What Is Seed Stratification? Plus, How to Stratify Seeds for Planting
Some seeds need to be kept cold before they will sprout.
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Plants often have ways to protect young seedlings from harsh environments. Many perennials produce seeds in late summer or fall; if they germinated immediately, their seedlings would likely not survive winter. So, before they germinate, the seeds of many hardy plants must first experience a period of cold temperatures, known as seed stratification (or cold stratification). In nature, winter temperatures satisfy this dormancy requirement; seeds germinate the following spring when the weather is more favorable. If you purchase or collect seeds that require cold stratification to break their dormancy, you must supply that cold period yourself.
Which Seeds Need Cold Stratification?
The seeds of most annual garden flowers and vegetables require no stratification. However, many trees, shrubs, perennials, and wildflowers will only germinate after their seeds are stratified. Over several weeks’ time, exposure to cold temperatures and moisture helps break down the hard seed coats so the seeds are able to sprout. If you sow these seeds outdoors in the fall, their dormancy needs will be supplied naturally during the winter months. However, if you plan to plant in spring, you need to stratify the seed before sowing outdoors. And don’t wait too long—some seeds need several months of cold moist treatment before they will grow.
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How to Stratify Seeds for Your Garden
When you purchase seeds from a reliable source, the seed packet will usually mention any stratification requirements that apply. If you collect your own seeds, do a little research to determine how long a cold period each plant will need. These may range from a few weeks to several months. Fortunately, the process is simple and can be accomplished in a couple different ways.
The most foolproof way to stratify your seeds is to plant them in a pot of moist potting mix, sand, or vermiculite—it should be damp but well drained. Space the seeds about a half inch apart and cover them lightly with the mix. Be sure to label each pot with both the plant name and the date, which really comes in handy in determining when to remove them from the cold treatment. You can also write the date when they’ll be ready to come out of the cold on the label so you don't have to remember that. Put the labeled pot in a plastic bag and punch a couple of breathing holes into the bag or leave a small opening in the top, then place it in the refrigerator for the recommended time. Check occasionally to see that the medium is still moist; water if needed.
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Small seeds can be sprinkled on a moistened paper towel and rolled up or covered with a second moistened towel and placed in a plastic bag. Label and place in the fridge for the appropriate time.
If you don't want to use your refrigerator for stratifying seeds, you can sow the seeds directly in the ground or in pots that you label and place outdoors, out of direct sun. Be mindful of periods with little or no precipitation—you may need to water the pots occasionally.
Planting the Stratified Seedlings
As spring approaches and the recommended cold period has been met, remove the pots (or towels) from your fridge. If you planted the seeds in pots, you may begin to see seedlings emerging even before they come out of the fridge. Place the pots in a partly shaded area of the garden—an uncovered cold frame works well—and allow them to germinate. For the seeds you stratified on paper towels, sow them in a moist medium, spacing them about a half inch apart. Keep the pots watered and be patient, the seeds may germinate gradually over a period of a few weeks.
Once the seedlings have developed a few true leaves, transplant them into separate pots or to a protected spot in the garden where you can keep an eye on them. Once they develop into robust young plants, which may take a couple months or a full growing season, transplant them to their permanent garden location.
