Cold frames are a clever way to add months onto your growing season, overwinter plants and to start seeds. Cold frames protect plants with four walls and a transparent cover for letting in warm sunlight and keeping out cold wind, freezing rain and snow.
Some plants do better in cold frames than others. Low-growing, cool-season crops such as lettuces work great.
Set Up a Season-Extending Cold Frame
- Choose a kit or build a DIY cold frame big enough to suit your needs.
- Select a south-facing site with good drainage.
- Dig out the top 9-10 feet of soil. Replace with 3-4 feet of gravel and top off with 6 feet of topsoil.
- Plant small plants or set pots or flats on top of the soil.
- Monitor the temperature inside the frame to keep it on the cool side. The temperature should be below 75°F for summer plants and below 65°F for cool-weather plants.
- Use the lid on your cold frame to maintain internal temperature. Raise the lid when the temperature hits 40°F or above, so the heat doesn’t build up and cook your plants. Close it in the afternoon to trap warmth overnight.
- Add extra insulation on the top of the cold frame to reserve heat on extremely cold nights. Use blankets, straw or newspaper. Snow also works as a natural insulator, but be sure to avoid a heavy pile-up.
Tip: Do not use glass for your cold frame if you live in an area that gets heavy snow, as it may break under the weight.