Vernalization Basics

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

| October 2015

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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