Most gardeners would thoroughly appreciate having a coldframe. It is a bottomless, boxlike structure on top of the ground, or with the sides several inches into the ground, covered by a slanting top of glass or plastic.
It utilizes the sun's heat; no artificial heat is applied. The soil is heated during the day and gives off its heat at night and keeps the plants inside it warm. A hotbed is similar to a coldframe except that it has a heat source, such as a heating cable, in addition to sunshine.
The coldframe can be used in the spring to harden seedling that were started indoors before planting them outside in the garden. The process of hardening reduces injury from unexpected temperature drops after transplating.
It also can be used to start seeds of annuals, such as zinnias, marigolds and alyssum, to root cuttings and to force tulips, hyacinths and daffodils into bloom during the winter.
The coldframe is easy and inexpensive to build. A southern exposure with protection from north and west winds is preferred. The south side of a heated building is a good place for it. It is better to dig a hole for the coldframe because less heat will be lost by those with sides several inches below ground.
The walls may be of boards, concrete blocks or poured concrete. The north wall usually is 8 to 24 inches high, and the south wall 2 to 12 inches high. If the frame is to be partially below ground level, the front wall should be only 2 to 4 inches above ground and the back 8 to 16 inches. If the frame is to be entirely above ground, the front wall must be at least 12 inches high in order to have room inside for the plants to grow.
Preferably, the frame should be a multiple of 3 feet. Cross pieces may be provided to help in sliding the sash across the frame and for added support.
Concrete blocks 4 inches thick occupy less space than 6-or 8-inch blocks and are recommended because they are easy to work with.
Glass or sheet plastic can be used to cover the coldframe. The cheapest plastic is polyethylene, the life of which is estimated to be 3 months during the summer and 9 months during the winter. The breakdown of this plastic is caused by the ultraviolet rays of the sun.
The frame should be tight and the outside edges may be banked with soil or other material to conserve heat. The sash (with glass or plastic) should fit snugly.