite different from the increased level of purity required for its foundation seed or stockseed. The precautions that are often taken when growing a seed crop for genetic preservation purposes will invariably meet a stricter standard than the isolation requirement for farmers saving seed for their own production needs. The relative nature of this determination must always be thought through and decided by the individuals who are upholding the quality standards for the farm, seed company, or public institution that they represent. The isolation distances may frequently vary based on the regional fluctuations of the biological and environmental factors discussed in this chapter. Those experienced in seed growing in any given region will be the most qualified to take the isolation guidelines proposed at the end of each crop section and amend the distances based on their practical knowledge of the regional factors that exist. This requires experiential knowledge to be collected among seed growers and seed-company fieldworkers who aren’t afraid to get their boots dirty, who have good observational skills, and who are real students of nature. Many of the minimum isolation distances given for the different crops in this book are greater than many experienced seed growers believe are necessary. The minimum distances presented here are only guidelines, to ensure relatively small levels of crossing between moderately large production fields, and should be treated thusly.
The fact that there is no such thing as perfect isolation can be intimidating to anyone seeking genetic purity in seed. But it can also be liberating once everyone involved in seed production realizes there is no such thing as absolute purity and that some genetic mixing is inevitable whenever seed is produced. It then becomes your responsibility to be much more involved in the process of determining the isolation distance based on the biology of the crop and the environment and topography of the location where you’re growing it. You must also get more familiar with each seed crop, realizing that your selection of off-types, outcrosses, and seed mixes is the only way in which the integrity of the crop will be maintained as it passes through your hands to farmers who will grow the crop now and in the future.
Knowing that genetic mixing and variation is inherent to the process puts us back in touch with our true role in the process. This is the same role that our ancestors who first domesticated these plants had. This is the way that we integrate these crops into our lives. The genetic integrity of the crop then becomes a reflection of our commitment and involvement in the process. The crops that we grow and use thus become woven into our communities and into the “culture” of our agriculture.
Factors That Determine Isolation Distances
First and foremost we must remember that what we are working with is a biological system that is dynamic and highly interactive with other biological entities in the ecosystem. The next thing to keep in mind is something all good farmers know: Crops interact with the surrounding environment. Certainly climatic factors of temperature, precipitation, wind, daylength, and relative humidity play a central role in the overall success of a crop, but they also can determine what amount of isolation distance is necessary. Along with these physical manifestations of climate are the physical elements of the surrounding landscape. The terrain—whether it’s hilly or flat, covered with lush vegetation or open—can play a very important role in determining the isolation distance that’s needed between crops capable of crossing. Lastly, the physical size of the crop, its configuration on the land, and the type of pollen, whether airborne or insect-transmitted, are all crucial in any determination of isolation distances. All these factors will be considered for the two reproductive classes of seed crops, selfers and crossers.
Is the Crop a Selfer
or a Crosser?
The crops that are grown from seed are usually categorized as either self-pollinated or cross-pollinated when referring to their mode of reproduction. The assumption is that selfers always self and crossers always cross, with only rare exceptions. In fact the scope of reproductive behavior for either of these reproductive types is a continuum, with some selfers crossing upward of 50% of the time and some crossers that routinely self at a significant rate under certain environmental conditions. (Indeed, all selfers will cross to some degree in almost any situation, and all crossers, even those with strong physiological mechanisms to enforce outcrossing, will self at some small percentage during reproduction.) As with many mechanisms in biology, discrete groups do not exist, and exceptions to any general category abound. As a seed grower you must learn the reproductive peculiarities of each seed crop for your environment through your own observation and experience, as well as the shared knowledge of others.
Self-Pollinated Crops
Common Misinformation: The literature concerning isolation distances for self-pollinated species usually has little more than cursory mention of the various ways that selfers naturally cross-pollinate (if this is mentioned at all). It is presumed that these crops faithfully self-pollinate when grown for seed, with rarely a mention of the possibility of crossing. When a distance is given it is usually specified as the distance necessary to prevent mechanical mixing between two varieties at harvest. In a number of instances in North American literature the minimum separation is given as 10 ft (3 m), 12 ft (3.7 m), or 25 (7.6 m) ft between varieties of selfers with this intent. However, this is not enough isolation between different varieties of a crop species to minimize the amount of crossing that is possible under certain environmental or biological conditions that may be present (see “Environment,” below).
Another serious flaw in the literature concerning isolation distances is that there is very little discussion of unusually promiscuous self-pollinated crops such as runner beans, favas, eggplants and peppers, which often cross at rates at or above 3 to 5%, with some types that can cross at rates above 20% or even 30% under certain environmental conditions. (See “Isolation Distances” under each of these respective crops for details.) These four crops, which are capable of outcrossing at such high rates, will be referred to as the “unusually promiscuous selfers” in this discussion.
