A cold frame is a place to grow plants that's enclosed with a transparent roof. Its purpose is to protect plants from cold weather while admitting sunlight and preventing heat escape. Think of it as a miniature greenhouse or season-extender. Cold frames are different from greenhouses, though, in that they're cold, not heated like greenhouses. The temperature difference between the inside and outside of the frame is generally not more than 5 to 10 degrees.
Cold frames allow plants to be started earlier in the spring and survive longer into fall and winter. They're often used to grow seedlings in the spring or late winter. Some crops suitable for growing in a cold frame include lettuces, parsley, salad onions, spinach, radishes and turnips. One vegetable crop can occupy the whole of a cold frame or you can grow a combination of crops that mature in rotation - so you'll get a wide range of different vegetables throughout the year from one cold frame.
Cold frames are also used to provide shelter for tender perennials or to start cold-tolerant plants such as pansies, even to overwinter summer-rooted cuttings of woody plants. So they're not just for edibles!
Cold frames can be purchased ready-to-use, but traditionally are made by do-it-yourselfers using an old glass window, which is attached as the top of a 1-2-foot-tall wooden frame. The roof is often sloped toward the winter sun to capture more light and to improve runoff of water, and hinged for easy access. Instead of glass you can use plastic, like my neighbor used in his homemade cold frame, shown in the photo right.
Success with Cold Frames
The best site for a cold frame 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 cold frame permanently in the garden, or make one that you put away when you're not using it. Before setting up a cold frame 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.
To get the best results from your cold frame, pay attention to the temperature, making sure it's cool, not warm. The temperature inside the cold frame should stay below 75°F for summer plants and below 60°F for plants that normally grow in spring and fall. To keep the temperature cool enough, just lift the lid according to this 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 close the lid in late afternoon to trap the heat inside for the cool night. You can also buy an automatic venting device.
On particularly cold nights, you might want to provide some extra protection to keep your plants from freezing. Since most heat escapes through the glass, old blankets, straw or newspapers should be piled on top. Snow insulates well, too, but brush heavy snow off the glass so it doesn't break.
For the handy gardener, there are videos about how to build one.