Although typically planted in the spring, harvested late summer, and stored for use until next summer, onions can have two major harvest cycles and you may be using only one of them. Begin overwintering onions in September and eat fresh onions in late spring!
Varieties like Candy, Walla Walla Sweet, and Olympic have all proved to be good overwintering onion options. Onion growth does not occur underground and away from the elements, therefore these plants benefit from protection like cold frames, polytunnels, or burlap structures in gardening zones that host colder temperatures and frozen ground for much of the winter.
Shoot for mid-September to begin your onion overwintering process. If you plant onions too early, the initial growth will be detrimental to the spring product. Direct sow onion varieties into the ground leaving about three to four inches between each bulb and cover with abundant compost and straw to protect the plants.
In similar fashion to overwintering spinach, you will want to slowly remove the protective straw layer in portions as temperatures steady above freezing. By March or April you can fully expose the plants, which should appear green and fruitful. Continue caring for your plants until June when they should be ready for harvest.