A cold frame is a clear plastic or glass-top box set directly on the ground to act like a miniature greenhouse.
The traditional form for a cold frame is a box with a sloping roof. The roof is highest along the north edge and slopes down an inch or two for every foot from back to front, with the lowest point no less than a foot high.
The larger the cold frame the more even temperatures remain inside. But a cold frame must be small enough so you can reach plants and soil from the outside -- after all, this is a cold frame, not a greenhouse.
The glass or plastic covering the cold frame is mounted in a wooden frame that can be lifted open. Advantages of glass coverings are their clarity and permanence.
Glass also is readily available because people always seem to be discarding old window sashes. But glass does break. Whether the covering is glass or plastic, you'll have to prop it open to vent heat on sunny days that are not too cold.
You can get year-round use from a cold frame. In winter, it can provide harvests of spinach or be home to plants just barely cold-hardy. Use a cold frame in fall to temporarily hold pots of bulbs for forcing.
In spring, get a jump on the season with early plantings of hardy vegetables and flowers followed by later plantings of tender ones. Remove the cover in summer and grow anything you want there.
The ideal site for a cold frame is well-drained soil with a slope facing east or southeast. At the very least the site must receive full sun in winter. A cold frame will keep even warmer in winter if it is snuggled near a fence, wall or dense, evergreen shrub on its north side.
Another way to hold extra warmth in a cold frame is by covering the sidewalls with rigid foam insulation and caulking all cracks. A double layer of plastic or glass reduces heat loss through the top.
A cold frame also stays extra warm if dug into the ground or if its sides are buttressed with soil.
Many homemade cold frames tend to leak air because the covers do not fit tightly and cold air blows in through cracks at the joints. This leakage is not all bad.
Although some heat is lost, you don't have to worry as much about the cold frame overheating if you forget to vent it or if you're not home when a cloudy day turns sunny.
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