What is a coldframe? Nothing more than four walls to trap heat and shelter plants, and a transparent lid that admits light. You can make the walls from any sturdy material—plywood, concrete, even bales of hay. An old window works perfectly as a lid, but you can also use Plexiglas or plastic sheeting tacked to a frame.
The lid's size usually determines the dimensions of the coldframe. Still, you'll want it to be larger than 2 by 4 feet to make it worth your while; you don't want it much larger than 3 by 6 feet, so that you can reach all the plants inside. Build the back 4 to 6 inches higher than the front to maximize the amount of light that reaches the plants inside and to allow water or melting snow to drain off the top easily.
The best site for your coldframe, according to Rodale's All-New Encyclopedia of Organic Gardening, is a south-facing, sunny spot with good drainage and some protection from the wind. Ideally, the site should get full sun from midmorning to midafternoon. You can set up a coldframe permanently in your garden, or make one that you put away when you're not using it.
Before you set up a coldframe in a permanent spot, dig out the top 3 or 4 inches of soil inside the frame and replace it with a layer of coarse gravel. Then put 6 inches of topsoil back. This will ensure good drainage.
You can grow coldframe plants in pots, flats, or, if you're growing just one type of plant (say, salad greens), plant right in the soil.
The key to using a coldframe successfully is paying attention to the temperature—and the trick is in keeping it cool rather than warm. The temperature inside the coldframe should stay below 75°F for summer plants and below 60°F for plants that normally grow in spring and fall. The way to keep temperatures cool inside a coldframe is to lift the lid. A good rule of thumb: When outdoor temperatures are above 40°F, prop open the lid 6 inches; when the outdoor temps clear 50°F, remove the lid. Be sure to restore the lid in late afternoon to trap the heat inside for the cool night. You can also buy automatic venting devices in some gardening catalogs.
On frigid nights, the plants inside the coldframe 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 or whatever is handy. Snow insulates well, too, but brush heavy snow off the glass so it doesn't break.