A coldframe—simply an enclosed area with a clear top to let in sunlight—is one of the easiest ways to extend your growing and harvest season. All you need are a few basic supplies and your imagination. (Imagine: crunchy fresh lettuce for the holidays!) Here’s what to do.
Start at the Top
The only essential for the frame’s cover is that light gets through. Almost any transparent material will work: glass, fiberglass, polyethylene, or flexible greenhouse coverings—the differences between them are insignificant.
Many people use old window sashes. Be aware, however, that some old frames may be covered with lead-based paint. If in doubt, pass on them, and keep looking until you find windows that the owner can assure you are lead-free. Also, make sure the wood isn’t rotting and the glass is secured firmly inside the frame. Be careful when handling the glass, and keep small children away from it.
In extreme northern areas, glass isn’t always the best option. “Here in Alaska, glass coldframes break under the weight of our heavy winter snow,” explains Jeff Lowenfels, a gardening columnist, “so we tend to rely on thick sheets of Lucite or other window-strength plastic instead.”
If you’re buying material to cover your coldframe, consider Lexan, an improvement over Lucite. Lexan stands up well to the elements (like rain, sleet, ice, or snow). And it insulates especially well.
Other gardeners prefer the corrugated fiberglass (4-by-8-foot panels) sold for greenhouse walls. Although it costs a little more than other plastics, it lets in a lot of light and doesn’t turn yellow with prolonged exposure to sunlight.