Cooperative Extension: Garden and Yard

The answer ultimately depends on what you’re sowing, but realistically you can do most of it now. As it happens, I just wrote an article for the Maine Home Garden News on winter sowing, so I’ll include the relevant portion to your question:

“Circling back to the types of seeds, warm season vegetable garden crops are not great candidates for winter sowing, though this technique can be used to “cheat” for slightly earlier germination in the early spring. But for true winter sowing, stick to brassicas (broccoli, Brussels sprouts, cabbage, cauliflower, kale can all be sown outdoors in February), lettuce, beets, and spinach (these three can be sown outdoors in March). If you want to experiment with using this method to get an early start on summer plants like peppers and tomatoes, it’s best to wait until April. If you’re interested in winter sowing ornamentals, there are some clues for good candidates on the seed packet–look for descriptions that include “hardy annual,” “will colonize,” “requires chilling,” “can be sown in Fall,” or “can be sown early,” among others (UMD Cooperative Extension, page 10). Options include asclepias, cosmos, pansies, zinnias, calendula, butterfly weed, echinacea, rudbeckia, and monarda. Hardy annuals and perennials can be started in January, standard annuals and tender perennials should be in March (Univ of IL Cooperative Extension video, time mark 15:52).”

Happy gardening.