A cold frame is a raised bed with a translucent lid; either glass or plastic used to retain solar radiation. Cold frames are used to grow produce when the weather is frigid.
Solar radiation warms the soil in the cold frame during the day, this heat is released but captured by the lid creating a warm micro-climate. This micro-climate allows crops to grow during the harshest times of the year.
A cold frame is best utilized going into spring and going into winter. Hardy greens can be started in late winter during warm spells, much like the one we just had. With a little moisture and a few warm days, germination happens. The warm environment allows the plants to grow, or at least stay alive through cold weather and temperature fluctuations. They will continue to grow and be ready for harvesting before a gardener can plant in their outdoor garden.
The second time to use a cold frame is going into the winter. Seeds are planted in late fall and the crops quickly grow to maturity. Crops will not grow during the harshest weather, they will sit waiting for the next warm day. With a cold frame this can be used to a gardener's advantage. In January when the weather is coldest, these mature crops sit in dormancy. As they are fully grown, gardeners can harvest them through the coldest month.
A cold frame can be made out of an old glass door, or using polycarbonate, a twin-walled plastic used to build greenhouses. Glass door or windows can be used by thrifty gardeners to make a successful cold frame. Gardeners must be wary of burning their crops when warm weather returns however. I build my cold frames out of polycarbonate because it is light, then put an automated lift arm on the lid. This lift, which is a wax cylinder, expands as the weather warms and when it becomes too hot, it opens the lid and vents some of the heat. This fail-safe is especially useful for gardeners who spend their day at work. With temperatures that can spike long after the gardener leaves the house, this fail-safe is a no-brainer.
Tune in next week for good crops to grow in a cold frame.