(Editor's Note: this article was originally published on April 3, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
The USDA Agricultural Library's Glossary defines the Winter Sowing Method as "a propagation method used throughout the winter where temperate climate seeds are sown into protective vented containers and placed outdoors to foster a naturally timed, high percentage germination of climate tolerant seedlings." This same winter sowing (WS) method can be used in early spring to give half-hardy annuals a good start.
My Groundhog's Day article, "Celebrate by Winter Sowing Your Seeds," details selecting, preparing, and planting WS containers. The method is just the same at this time of year. You need a covered, vented container that holds several inches of potting mix and provides seedlings with light, water, and drainage. With the stronger spring sunshine, it's even more important to find a location where your containers will be shaded from hot afternoon sun. A pinch of polymer moisture crystals added to your potting mix can help keep it from drying out, but you may have to do a little watering if you get hot days with no rain.
What annuals do especially well with the WS method? If you drop by the DG Winter Sowing Forum, you'll pick up all sorts of ideas for annuals that can be successfully winter sown. Some people start on the Winter Solstice in December; I like to wait until Groundhog's Day in February. Have you had any annuals reseed for you? When I find scattered volunteers of Petunias, Sweet Alyssum, Amaranthus, or other annuals I planted the previous year, I know I've got another great candidate for my winter sowing containers!
The timing of using the WS method for annuals depends on how the relative hardiness of the plant. Some of the annuals in our summer gardens are tropical plants that can't tolerate a hint of frost. Others, like pansies, may even be planted in fall to over-winter and come back the following spring, blooming like crazy. The more hardy the annual, the sooner you can sow it.
You may already have winter sown some hardy annuals. Hardy annuals can handle some frost and may even need cold stratification to germinate. Bachelor Buttons, Poppies, Violas, Snapdragons, Calendula and Cleome are some common hardy annuals that germinate readily with the winter sowing method. If you didn't sow them in February, go ahead and "winter sow" them now. If it's cold enough that you grab a jacket when you go out at night, it's cold enough to give those seeds the chilling that they like.
Half-hardy annuals can tolerate some chilly weather but may be damaged by frost. Marigolds, Love-in-a-Mist, Four O'Clocks, Cosmos, Petunias, and annual Salvias can be started by the WS method in early spring.
Tender annuals often wither at the least touch of frost. They can also be started in WS containers, but it's prudent to wait until the time for hard freezes is past. You can also direct-sow tender annuals after the soil warms up. Using the WS method gives them several weeks' head start, as the container serves as a sort of miniature cold frame. Morning Glories, Nasturtiums, Zinnias, Basils, and even Tomatoes can be started this way, especially cherry or other early bearing varieties.
The mini-greenhouse environment under the lids and domes of WS containers gives your seedlings extra protection against cold. Even so, if a hard freeze is forecast, I'd suggest covering containers with sprouted seedlings or moving them to shelter for the night. An unheated garage is a better choice than a warm house, as they'll have to re-adjust to outside temperatures when you move them back. Seedlings that have grown large enough to touch the sides and tops of their containers seem more likely to be damaged by cold.
Are there annuals that shouldn't be started with the WS method? Some annuals germinate best in warmer temperatures and may simply take too long to sprout outside, even in their little "greenhouse" containers. Annuals that take 10 or 12 weeks to bloom from seed should probably be started inside on light shelves. Otherwise, you'll only have a few days to enjoy their flowers before frost. Examples of common, slow-to-mature annuals are annual Geraniums, Wishbone Flower, Impatiens, Begonias, Heliotrope, some Petunia varieties, and Coleus.
This is about the time when I realize I will never have enough room on my light shelves for all the seeds I'd like to start this year. And it's such fun to have flats of annuals to plant all around the yard for a riot of color! I was delighted to learn I can continue to use the WS method as winter turns into spring.
Now my supply of annual bedding plants is limited only by my ability to scrounge milk jugs and other suitable containers. It's hard not to go overboard. I try to keep in mind that more plants = more digging and planting in a few weeks. But when it comes to having a rainbow of color in the garden, I'll just never believe that "less is more." Sometimes, more is more! Keep using the WS method this spring, and grow a rainbow of your own.
Photos by Jill M. Nicolaus.