A cold frame is an enclosed wooden box, usually with a glass lid, that gives less hardy vegetables a chance to grow in cold and harsh weather, protecting them from snow or strong winds that they would otherwise not survive. This miniature greenhouse traps the heat of the sun through the glass and retains it, thus keeping contained vegetables 10 to 20 degrees warmer than outside. A cold frame is filled with rich soil that promotes healthy vegetable growth.
Step outside and select a suitable spot to place your cold frame, preferably one with a southern exposure. Make sure the spot gets plenty of sunlight and has well drained soil. You will be placing the base-less cold frame directly over the soil, so drainage is very important to prevent root rot.
Construct the framework for the cold frame. Use an old window sash for the top and 2-by-6 wooden posts to match its dimensions for the frame. Keep the back posts 18 inches high, and the front 12 inches. This angle allows for maximum sun exposure and for water to drain off the top. Insert the posts into the ground, and attach the sash to the tall end with hinges.
Unravel the polyethylene plastic and staple it securely to the posts to form walls for the cold frame. Also spread it over the roof and staple it into the edges of the sash.
Mix good quality soil with an equal amount of soil amendments such as peat moss, manure and compost, and add it to your cold frame until the soil level is 12 to 18 inches high. The resulting nutrient-rich soil will hold moisture and further assist in drainage.
Follow packet directions for appropriate depth and spacing when planting vegetable seeds in the soil. Water the soil lightly with a watering can. Check the seeds regularly for signs of germination, and keep the soil evenly moist at all times.
Cover the cold frame with burlap, blankets or any material for added protection if temperature falls below 20 degrees F. Remove the cover once the cold spell is over. On a warm, sunny day, place a stick strategically to prop open the glass cover of the cold frame.