What's snow got to do with it?
Snow may make you want to curl up under the blankets, or bake up a storm of carb-laden goodies. Snow makes some seeds wake up and want to grow. Snow is wet and cold. Happily, it is the moisture and cold that some seeds need to get busy. When the weather report has you getting the snow shovel and salt bucket handy, let it also remind you to look through your seed stash for seeds that need cold stratification.
Certain seeds need cold and moisture to get germination started. These seeds require stratification.
Cold stratification means exposing seeds to moist, chilly conditions for some period of time before planting. But why do some seeds need this treatment, and others do not? Some plants have a longer, slower life cycle, and they need to ensure that their seedlings develop well for the long haul. Many wildflower, perennial, shrub, and tree seeds need stratification. Seeds that "sleep" until very early spring grow into strong seedlings during spring's milder temperatures and steady soil moisture. Sturdy seedlings can endure the stress of summer. They are well grown before winter returns.
Certain seeds need weeks or months of steady cold, or multiple, up and down, short cycles of cold. Some tree seeds need two "winter" periods to get growing. It has even been found that daily temperature fluctuation stimulates germination in some species.
Frost germination and natural cold stratification
Frost germination is a traditional term for cold stratification. And frost germination says what it means. Exposing sown seeds to natural frost can often give them the right signal to sprout. Haven't we often found that Mother Nature knows best? Allow Mother Nature to work, with a little help, by planting seeds in prepared pots or bed, early in winter. Then let snow, frost, cold and sleet give seeds what they need.
Aquilegia (columbine) are popular plants and easy to grow from seed, using natural cold germination. Many perennials, wildflowers, and woody plants can be grown from seed using natural cold stratification. Plant Propagation A to Z by Geoff Bryant contains a table of germination requirements by genus.
Choosing seeds for natural cold stratification
Seed packets and seed catalogs should specify when seeds need cold stratification. Advice to refrigerate seeds in moist medium should be read with "or frost germinate" added. Often these seeds will be for perennials, biennials, wildflowers, shrubs or trees. (Most warm season annuals like marigolds do not need frost to start growing, although many annual seeds can survive frost in your garden.)
How to sow directly in the garden for cold stratification
In milder cold areas such as in the Mid-Atlantic, garden soils stay unfrozen well past first frost. Prepare the soil surface by raking, breaking down clumps and removing large, loose litter. Sprinkle seeds, rake lightly again, press lightly for good soil contact. Even when the ground has frozen, you can sow seeds. Sprinkle seeds on bare ground. Cold and moisture will work on them, and spring freezes and thaws will open the soil for tiny roots to gain entrance. Add a sturdy label. You'll want to know what seeds are where, months after planting the seed. Cover it all with a light open mulch. Straw, pine straw, pine boughs; these will protect the seeds from strong rain but let light through.
How to sow in pots outside for cold stratification
Use pots three or four inches deep; they are less prone to drying out when spring arrives. Mark the pots with Sharpie, or prepare labels. Prepare any number of nursery pots with good, moistened potting mix. For medium to large seeds, fill pot to top and press lightly, place seeds, add one-quarter to one-half inch of soil, then press again. For tiny dust-like seeds, fill the pot, sprinkle seeds, and press lightly for good contact.Place the pots where snow, sleet, or rain can fall. You may want to protect them with light mulch, or by enclosing in mesh or a well perforated plastic bag (squirrels seem to like digging in freshly prepared pots.)
As spring arrives...Now for the waiting game. Your chosen seeds will spend a month or more getting their internal chemistry going before emerging. Snow is not an absolute requirement for germination, but moisture is. As spring arrives, you see sprouts in your pots, or a group of like seedlings in your prepared garden bed. You may see weeds sprouting in the garden as well. Weeds can be recognized because they appear in other areas, too. Remove weed seedlings to favor growth of your chosen plants.
If the seeds sprout thickly, you may thin or move them. Let them develop a few sets of true leaves, and move them while temperatures and rainfall are favorable. Various species' seeds will sprout at different times. Some seeds, especially of woody plants, may stay dormant through two winters.