A cold frame is a four-sided mini-structure with a clear top. It is one of the easiest ways to extend your growing and harvest season. It is placed on the ground and is used to house, protect, and harden off seedlings and small plants when it is still cold outside in late winter or early spring. It usually does not have artificial heat. The cold frame will also provide the plants with protection from cold winds. It is usually 12-24 inches tall.
All you need are a few basic supplies to make a cold frame. The only essential for the frame’s cover is that light gets through. Almost any transparent material will work: glass, greenhouse plastic, or clear plastic.
A simple frame can be made from hay or straw bales. Just arrange the bales into a rectangle shape to make the sides of your coldframe. Then put the glass or plastic cover on top of the bales. To make a more permanent and easily vented structure, build the sides from wood and attach an old window frame to it with hinges. Cedar, cypress, and redwood are naturally rot-resistant, but you can use almost any kind of wood—as long as it isn’t pressure treated. Cinder blocks could also be used.
Be sure to vent the frame on sunny days. Set an alarm to remind you if that helps. Otherwise you will come out and find burned up plants on a sunny day. Automatic vents can be purchased to help if you are memory challenged. This one starts opening between 55 – 75°F, depending on how the vent is adjusted.