Follow this tip to grow green vegetables during winter.
Q: It's mid November, USDA Zone 4, and I'm growing lettuce and other greens in my well-insulated cold frame. How long can this last? Can I add a light bulb, a string of Christmas lights or other heating device and continue to grow greens through a Minnesota winter?
A: Depending on how well insulated your cold frame is, the microclimate of its location, and on the weather, you should be able to keep cold-tolerant greens growing for quite a while into the cold weather. Some may slow down almost completely and then start growing again in very early spring.
You are right in thinking that a simple heat source such as a light bulb can extend the season significantly. I would caution you that whatever you use should be of good quality and made for long-term operation so that it's as safe as possible; there are in fact heating cables made for such a purpose. In the old days gardeners would build a cold frame over what was essentially a hot manure pile and capture the heat from that. These days we have more options!