(Editor's note: This article was originally published on February 2, 2008. Your comments are welcome, but please be aware that authors of previvously published articles may not be able to respond to your questions.)
Winter sowing may seem like a contradiction in terms, but it's something Mother Nature has done for ages. Seeds drop to the ground in summer or fall, and when conditions are right, they start to sprout. The lucky ones sprout in good soil, in sheltered locations, at just the right time, and they grow into sturdy spring seedlings. The extra protection of the winter sowing method creates ideal "natural" conditions, increasing the chance of success for any given seed.
The USDA National Agricultural Library 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."
If you look through some of the threads in the DG Winter Sowing forum, you'll see that people use a creative variety of containers. Ideally, containers should have space for at least 3-4 inches of potting mix plus several inches of headroom for the seedlings. They must have drainage holes in the bottom. Vent holes in the top are also needed to let in rainwater and to release built up heat. Containers should be clear or translucent so light can reach the seedlings.
Milk jugs and plastic soda bottles make great winter sowing containers. Remove caps for ventilation. I use a serrated bread knife to cut them (be careful). With milk jugs, I cut most of the way around, leaving a "hinge" at the handle and adding punch holes for a twist-tie to hold the top and bottom together. With soda bottles, I cut two slits to form a tab on the top so the two halves nest firmly together with no need for tape.
Using bins, boxes, or laundry baskets to corral your winter sowing containers helps in several ways. They keep the wind or the neighbor's dog from scattering the containers up and down the block. Containers can be grouped according to their contents. An appearance of organization also counteracts the hilly-billy look of hundreds of assorted recyclables.
I've used cardboard boxes or old propagation trays with small holes in the corners to hold containers. Water from the hose or from rainfall drains out slowly, giving the seedlings a chance for a good drink without leaving the containers sitting in water. This year, I'll also sow seeds in pots and put them inside clear storage boxes, with holes for ventilation and drainage.
Find a good location for your containers. They're meant to go outside in the cold , the rain, and the snow. But you don't want them to be flipped over by windy weather or to be cooked by bright afternoon sun. A somewhat sheltered location that gets morning light is ideal. Most of my winter sowing containers get tucked under the bench on my deck. They're out of the way there, and they're protected from strong winds and harsh sun.
Sow seeds into moist, good quality potting mix. Garden soil or cheap potting mix can end up compacting into dry little bricks. Polymer moisture crystals help to keep the mix from getting too soggy or too dry. Adding crystals lets you successfully winter sow in more shallow containers, also. I treat shallow containers similarly to indoor seed starting trays -- once seedlings are well started, they're transplanted to larger, deeper containers.
Germination isn't necessarily lower with winter sowing, so don't sow seeds too thickly. Since I tend to sow with a heavy hand, I often sow seeds in rows so they're easier to thin or transplant.
Label your containers! Markers fade, and sticky labels fall off with winter weather. Writing directly on the container or on vinyl labels with a paint pen seems to work best for many people. Using more than one method to label your seedlings is a good idea. Trying to identify dozens of lost-label seedlings in spring is a challenge you'd probably rather do without.
What seeds can be winter sown? Many seeds actually need cold treatment, often called cold stratification, in order to stimulate germination. Winter sowing provides those conditions naturally. Try winter sowing for those seeds that Tom Clothier's germination database suggests moving between warm and cool temperatures to stimulate germination, as well as for seeds with the notation "Out" on The Seed Site's germination database. Seeds that can be direct sown in fall, or plants that often self-sow or naturalize in your area, are great candidates for winter sowing,
February is a good time to start winter sowing around here. The usual January thaw has come and gone, and with luck the more tender seeds won't sprout until after the late spring freezes. Following experienced winter sower Illoquin's advice, I start with tree and shrub seeds and with perennials hardy enough to reseed in my zone. Hardy annuals and other perennials will follow, and by March I'll be sowing half-hardy annuals.
In addition to the advice and encouragement to be found on the Winter Sowing Forum, DG has a brand new resource! The Winter Sowing Database is under development, and more information is being added all the time. Currently, reports are being entered under either common names or botanical names but are arranged alphabetically by botanical name. To add information to the database, go to the link at the bottom of the PlantFiles entry for the plant. For ongoing discussion, see the series of "Winter Sowing Data Base" threads in the Dave's Garden forum. 
So don't be discouraged by a groundhog's shadow! Cut up a milk jug, add a few inches of moist potting mix, and sow a pinch of hardy perennial seeds today. Celebrate six more weeks of winter by dreaming of hundreds of spring seedlings. Happy Groundhog's Day!
Photo of Phil courtesy of Alan Freed, c. 2006. See PunxsutawneyPhil.com for details of the annual Groundhog's Day festivities in Punxsutawney, PA and more!
All other photos by Jill M. Nicolaus. Flowers are all plants grown in my garden from seeds I winter sowed.
 Thanks to Illoquin for sharing her winter sowing expertise. What I thought had been annual procrastination on my part turns out to be a good thing!
 Thanks to all the enthusiastic gardeners who got first the Winter Sowing forum and then the new database underway. Without your generous sharing of information, experience, tips, photos, and seeds, our gardens would be far less fabulous!