Elliot Coleman is a farmer and gardener on the coast of Maine. He wrote Four-Season Harvest, a lovely and remarkable book about year-round gardening in snowy climates. (You definitely want to look at the drawings.) Even in the middle of January, he's harvesting fresh salad greens and sweet carrots from old-fashioned cold frames. Coleman relies on low-tech solar heating and cold-tolerant vegetables.
Planting and Harvest Schedule
This winter, I'm hoping to grow leafy endives, mache (a salad green), spinach, carrots and claytonia (another salad green). I need to plant most of these vegetables in mid-summer and early fall. This gives them plenty of time to grow before the real cold sets in.
These vegetables should be harvestable from mid-fall well into the depths of winter. Coleman suggests that only the mache will be harvestable in late January and early February.
Several of these vegetables benefit from the cold. In particular, Coleman claims that the carrots are sweetened by frost, and their starches begin to turn into sugars.
A cold frame is basically a foot-high wooden box with a glass lid. It traps solar heat during the day, and provides a small amount of insulation at night. Unfortunately, traditional cold frames must be opened by hand on warm days to prevent overheating. Some modern designs use special, temperature-sensitive arms to raise the glass.
Good, pre-built cold frames seem to be expensive; I've been quoted over $300 for a small 4'x4' model. According to Coleman, two 4'x8' cold frames will provide fresh vegetables for one person throughout most of a New England winter, so some pre-built models work out to $1200 per person, which is insane. The raw materials for a homemade 4'x8' cold frame with lexan panels and a single automatic opener run about $150/frame, or $300/person.