Used to protect tender plants or rooted cuttings during the colder months, a cold frame is simply a box with a transparent lid or cover. It acts as a passive solar energy collector and reservoir.
During the day, the sun's rays heat the air and soil in the frame; at night, the heat absorbed by the soil radiates out, keeping the plants warm.
A cold frame is useful at other times of year as well. In spring, it provides an ideal environment for hardening off annual flower and vegetable seedlings started indoors. Seeds of many plants can be sown directly in the frame and grown there until it's time to transplant them to the garden. In summer, you can replace the cover with shade cloth or lath, creating a nursery for cuttings.
Set up your cold frame in a site protected from harsh winds by trees, shrubs, a fence, or a wall. To ensure that the frame will receive as much sunlight as possible, orient it to face south or southwest.
Sinking the frame 8 to 10 inches into the ground increases heat retention significantly. Make sure the location has good drainage, since you don't want water to collect around the frame after every rain.
Building a Cold Frame
1. Start by selecting a cover, since its size will often determine the dimensions of the frame. Good choices include an old window sash or storm window; if you don't have one on hand, look for recycled windows at garage sales. You can also make a cover out of clear acrylic or fiberglass sheets sandwiched between narrow strips of wood and reinforced at the corners with metal corner plates. Polyethylene film stapled to a wooden frame is another option; it's quick and inexpensive, though it lasts only a year or so. Make sure the cover isn't too heavy to lift easily. Don't make it too wide, either, or you'll have a hard time reaching the plants inside the frame; a width of 2-1/2 to 3 feet is ideal. A length of at least 4 feet will allow you to grow a variety of plants.
Build the frame from lumber, such as rot-resistant redwood or cedar or less expensive plywood or scrap lumber. The frame should slope from about 1-1/2 feet high at the back to a foot high at the front; this traps the most heat and lets rainwater run off. For strength, reinforce the corners of the box with vertical posts. Attach the cover with galvanized steel hinges and apply weather stripping around the top edges of the box.
2. Ventilation is vital to prevent overheating. A minimum-maximum thermometer is useful for keeping track of temperature fluctuations. Plan to prop open the cover when the temperature inside reaches 70 degrees to 75 degrees F/21 degrees to 24 degrees C. Close the cover in late afternoon to trap heat. (If you won't be around during the day, you can buy a nonelectric vent controller that will automatically open and close the cover at a preset temperature.) On very cold nights, drape the frame with an old blanket or piece of carpet to provide extra insulation.