The vegetable crops with a biennial life cycle have a special set of circumstances that must be met to successfully produce a seed crop. A biennial is defined as a plant that takes 2 years to complete its life cycle. Biennials grow vegetatively during the first growing season, storing carbohydrates in storage tissue. In the second season this stored food is used to quickly establish a robust plant that has the potential of producing prodigious amounts of seed before the end of the season. The storage organ of a biennial is usually a root (for example, carrot or beet) or a bulb (onion), but can also be the stem (celery), leaves (cabbage or kale), swollen stem (kohlrabi), or the unusual enlarged fleshy hypocotyl and upper root tissue that makes up the root crop of turnips, radishes, and rutabagas. After storing food in their varied storage organs at the end of the first season, all biennial plants must go through a period of relatively cold temperatures in order to promote flower initiation the following season. This cold treatment or vernalization requirement (see “Vernalization,” below) is the reason that seed of these crops is almost always grown in temperate climates and is not easily produced in tropical climates. Unfortunately, there are relatively few temperate climates where the winters supply enough cold to fully vernalize the crop without destroying it if the crop is overwintered in the field.
The Biology of
the Biennial Life Cycle
The evolutionary strategy of the biennial life cycle is simple. In the first season the plant grows a substantial amount of foliage and stores much of the photosynthates that it gathers in the form of carbohydrates in storage tissue for the next season. The plant then goes into a resting phase for the winter at a low level of respiration (the oxidative breakdown of stored food within the cells to maintain cellular processes), using as little of the stored carbohydrates as possible. During this period the plant is naturally exposed to a prolonged period of cold temperatures, and this is a critical factor that contributes to the induction of flowering in the plant’s second growing season.
The term vernalization essentially refers to a particular length of time at or below a certain temperature that each biennial crop requires for flowering in its second season.
This cold treatment or thermal induction is essential in the initiation of the floral stem in the plant that will become the inflorescence soon after the plant resumes growth in spring of the second season. Without the proper vernalization period the crop will not flower.
The length of the vernalization period varies from one biennial crop to another and is usually measured in weeks by seed growers. It differs from species to species, from crop type to crop type within a species, and even from variety to variety within a specific crop type.
Carrots are a good example of a crop species that has a wide variation in the time required for vernalization. For example, carrots adapted to the subtropics usually require 2 to 4 weeks of low temperatures to initiate flowering, whereas most varieties of the western type require at least 8 weeks of vernalization. There are also differences between carrot varieties within these types: Among the western, temperate-adapted carrots some of the older true Nantes varieties are completely vernalized after 6 weeks, whereas some of the Northern European Flakkee storage varieties need a full 10 weeks (and sometimes more) of cold treatment to be fully vernalized.
The temperature range at which vernalization occurs will also vary from one crop to another. The vernalization temperature requirement for most biennials is somewhere between 32 and 60°F (0 to 16°C). There is a considerable range in the optimum vernalization temperature for different crop species. For example, sugar beets have the quickest flowering response when vernalized at 54°F (12°C), while a number of onion varieties from northern Russia have an optimum near 37 to 39°F (3 to 4°C). In general, an optimum temperature range for most biennial crops is between 41 and 50°F (5 to 10°C).
In practical terms most experienced biennial seed growers know that when most biennial crops are exposed to temperatures that are consistently below 50°F (10°C) for at least 8 to 10 weeks after the first season of growth, they will reliably flower the following season.
Floral Initiation
Plant growth regulators that stimulate flowering increase as a response to vernalization, long before any outward manifestations of floral initiation become evident. The first morphological change that is visually apparent (without cutting into the growing point of the crown) is the formation and elongation of the flower stalk. In most biennials, the flower stalk initiated by the bolting response is first evident at or near the crown, or apical growing point, of the plant. When flower initiation has occurred, the physiology of the storage tissue will change, directing nutrients to flower production. Once this biochemical cascade has started there is nothing you can do to reverse the flowering process and force the plant to return to the vegetative stage. In other words, the misguided practice of cutting off the flower stalk of any biennial will not stop the biochemical processes once they have been initiated. Even though flowering may be prevented when the inflorescence is cut from the plant, the re-initiation of vegetative growth necessary for a commercial vegetable crop will not occur at this point.
All biennials have a period of growth, early in their development, before they are responsive to the effects of low temperatures that induce vernalization. This period is known as juvenility. This period is different for each biennial crop species and for different varieties within each species. While chronological age and size are not the sole determinants of juvenility, most biennial crops are in fact quite small, often only 4 to 6 weeks old when they are physiologically mature enough to begin the vernalization process, if temperatures are low enough. Information on when this point of maturity occurs is not easy to determine for most crops; experience in growing the crop often serves as the best tool to judge the proper stage at which a biennial plant is ready to begin receiving the vernalization treatment. As an example, more is known about carrots and their approximate size for vernalization than for most other biennials. Carrots must have at least 8 to 12 leaves and a storage root with an approximate diameter between 0.15 and 0.3 in (4 to 8 mm) to start vernalizing (the common diameter of a pencil is 6 mm). While this may seem quite small, it is important to realize that carrots must usually have between 6 and 10 weeks of steady, initial growth to achieve this kind of size. Other biennial crops probably need a comparable period of growth before the vernalization process can commence; anything less than this in the first season of growth, especially when producing a seed crop using the seed-to-seed method, could lead to less than complete flowering of the crop during the second season.
There are two options when it comes to having a biennial crop survive from the first season of growth to the second season. The first option involves two techniques of keeping the crop in the field under ambient environmental conditions during the winter. In this first option the crop is either left standing in situ or is placed in pits and covered by soil. In these two field options the environment cannot be too cold, as there will be a risk of frost damage, or too mild, as the crop may grow excessively and not receive a full vernalization treatment. The second option involves bringing the propagules, the parts of the biennial plant with the stored carbohydrates that are used to produce seed, out of the field and storing them in a controlled environment. In this second option the propagules of the crop are placed in either a cooler or some form of root cellar. In these controlled environments you must make sure that the artificial storage conditions maintain a sufficient temperature and humidity to properly store these plant parts till spring. You must also ensure that the storage environment is sufficiently cold (see “Overwintering the Crop in a Coldroom”) and that the duration of storage is of an adequate length of time to fully vernalize these plants. Remember that the ideal temperature for storing the propagules for an extended period of time may be different from the required range of temperature required for vernalization, depending on the type of crop stored.
Overwintering
the Crop in the Field
The challenge is successfully overwintering the plants after the first (vegetative) season through to the second (reproductive) season. Most of the common biennial vegetable crops originally come from mild temperate regions with mild winters. They are frost-tolerant, but depending on their size, stage of growth, previous exposure to cold, and duration of cold, many of these crops can be damaged at temperatures below 29°F (–2°C). Conversely, a number of biennial vegetables can tolerate temperatures much lower than this if they are at an optimum size for overwintering and have been slowly hardened off to the cold. Under these circumstances there are some biennials that can consistently tolerate temperatures between 14 and 18°F (–10 to –8°C), including some varieties of Brussels sprouts, European kale, escarole, and leafy chicories. In fact, many parsnip varieties are famously cold-tolerant to temperatures of at least 10°F (–12°C) or below.
There are a number of factors to consider concerning both the plant and the environment that determine whether you can successfully overwinter the crop outdoors. Many temperate regions are excessively cold during peak winter weather, and it will not be possible to overwinter biennial crops in the field. An alternative to this is storing the crop by “pitting,” or by storing it in a cooler or root cellar. Pitting—long used in many temperate regions—involves storing root vegetables in a pit that is dug below the frost line and is backfilled with soil. In seed production of certain root vegetable crops, like beets and turnips, the roots are dug, topped, and windrowed; then enough soil is mounded over them to prevent freeze damage for that specific climate. While this is still called pitting, it could more accurately be described as “mounding,” and it involves the use of soil as insulation (see Garden Beet in chapter 5).
Overwintering
the Crop in a Coldroom
If the seed crop is of a great enough value you may choose to store the roots or plants at the end of the first season in a cooler or root cellar. The advantage of storing the plants in some type of artificial environment is the ability to avoid extremes of cold in the field for many temperate climates and also to protect the plants from potential damage from other environmental factors such as diseases, insects, or animals. Of course the size of the crop and availability of storage space are limiting factors in many instances.
