There is a significant lag between the annual amount of sunlight we receive and the temperature we experience. Mid-March gets as much sun as mid-September but the temperature difference is at least 30°F. Growers can get a jump on the season by taking advantage of this discrepancy by constructing small enclosures known as cold frames which trap the heat and also insulate new plants.
Construction: The basic cold frame is a wooden box with framed glazing on the top. The box must be tall enough to accommodate the plants and not be larger than three by six feet so you can reach all the plants inside. The side may be constructed of any sturdy material, including plywood, concrete or even bales of hay and the top could be any clear medium (e.g. Plexiglas, plastic sheeting tacked to a frame or an old window).
Location: Set up the cold frame in a wind protected, sunny location in order to take advantage of as much sunlight as possible yet cut down on wind.
Protection: On frigid nights, the plants inside the cold frame may need a little extra protection to keep from freezing. Most heat escapes through the glass, so pile insulation on top. You can use old blankets, straw, newspaper, Styrofoam, or whatever is handy. The most significant heat loss can be from infiltration by the wind. Seal up cracks and, if necessary, berm up the sides with dirt.
Heating: One of the best ways to heat your cold frame is to use passive heating. Use gallon size plastic milk containers filled with water to line the north rear of your cold frame. During the day, the water will heat up and then later it will give up the heat during the night. You will also get even more protection as the temperature dips below freezing since the large amount of water has to freeze before the plants in the cold-frame freeze. Users want the cold frame to get quite warm during the day and stay above freezing at night. The coldest time is just before dawn. However, cold frames can also get too warm for tender plants and then the operator must crack open the frame to let some heat out.
Solar Option: For a little more work and cost you can build a solar cold frame. This structure has the glazing facing the sun at approximately a 45° angle. This will catch as much of the sunlight as possible. The north-facing portion should be insulated.
The Solar Greenhouse Book, edited by James C. McCullagh, Rodale Press, Emmaus, PA, 1978.