Seeds from plants native to temperate zones possess a remarkable survival mechanism: they require a cold period to germinate, a tactic that occurs naturally in the garden during our winters. This makes evolutionary sense; otherwise, fresh seed would germinate in the autumn, and the young seedlings would succumb to icy blasts. By imitating nature, gardeners can get a jump-start on the season at any time of the year by artificially chilling seeds—a process known as stratification. By spring, gardeners who had germinated seeds indoors will have good-sized transplants rather than a patch of naturally germinated, smaller seedlings. Most commercially available seeds have already been stratified, but those you’ve collected yourself or obtained through seed exchanges will need this pre-treatment before they will germinate indoors in pots.
How to stratify seeds
Several soilless mixes are suitable for stratifying seeds. Peat moss works especially well for small seeds, but sift it first and use only the fine particles. Once sifted, dampen the moss and add one part horticultural sand or vermiculite to four parts peat moss to improve aeration; for larger seeds, use a mix of half horticultural sand and half vermiculite.
Place a handful of the soilless mix in a small bowl. Make a wide depression in the centre and add as many seeds as desired. Cover with a little more mix, then remove the seeds and mixture from the bowl and gently squeeze out any excess water, but do not compact.
Place the mound into a resealable plastic bag, label with the species’ name and the date and leave it in a warm place (the top of a refrigerator, for instance) for three days to allow the seeds to take up water and swell. They’re now ready for chilling.
Place the bag in your refrigerator’s meat drawer, which is usually the coldest area (about 4 to 5°C is ideal), but make sure the seeds don’t freeze. Shake the bag once or twice a week to keep them aerated.
After the required chilling period (see “The big chill”), remove the bag from the refrigerator. Large seeds can be removed and sown five millimetres deep in pots filled with growing medium; cover with seed-starting mix.
Leave small seeds in the soilless compound and sow directly into containers filled with commercial seed-starting mix. Move the pots or flats to a warm, sunny window until ready to be transplanted outdoors in the spring.