The heat source of the cold frame is the sun. When the sun shines, this concentrated heat is trapped inside the greenhouse. The glass covering blocks the escape of the heat. This collection of heat accounts for the build-up of temperature during the day and the higher temperature inside the cold frame during the night. You will want to place a thermometer inside on the soil, but not in direct sunlight. The ideal temperature is about 75◦F; when the temperature is higher, open the lid to permit air circulation. When it starts to drop, close the cover to conserve the heat that has been absorbed by the soil. This is why cold frames need almost daily attention.
Most cold frames are about 3x 6 feet in size. To construct your new cold frame, the sides and ends should be an inch thick. If you use wood, use treated lumber (not creosote) for a longer life. The frame should be sunk into the ground about six inches deep. The back or north side of most frames is usually 12 to 15 inches high and the front or south side is about four inches shorter. Be sure the plastic or glass covering is slightly larger to allow water runoff. The sides and ends may be nailed together, but you may want to use strap hinges to secure the top to the cold frame.
Before sowing your seeds or plants, provide a mixture of four to six inches of well prepared soil medium composed of one part compost or organic matter, one part garden soil, and one part sand.
For information on cold frame construction, call Delaware Cooperative Extension at 302-857-6426 or 302-730-4000 in Kent County, and at 302-856-7303 in Sussex County.