A cold frame—a low, bottomless box with a clear glass or plastic top, that is set on the ground or over a sunken bed—can extend the growing season by 1 to 3 months.
Cold frames can be used for year-round gardening:
• Spring: Use the frame to start seeds early and harden off seedlings.
• Summer: Use the frame as a nursery bed to start fall crops from seed: simply remove the lid.
• Autumn: Extend the summer harvest by protecting low-growing warm-weather crops or grow compact cool-weather crops.
• Winter: Extend the harvest of autumn crops and grow cool-weather crops in cold weather.
Making a Cold Frame
A cold frame consists of a bottomless wooden, masonry, or plastic box—the sides can be wood, cinder block, or brick with a clear plastic or glass top. Construct a wood-sided cold frame with 2-by-8- or 2-by-10-inch boards forming a box. The box can sit on a raised bed or directly on the ground or over a sunken garden bed. The top or cover can be made out of 1-by-3-inch boards with one or two layers of polyethylene plastic or glass–two layers offer greater insulation from cold weather. Old storm windows make excellent tops.
Portable cold frames commonly have aluminum frames and clear plastic or polyethylene sides. Small- to medium-size cold frames kits are relatively inexpensive and easy to assemble and disassemble for storage. Portable frames with clear covers and sides are made from acrylic, Plexiglas, UV-stabilized polyethylene attached to a frame, or glass. Polyethylene and plastic are less expensive than glass but usually last no more than five seasons—they deteriorate when exposed to the sun.
Polyethylene transmits 90 percent of available sunlight and protects plants to 25°F.
Siting a Cold Frame
Cold frames should be slanted so that plants receive the maximum sun exposure. (In the northern hemisphere, the frame should face south.) The back wall should be a few inches higher than the front wall. A rule of thumb is to add 20 degrees to your latitude to arrive at the best angle from the horizon (for example, if you live at the 40-degree parallel, the top should slant 60 degrees from the horizon).
Good drainage is essential for a cold frame. Make sure the site is well-drained; do not allow water to stand inside a cold frame–roots will drown.
Ventilation is important. A thermometer inside is helpful. The frame top should be opened when the temperature inside reaches 65° to 75°F. Direct sunlight can overheat a cold frame and harm plants.
Where winters are very cold, a cold frame can be insulated. Line the sides of the frame with 2-inch-thick foam sheets and use double-thick plastic or glass for the top.
For additional tips on growing vegetables in cold frames, click on these articles: