Learn about vernalization and why it’s necessary for flowering plants.
The Seed Garden (Seed Savers Exchange, 2015) by Micaela Colley & Jared Zystro and edited by Lee Buttala & Shanyn Siegel brings together decades of research and hands-on experience to teach both novice gardeners and seasoned horticulturists how to save the seeds of their favorite vegetable varieties.
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Biennial crops require a cold treatment, or vernalization, in order to progress from the vegetative to the reproductive phase. Without this stimulus, biennial plants will not flower (nor will they set seeds) even if they are grown under conditions (light, temperature, moisture) that would normally support flowering. A plant’s vernalization requirement helps ensure that the plant will not begin to flower at a time of year when weather conditions do not support pollination and reproductive development. The amount of time required for vernalization is cumulative but not necessarily consecutive in nature, beginning in fall as temperatures begin to drop and possibly continuing into the spring.
Three criteria must be met in order to successfully produce seeds of a biennial crop. First, plants need to be an appropriate size for vernalization. Second, the crop needs to be exposed to the proper temperature for the proper length of time to fulfill vernalization requirements. And third, the plants need to survive the winter and avoid succumbing to disease in order to flower and set seeds the following season.
Most biennial plants must reach a certain minimum size or age before they are competent to flower. For example, collards and kale (both Brassica oleracea), become receptive to vernalization when they have approximately eight leaves or stems that are about half an inch (13 mm) in diameter. If they are smaller than this, the plants will not register the cold stimulus and will not be induced to flower, but instead will continue to grow vegetatively.
On the other end of the spectrum, plants that are too large are susceptible to problems during vernalization. Full-sized plants may be more vulnerable to cold injury, particularly at the apical meristem, or growing point, where damage can prevent growth of the flower stalk. Large plants, particularly root crops, are also often more prone to cracking, which makes them more susceptible to disease and rot and less likely to survive long enough to produce mature, healthy seeds.
Different plant species—and even cultivars within a species—vary in the temperature and the length of time required for vernalization. In general, exposure to temperatures below 50°F (10°C) for 8 to 10 weeks is adequate to induce flowering in most common biennial vegetable crops; however, most biennial plants also have an optimal temperature range for vernalization. If temperatures are above or below this optimal range, but still within the larger range appropriate for vernalization, plants will require a longer period of exposure to these temperatures to initiate flowering.
For example, carrots held between 36 and 41°F (2 and 5°C) are expected to reach the reproductive phase in approximately six to eight weeks. However, in a garden where the temperature fluctuates above and below these optimal temperatures, vernalization may require more time. If carrots are exposed to temperatures below 59°F (15°C), but not consistently within the ideal 36 to 41°F (2 to 5°C) range, floral induction could take 10 weeks or longer.
Because the total duration of cold exposure does not need to occur uninterrupted, cool fall nights and spring days contribute to the vernalization process. In fact, some biennial plants with a short vernalization requirement can be induced to flower solely in the cool weather of early spring provided the plants are large enough to be receptive to vernalization. With many crops, the size or age of a plant affects not only whether vernalization can proceed, but how long it will take. Younger plants may require a longer exposure to cold than mature plants before they will flower.
A biennial plant’s ability to survive the winter is the last important consideration in growing biennials for seed. Winterhardiness in plants is not based solely on temperature. The age and size of a plant often affect its cold tolerance, as do environmental factors, including the total duration of low temperatures, the occurrence of extreme or repeated fluctuations in temperature, the speed with which cold weather initially comes on, and winter precipitation and drainage. Although the winterhardiness of ornamental perennials is fairly well documented, the low temperature tolerance of biennial vegetable crops—which are typically only vernalized and grown for seed production in favorable climates—is not as widely known. Often the lowest temperature a biennial species is known to survive is based on brief exposure to such conditions. For example, a kohlrabi plant may be expected to survive a winter in a relatively mild region where temperatures drop to near 15°F (-9°C) on just one or a few nights, but the likelihood of winter survival decreases if the same plant is grown in a region that experiences extended periods of time at or near that same low temperature.
There are some circumstances in which a biennial variety will flower in the same year it is planted. This can occur in varieties that were developed in warmer climates and naturally require a shorter vernalization period. For example, carrot varieties such as ‘Kuroda’ that evolved in the subtropical regions east of Afghanistan tend to bolt much more easily than their western counterparts. However, it is also possible for just one or a few biennial plants within a population to flower prematurely. These individuals should be rogued from the population to avoid inadvertently selecting for an early-bolting population.
It is useful to know that successful vernalization initiates the flowering process, or development of the flowering stem. However, the visible stage of flowering, or bolting, may not be immediately apparent. Some biennials will begin to flower quickly in the spring, while the flowering stalks of others, even after proper
vernalization, will not be visible until later in the growing season.
Plants can be vernalized in the ground where weather conditions satisfy a variety’s cold requirement without killing the plants, but in regions where these conditions do not naturally occur, plants can be vernalized in storage. Vernalizing crops in storage entails digging up the plants, storing them in the right conditions, and replanting them the following season to flower and produce seeds.
Where conditions allow, in-ground vernalization is the simplest method for saving the seeds of biennial crops. In milder climates where plant loss to freezing temperatures is less likely, biennial plants are usually grown to market maturity
in their first season. In areas where they are marginally winter hardy, they are often sown later than they normally would be for eating because smaller, less mature plants are better able to withstand cold winter temperatures. Additionally, plants can be protected with a winter mulch of straw, leaves, or floating row cover. Where winter snow cover is consistent, snow can act as an insulator against cold.
Biennial crops being vernalized in the garden can be left in place as they were sown, or they can be dug up and replanted at appropriate spacing for their second season of growth, which takes into consideration the size of the plants at seed maturity. They can be moved in fall, before vernalization, or in spring, when plants that have succumbed to winter weather can be rogued out of the population. In the case of root crops, this step can also serve as an opportunity to confirm that the roots are true to type, and any off-types can be rogued.In regions where vernalization cannot occur in the ground, plants must be vernalized in storage. Plants can be held in a root cellar, in an unheated space that meets a crop’s vernalization temperature requirements, in a refrigerator, or even buried in a pit that is layered with straw for insulation from winter cold. After storage, the plants are planted out again the following spring to flower and produce seeds. This method is easiest with storage crops, such as beets, carrots, and onions, but—with a little more effort—can also be applied to other non-root biennial crops, such as Swiss chard and kohlrabi.
For more on seed saving, see our Seed Saving Guide.
Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.