That's where more permanent season extenders come into play. One of the most popular of these options is the cold frame, both to get a jump on the season in spring, and to help extend the season in fall. Cold frames can also be used to overwinter less-hardy perennial plants by providing additional protection from wind and cold. A cold frame is an outdoor structure that works much like a miniature greenhouse. The cold frame has a lid made of a transparent material such as glass or plastic to admit light.
Most cold frames face south, to admit as much light as possible.
Insulation from both the frame's construction and the surrounding earth can provide protection to enclosed plants for far longer than those same plants would survive if planted in an unprotected garden. A variation on the cold frame is something known as the hotbed. Similar to a coldframe in structure, hotbeds provide even more protection because they have an internal source of heat. Today gardeners who employ hotbeds may use electric cables to heat the growing area. As the manure rotted, it produced heat.