Article and photos by Diana Westland, Mississauga Master Gardener
Winter sowing is a way of forcing the dry plant seed out of dormancy, by simulating the freeze / thaw conditions found in the wild. Many native plant seeds in Ontario require a period of what is called ‘cold stratification’ before germination can take place.
Seeds are sown in artificial individual greenhouses. This protects them from foraging by birds and animals in fall. The greenhouses also magnify the weak spring sunlight to the seed, warm the seed bed faster, and encourage earlier germination for seeds needing sun to germinate. The mini greenhouse also protects the tender seedlings from wind, frosts, and foraging in early spring. As the conditions are more natural than indoor growth, the seedlings are not leggy, and no hardening period is needed as the seedlings are exposed to natural sunlight from the beginning.
First, create the greenhouse. My greenhouses are empty distilled water bottles. Put rain water and air holes in the top, and drainage holes in the bottom. Then cut the bottle such that there is at least 6 inches for soil. The soil may compact slightly when snow gets in the bottle, and many native plant seedlings typically have deeper root systems than the seedling height would suggest. Make sure you give them space.
I use purchased growing medium with mycorrhizae. The soil and roots should be disturbed very little during the transplant in spring. The bottle will be cut off and the root system planted whole. The root mycorrhizae will give the roots and plant a good start in their new home. Pre-moisten the soil before adding it to the bottle. Carefully smooth the surface without compacting.
Next, sow the seeds. As the seeds were gathered in the field, and the germination rates may vary, I lay down several seeds spaced in the appropriate bottle. If more than one germinates, thinning will be needed. Some native seeds require burying to keep them in the dark, some need to be exposed to sunlight to germinate. I use vermiculite to cover the seeds to the depth that is needed, or just scatter lightly to anchor the finer surface seeds. Coarse concrete sand can also be used as this will not block sunlight, and will help the very fine seeds stay in place during periodic wetting. Sowing seeds too deep is a common mistake.
Water the container thoroughly with a fine water spray, provide a label (with paint pen is best), and seal the bottle top to the bottom section with duct tape. Then set it outside in a sunny protected spot that is open to natural precipitation. Periodically check the greenhouses to make sure the soil isn’t drying. If all goes well, you will have effortless young native plants to add to your garden in spring.