Storage Temperature: The storage facility must be cooled to at or near the appropriate storage temperature when the crop is first put into the room. This should be the same temperature that the crop will be stored at for the duration of its time in storage. This is easy if you’re using a refrigerated cooler with temperature control. Using a root cellar can be much more challenging, as the ambient temperatures of these earthen structures are often not cool enough at the time of the fall harvest to be acceptable, thereby limiting their use to certain geographic districts where the onset of cold weather is early enough in the fall to coincide with harvest.
In general, for long-term storage, most biennial propagules that are stored for replanting in the spring require a temperature as close to freezing as possible without the risk of damaging the plant’s tissue by freezing at any time during the storage period. For this reason most biennials are stored at temperatures from 34 to 37°F (1 to 3°C) to avoid the chance of the temperature dipping below 32°F (0°C) with the normal temperature fluctuation of the cooler. Some growers use makeshift coldrooms—say, insulated outbuildings—that don’t maintain an optimum temperature range, fluctuating with ambient conditions. Coldrooms where temperatures drop below freezing cannot be tolerated, but there are instances when coldrooms that occasionally warm up into the 37 to 45°F (3 to 7°C) range have successfully been used for storing roots for some period of time. The problem with storing roots at temperatures above 38°F (3°C) is that they will respire at a higher rate, using more of their carbohydrate reserve, and will prematurely put on both root and apical shoot growth, often long before the appropriate spring weather for replanting in the field has arrived.
Storage Relative Humidity: In addition to being at the right temperature, the other primary concern when storing any biennial propagules in any type of coldroom is maintaining the relative humidity at a level that is appropriate for long-term storage of the crop. Most biennial propagules, both the root crops and the biennials that store energy in leaves, stems, or petioles require a very high relative humidity of at least 95% to remain in optimum health for replanting at the end of the storage period.*
The biennials that are not propagated by their roots are not well suited to storage and are rarely stored in coldrooms over winter, especially when they are fully developed at the end of their vegetative cycle. Most of the non-root biennials are large, bulky plants such as cabbage, kale, broccoli, or cauliflower, and they are best stored with their roots “healed-in” into healthy soil. Some of these non-root biennials have lots of vegetative tissue that can easily rot during long-term storage (the petioles or stems of Swiss chard and celery, the curd of cauliflower, the leaves of cabbage or kale), and so the commercial production of seed in almost all instances for these crops is done in climates suited to successfully overwintering these species in the field. Specifics on the best practices for all non-root-propagated biennial crops can be found in the individual chapters devoted to each crop species (see part 2).
* Biennial bulbing crops of the Alliaceae family (e.g., onions and shallots) are a notable exception requiring temperatures between 34 and 38°F (1 to 3°C). Onions and shallot bulbs also have dry outer scales, which require a drier relative humidity (RH) in the 65 to 70% range for long-term storage.
Storage Relative Humidity for Root Crops: Achieving and maintaining a relative humidity (RH) of 95% or greater is essentially done in two ways for root crops. You can place the roots directly into either totes in a high-humidity environment, bins with a storage medium for root cellars, or specially prepared plastic bags that can be placed into any coldroom that may not supply ambient RH. In all cases you must consider the extent to which the roots or “stecklings” must be prepared for each different storage option. (See “Preparing Roots for Storage”.)
If you’re using a cooler that is equipped with a reliable humidifier capable of producing 95% RH, then the roots can be stored directly into open wooden or plastic totes with small openings between the slotted planks on the sides to allow the free flow of humidity to reach the roots. Covering the totes with a 2 to 3 in (5 to 8 cm) layer of clean wood shavings (not sawdust, as this will cake, preventing air circulation) will keep the roots free of standing water that can accumulate on the surface of the uppermost roots under high-humidity conditions. This helps eliminate a source of potential rot. Cedar wood shavings may be superior for this purpose as cedar is reported to have a higher level of antimicrobial factors than most other woods.
Root cellars or other earthen storage can supply a high level of RH naturally, though it can be quite variable from cellar to cellar and may also vary across seasons. If you are unsure of the ability of the storage environment to supply humidity constantly at this level, then it is wise to place the roots in some type of medium or container that helps keep the roots at a constant RH. In root cellars, roots are traditionally stored in wooden bins with layers of clean, slightly damp sand or clean, undecayed deciduous leaves. (In New England maple leaves, which are low in tannins, are sometimes used.) The roots are laid carefully between layers of this material so as not to touch, thus slowing the spread of rot through the lot of roots should any decay start. (Remember the old adage that states “one rotten apple can ruin the whole barrel.”) These strategies may require some tinkering depending on the materials you have available and the often variable temperature and humidity conditions of each root cellar.