Environment: Environmental factors such as heat and humidity can influence how easily the cleistogamous flower of a selfer will open when fertile. Under high temperatures, several crops of the Fabaceae may open slightly at pollen maturity, allowing any pollinator more access to pollen and nectar than usual. Also, the degree to which cleistogamous flowers will open when a probing insect in search of pollen or nectar attempts to pry the petals apart for access can vary with temperature and humidity. Crossing rates in field peas are higher under cool, clear conditions than when temperatures are excessively hot and dry.
Physical Barriers: The presence of physical barriers can play an important role in determining the isolation distance for a particular crop. Physical obstructions to the movement of insect pollinators (the main vectors of pollen in cross-pollinations among selfers) such as hills or forests can be very effective barriers that can cut the isolation distance between crops to half of what it should otherwise be in open terrain. However, when physical barriers are less than complete, the isolation distance needed should be somewhere in between the distances recommended for those with barriers and the standard distances given when no barriers are present (see “Isolation Distance” under each individual crop type).
Genetics: Different varieties of the same self-pollinated species may differ in their response to environmental challenges. In certain varieties of several crops, low relative humidity levels may result in the failure of pollen to germinate on the stigma. This allows a window of opportunity for pollen from a less humidity-sensitive variety to fertilize these flowers during an excessively dry period. The take-home message for all seed growers to remember is that there are almost always genetic differences between varieties of any given crop in their susceptibility to environmental stresses. In other words, some varieties of a particular self-pollinated crop will outcross more readily than others, especially in the face of challenging environmental conditions. With the likelihood of environmental extremes increasing due to global warming and regional climate change, the precautions dictated by the isolation distances listed for self-pollinated species are warranted.
Recommendations: So what distance should you use as the minimum isolation distance for selfers? Because the potential influence of biological factors is quite variable and dependent on the environment of production, it is important to set a standard isolation distance that ensures a high degree of purity across most environments and production situations. Depending on whom you speak with in the seed industry, this can be a distance anywhere from 20 to 150 ft (6 to 46 m) to minimize possible crossing in most selfers for commercial purposes.
If we want to set a rigorous standard for isolation distances between selfers, then we must consider the standards set during the implementation of the Lend-Lease Act of 1941, when the United States greatly expanded its export of seed with the onset of World War II. Because Germany occupied many of the major seed production areas of Northern Europe during World War II, the United States was thrust into the role of a major supplier of seed, especially the seed of several important species of vegetable crops. In an attempt to ensure high-quality seed the isolation requirement for selfers was set at 150 ft (46 m) between crops. It must be remembered that this distance was established for large-scale productions where the risk of crossing is always greater due to the sheer numbers of plants. While this distance may seem excessive to many seed growers with experience in producing selfers, it will certainly eliminate most potential crossing events under most climatic conditions across the varied agricultural regions of the world. There are many climates where excesses of heat, humidity, or the presence of certain insects can cause a much higher rate of crossing in all selfers. Therefore you can confidently use this strict 150 ft (46 m) minimum isolation distance for most selfers in most cases (excluding the unusually promiscuous selfers, which require greater distances). If you are producing a particular self-pollinated crop in a climate where the isolation distance has proven to be reliably less than this recommendation, such as the production of common bean seed in the Treasure Valley of Idaho or lettuce seed in the Sacramento Valley of California, then you should use the locally established isolation recommendations. Always remember that seed production under organic conditions often favors a higher rate of crossing due to increased biological activity.
Common Misinformation: While information on isolation distances for cross-pollinated crops can usually be found in the seed-growing literature, it is usually too general and doesn’t make any distinction between insect-pollinated crossers and wind-pollinated crossers. There is a definite tendency for the isolation distance to be given as a “one size fits all” number for all crossers in many of these publications. In American publications the one number recommendation is typically either 0.5 mi (0.8 km) or 1 mi (1.6 km) for all crops, or sometimes 0.5 mi (0.8 km) is used for some of the crossers and 1 mi (1.6 km) is used for others, often in a seemingly arbitrary fashion. In a sample of the European literature published in English, the distances are usually either 1 km (0.6 mi) or 1.5 km (0.9 mi). While there is often a bit more reasoning behind the increased distance for some crossers in this group, there is still less-than-adequate information on the need for some crops in some situations to have upward of three to five times this much distance between them! (See specific recommendations as listed under “Isolation Distances” for each crop in chapters 4 through 12.)
Environment: Environmental factors such as temperature and relative humidity can have a marked effect on the extent of cross-pollination in crossers. Sometimes environmental extremes can diminish cross-pollination. If it’s hot enough, the pollen may be damaged quickly at pollen shed or anytime during anthesis (the period of floral fertility and receptivity). If the relative humidity is too low, the pollen may desiccate and die before reaching another fertile flower. Cold weather can also diminish the viability of pollen and the receptivity of the female stigmas of a flowering crop. In contrast, some environmental conditions can increase the chances of pollinations occurring over time or space. High humidity can contribute to pollen’s longevity, helping it remain viable longer and enhancing its chances of reaching a sexually compatible plant that is farther afield than it would normally travel, carried either by an insect or the wind. The wind, too, can contribute greatly to cross-pollinations, occurring far beyond the normal range of the wind-pollinated crops when the conditions are just right.
