Vernalization Basics

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Providing good drainage, a protective layer of mulch, and a site against a heat-radiating wall can help plants such as these cardoons survive the winter so they can flower and set seeds.
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Biennial crops, such as Swiss chard, require exposure to cold in order to flower and set seeds, but the result is not immediately apparent. Evidence that a plant’s vernalization requirement has been fulfilled—in the form of a flower stalk—may not be visible for weeks or more after vernalization has occurred.
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Roots, such as those of carrots that are being stored for replanting, are brushed clean of soil and trimmed of leafy growth, taking care not to damage the growing points. They are then layered in slightly moist sand or wood shavings to help maintain desired relative humidity levels. The short roots of ‘Paris Market’ carrots are not only well-suited to shallow soils, but they also require less space if being vernalized in storage.
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The apical growing point of some plants, such as this kale, is at the top of the main stem and needs to be left intact when foliage is trimmed for winter storage.
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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.

Methods of Vernalization

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.