Answer: Depending on how well insulated your cold frame is, the microclimate for 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 even 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 to make sure it is as safe as possible; there are in fact heating cables made for such a purpose. In the olden 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!
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