What are cold frames and hotbeds for plants?
Cold frames and hotbeds are similar; both are glass-covered wooden boxes set over the ground. Both protect plants from frost, wind, and cold weather. A hotbed has an electric heating cable buried in its soil, extending its usage even further.
Seeds can be started and seedlings planted in a frame several weeks before garden planting is possible. Lettuce, radishes, spinach, and scallions can be grown in frames for an additional month to 6 weeks in fail.
A frame should be 1 to 2 feet high in front and 2 to 3 feet in back, producing a sloping top that maximizes the sun's warmth. If possible, locate a frame facing south.
A serviceable frame can be constructed of 2 x 4's and scrap 1-inch boards with discarded window sashes as covers. Much neater is this frame made from a sheet of 3/4-inch exterior plywood. Saw the pieces to size, making 15-degree beveled cuts on the top edges of the front and back (to match the slope of the sides).
Assemble the parts with waterproof glue and countersunk No. 8 x 1 1/2-inch flathead wood screws electroplated with zinc. Around the top, nail a frame of 1 x 2 furring strips, using 6d galvanized finishing nails.
Make m 2 x 2 pine with lap joints at the corners. Add wire crossbraces with 2-inch turnbuckles to give the frame rigidity. Use two 5inch T-hinges to secure the cover to the frame. Paint with an exterior paint.
For low-cost glazing, staple two-mil clear plastic film to the cover; replace it yearly. For a longer lasting glazing, use I/4-inch acrylic plastic sheet or clear corrugated fiberglass roofing. Fasten it to the frame with No. 1/2' x 8 roundhead plated screws. If you use the fiberglass, seal its edges with caulking.
Spade the soil where the frame is to be placed to a depth of 1 foot; add peat moss or compost and builder's sand as conditioners. Dig a trench 3 to 4 inches deep to set the frame into.
When the frame has plants in it, keep a thermometer there. If the temperature reaches 75°F, prop up the cover. At night conserve heat by putting an old blanket over the cover.
To convert a cold frame to a hotbed, purchase an electric heating cable at a garden center. Bury the cable about 8 inches deep in a back-and-forth grid pattern with the loops 1 foot apart. If the soil is damp, lay gravel 4 to 6 inches deep beneath the cable. Lay hardware cloth on top of the cable.