Vernalization Basics

Learn about vernalization and why it’s necessary for flowering plants.

  • 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.
    Photo courtesy Seed Savers Exchange
  • 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.
    Photo courtesy Seed Savers Exchange
  • 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.
    Photo courtesy Seed Savers Exchange
  • 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.
    Photo courtesy Seed Savers Exchange
  • Filled with advice for the home gardener and the seasoned horticulturist alike, “The Seed Garden: The Art and Practice of Seed Saving” provides straightforward instruction on collecting seed that is true-to-type.
    Cover courtesy Seed Savers Exchange

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.

You can purchase this book from the MOTHER EARTH NEWS store: The Seed Garden.

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.

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