Physical Barriers: Physical barriers of the terrain, such as hills, vegetation, or even buildings, are just as important in reducing the flow of pollen between crops in crossers as they are with selfers. These obstructions will reduce or stop the insects or wind that can carry pollen. As with selfers, a solid obstruction such as a forest with dense vegetation between two crops will enable you to cut the isolation distance in half. Incomplete or partial barriers require you to make a judgment call as to the degree that you can reduce the isolation distance and avoid the vast majority of potential crosses. The isolation distance for partial physical obstruction must be gauged somewhere between the full recommended distance and half that distance.
Genetics: Genetic differences among the crossers include a number of traits that determine the quantity and possibly the quality of the pollen produced by a given cross-pollinated crop variety. Over the years many seed growers have recognized that some varietal populations are more robust in a number of floral characteristics than other varieties of the same crop: the number of flowers produced per plant, the average duration of flowering for the variety, and the amount of pollen produced by individual flowers at the time of pollen shed. All of these characters can vary greatly from variety to variety, with genetically healthy, robust populations producing more copious amounts of pollen due to the combination of the three. This increased quantity of pollen can have an impact on the potential for outcrosses, based on the idea that more pollen grains in a specific environment will increase the chances of wind or insects carrying it to another location.
Plant breeders have also long known that pollen from different genetic backgrounds within a particular crop species can be quite different in its duration of viability and in the vigor expressed in the growth of its pollen tube after landing on the stigma of another flower (see chapter 2, Reproductive Biology of Crop Plants). The genetic variability of the male sexual cell or gamete’s ability to reach an egg cell (some see it as a competition of the male gametes) can definitely influence the number of outcrosses between two varietal populations when all else is equal.
Wind- or Insect-Pollinated?: The two different types of pollination found in the cross-pollinated species—wind or insects being the primary mode by which pollen is transferred between plants—generally require two different sets of numbers for the isolation distances needed between crops of the same species. They also require a different mind-set and strategy when you’re making decisions as to the placement of isolation plots. Most cross-pollinated crop species are primarily pollinated by insects. The notable wind-pollinated vegetable crops are sweet corn and the members of the Amaranthaceae (beets, orach, spinach, and Swiss chard). The wind-pollinated crops have pollen that is lightweight for its size and has a surface morphology that enables it to become airborne easily and potentially travel far distances. The classic story that is told to put one in awe of how airborne and travelworthy this pollen can be is the tale of beet pollen being collected at 5,000 ft (1,524 m) in an airplane flying over a sugar beet seed production area. How high and how far this windborne pollen can go before desiccating and losing viability is the real issue in this case, but the main point—that windborne pollen can travel quite far—is well taken when considering the greater isolation distances that are recommended for wind-pollinated species. These are the questions that must be answered when placing wind-pollinated crops. Is there a predominant wind pattern in the cropping area under consideration? Is the crop directly downwind from another crop that it can cross with? Is it upwind of a crop it can cross with? Also, the issue of crop type within a species comes to the fore. In commercial seed production two different crop morphological types are often separated by a greater crop isolation distance. For instance a smooth-leaved spinach and a savoy-leaved spinach require greater minimum isolation than two smooth-leaved spinach varieties. Why? Because if a cross does occur between a savoy and a smooth type, the savoy trait will become obvious in the outcrosses that appear in the next generation of the smooth-leaved variety when it is grown by the produce farmer!
The extent of crossing possible in insect-pollinated crops can increase depending on the type and number of insect pollinators present. There is very little information on the distances that wild pollinators will travel when foraging for pollen. We now know a good deal more about the range of honeybees and the other semi-domesticated bees, which are currently becoming even more important as pollinators as modern problems like colony collapse disorder keep mounting for the honeybee. Certainly the density of pollinators present on a per-acre basis is quite important when considering chances of crossing with nearby crops. If pollinators are numerous, then their incentive to travel farther to forage is increased, and this may increase the rate of crossing at what may otherwise be an adequate isolation distance.
Key to Isolation
Distance Terminology
Used in Each Crop Section
The distances given are the minimum distances that are necessary to eliminate most naturally occurring chances of crossing between two crops of the same species. Crosses may still occur at these distances due to insects or other animals, wind, and sometimes even human activity. Beet, Swiss chard, and spinach seed workers will need to change clothes between fields of the same crop if working in more than one field per day, since pollen clinging to cloth can be transmitted and cause crosses.
Barriers are physical obstructions to pollen flow on the terrain between two crops of the same species. These may include hills, mountains, rock formations, forests, or other forms of dense vegetation of some height and stature. They can also include buildings or other structures that have a significant height or stature. When these obstructions are prominent on the landscape and occur between two seed production fields, the seed grower can often reduce the minimum isolation requirement by half. If the barrier is partial, then the grower or seed company must decide if it will be significant in slowing the flow of pollen and insects. In any event, the reduction in isolation distance will be less than the one-half reduction that is possible when the barrier is prominent on the landscape.