When a high-RH environment isn’t available or economically feasible, then biennial roots can be placed in plastic bags and stored with great success in coldrooms with a lower-than-optimum RH. A high-RH environment can be maintained within a plastic bag for a long period of time; however, there is a risk of the roots rotting if you don’t make provisions to allow moisture to escape and prevent the formation of standing water. This can be accomplished through several simple and inexpensive techniques that a number of seed workers have perfected over the years. Most seed growers use relatively large plastic bags as described here when holding enough roots for a sizable seed crop. Any grower or breeder wanting to store smaller numbers of roots will need to extrapolate down from these sizes.
Large plastic commercial produce bags (25 lb/11 kg) should have at least three to four rows of small, circular, penny-sized holes across the middle section of the bag. These holes allow some moisture to escape the bags over the course of the storage period. This is necessary because the roots are respiring and water is released in the process. While much of the moisture can escape the bags through these holes, there is always some that condenses and remains in the bag, potentially promoting rot. To trap this free moisture it is recommended that you spread three large handfuls of wood shavings evenly among the vegetable roots in each bag. As in the humidified cooler, cedar wood shavings may prove superior for this purpose: Cedar is reported to have a higher level of antimicrobial factors than most other woods. Care should always be taken not to place warm roots from the field into any of these various cold storage situations directly, as this will cause excessive condensation that may promote rot. These plastic bags should be checked every 6 to 8 weeks during the storage period to identify any rot and remove it from the bags before it spreads. The bags should also be turned and given a shake at this time to redistribute the shavings.
Preparing Roots for Storage: For long-term storage in a cooler or root cellar, the roots need to be properly prepared. Roots that are stored for replanting to produce a seed crop are called stecklings. Stecklings are much more likely to rot in these simulated storage environments than they are when stored in pits in the field. Therefore, several methods are used in preparing the stecklings for storage in a controlled environment. First, the roots should be cleaned of most of the soil that is still attached to them, but without the use of water, since getting the roots wet before storing them only encourages decay organisms to grow. In fact, some researchers believe that the balanced microbial community that exists in healthy soil may act as a deterrent to many destructive pathogens that might otherwise grow in storage. Soil that is not easily removed by hand or shaking should not be removed by vigorous rubbing; this can damage the root, making it more prone to disease in storage.
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PREPARING STECKLINGS FOR WINTER STORAGE: (A) Make an initial cut to trim back the leaf tops; (B) and (C) Using a sharp knife, carefully trim the petiole mass farther, to about 0.5 in (1.3 cm) of the base of the crown, but leaving the apical bud intact; (D) The final appearance of the trimmed carrot tops; (E) The last step in this process for carrot stecklings is to remove, with a clean cut, the bottom portion of any taproot longer than 3 to 4.5 in (8 to 13 cm). It is best to do this after winter storage in spring. Photos courtesy of Micaela Colley
The next step is to remove any portion of the taproot that is longer than 3 to 4.5 in (8 to 13 cm) and also remove most of the petioles (leaf stems), as both of these tissues are the most susceptible portions of the root to rot. Remember that both the taproot and the apical shoot enclosed in the deepest layers of the petioles are where the active growth will emanate in the spring. The taproot, or the lowest portion of the bulb, is where the roots will emerge during the second year; hence the need to retain the 3 to 4.5 in (8 to 13 cm) taproot. The petioles will slowly rot away in storage, and because they enclose the apical bud for next year’s shoot growth, this rotting, if excessive, can infect and destroy the bud before the root is replanted. A preventive measure is to trim away as much of the petiole as possible at the beginning of the storage process without damaging the apical bud. This requires deft handwork, a sharp knife, and knowledge of where the apical bud resides. Essentially you will cut much of the petioles off, starting very close to the crown of the root, near the base of the petioles, and cut upward, so that the center of the petiole mass is about 0.5 in (1.3 cm) above the base of the crown (see chapter 6, Carrot, “Root-to-Seed Method”). In essence you are eliminating as much of the petiole as possible without damaging the apical bud.