A cold frame has a funny name. The idea behind it is warmth -- and it is actually an insulated frame that uses solar power to create a warm environment to give plants a new lease on life in cold weather.
It's called cold to distinguish it from a hot bed, which is basically a cold frame with a heat source other than the sun. With adequate insulation, it will allow cold weather crops to grow right through the winter in many parts of the country.
The mechanics of a cold frame are simple. During the day, the sun heats up the dark soil, which acts like a heat storage tank. When the sun goes down, the panes of glass are closed down over the frame, to hold the heat in.
The most basic cold frame is one made of bales of hay, stacked and covered with old storm windows. This is just a temporary fram, though, and it won't take plants through winter, except in warm places. Even in cold climates, though, it will make the growing season a little longer.
One of the more complex cold frame plans calls for the area to be dug out to a depth of two or three feet, and lined with urethane foam or insulation board. This keeps the cold from creeping up from underneath. A well-insulated frame, built on top, keeps the heat from escaping upward.
The frame can be made of concrete, but most people construct theirs of wooden planks, fitted into the earth, so no air can flow betwen the two. It should face south, for maximum sun. And the sides should slope, with the north side twice as high as the south, for the same reason.
Old storm window panes make fine, and inexpensive, lids. In fact, if you check out the local curbs during clean-up week, you're likely to find plenty of these. If you staple clear plastic to the insides, you'll create the equivalent of expensive thermal panes. Ou'll get more insulation, including a layer of warm air trapped between the two layers.
Make sure the frame contains at least a foot of fertile soil, rich in compost, sweet, and maybe with a little sand, mixed in for texture. Once you've got plants growing inside, a mulch of black plastic will catch even more of the sun's heat.
A friend of mine warms his cold frame with plastic gallon jugs, filled with water and placed so they're in direct sunlight. The jugs heat up during the days and share their heat with the plants when nights are cold.
At the Rodale farm, I saw this idea taken a step further. The jugs were painted black for maximum absorption of the sun's rays.
Make sure that there are no holes in your frame, because large quantities of heat can escape through tiny spaces. Seal all the holes, and the cracks near the hinges with caulking, weather stripping, paper -- anything that will do the job.
For extra insulation, surround the frame with bales of hay. And keep the heat from escaping at night by covering the glass with a tarp, old blankets, rugs, or more hay.
A cold frame makes certain demands on a gardener. You have to be there to open it on warm mornings and to close it tight on cold nights. But, even this is flexible. You can cover it in the afternoon on cool days when you won't be around at dusk.
It will meet your demands by keeping you in cool crops, like lettuce, greens, radishes and a whole array of cold hardy Chinese vegetables, when nothing grows outside.
In spring, you can use the cold frame to get a strong and early start on garden plants -- and you won't have to worry about finding enough space on your windowsills to accommodate a young garden.