Winter sowing is simple and requires no special equipment. You can winter sow any seeds. The difference is when and how you sow them.
Woody species: Trees and bushes may need extra time to germinate so can be started in a mixture of coarse sand and cocoa fiber. Cocoa fiber is not necessary strictly speaking but it retains moisture for long periods of time so can help with moist stratification. Moist stratification refers to the amount of time seeds need in cold moist soil otherwise known as winter. Some trees and bushes need two (2) years cold moist stratification.
Start woody species like hawthorns, some viburnums, prickly ash and the like in slightly moistened coarse sand/cocoa fiber in a plastic zip lock bag in the refrigerator. Keep the bag with seed in the refrigerator for two (2) months and then two months on the counter (shade counter) for two (2) months. (I started mine in October in the fridge and can sow the seed in a container outside in February.)
Perennials can be sowed any time from the winter solstice in December to January. Not all perennial seed requires cold moist stratification but most native species (like milkweed) do.
Annuals can also be sown outside but later in the year. I start my annuals outside in February or March. Any annuals can be sowed this way – vegetables or flowers.
Find a plastic container, such as milk jugs, vinegar jugs, soda bottles or deep foil pans with clear lids.
Native perennials or grasses do not need a top as long as you protect them from furry creatures but the “greenhouse” aspect of this method means there has to be a top on the container.
Poke drainage holes in the bottom of the container.
You will need to poke drainage holes in the bottom of the container as well as the top of the container. The bottom of the container needs to drain the water so the seeds don’t swim and the top requires holes so that water can seep into the container.
Fill the container with potting soil and sprinkle seeds on the seed starting soil mix.
I fill the bottom half of the container about ¼ of the way up the container with an organic seed starting mix with some cocoa fiber and worm casings. All you need is the seed starting mix but it dries out easily and there are no fertilizer in it. This mix should be good for the seed from the time you put it out to the time you open the container in the spring. Sprinkle the seed in the soil mix and then cover lightly with a soil mix and coarse sand. The coarse sand stops the seed from moving around when you do have to water it. Soil depth for covering the seed can depend on the seed but a light coat of soil seed and coarse sand should work for most seed.
Put the lid/top back on and secure with duct tape.
Many websites show the duct tape method but I jam the top of the container into the bottom leaving the opening of the jug open so there are plenty of openings and vents for air to circulate and water to seep in while keeping the “greenhouse” aspect when the sun starts to warm the seed.
Label the bottom of the container with the seed type and position the containers somewhere outside where they can get snowed on and are protected from the wind.
You can monitor the seed by looking into the jugs and open the jug during the spring on warm sunny days. After frost danger has passed you can divide the plants and pot them or plant them.