Cold frames and hotbeds are essentially the same thing, except a hotbed has added heat.
Cold Frames
A cold frame is a small greenhouse. Structurally it is a bottomless, glass-covered box, heated only by the sun, for the propagation, growing, and protection of plants.
The wooden box is of airtight construction, treated with wood preservative, painted, and sunk into the ground. A typical cold frame is 3 feet wide and 6 feet long; the back is 18 inches high and the front is 12 inches high. Its hinged transparent window can be a stock cold frame sash (available from greenhouse supply firms), an old window or glass door, or a wooden frame covered with polyethylene film. To further insulate the cover, stretch a second layer of polyethylene over the glass or plastic, with a dead-air space of no more than 3/4″ separating them.
Use hinges with removable pins to facilitate changing covers, keeping two or three replacement covers on hand for different purposes. Use glass or plastic covers for warmth during the coolest weather, bird netting for protection during pleasant weather, and lath or shade cloth covers to break the force of the sun during the hottest part of summer.
Fill the planting bed with layers of gravel and sand for quick drainage. Cold frames are frequently used to start vegetables and flowers in early spring, then squash are planted in them for the summer. To facilitate this use, prepare the soil inside for planting, then cover it with two inches of sand to avoid mud. Start the seedlings in flats set on the sand, then plant squash seeds directly in the sand when the seedlings are removed.
Orient the cold frame to the south sun, on a well drained site. A fence or wall on the north side is excellent wind protection and reflects additional light and heat into the box. White or silver paint on inside walls also reflect more light to the plants.
Equip the cold frame with a thermometer. When it reads above 75 degrees, open the sash to permit air circulation to reduce it. Close it again when temperatures start to drop, to conserve radiation absorbed by the soil.
Self-opening vents are available to avoid the necessity of opening and closing the lid. They require no electricity, being driven by a piston in a cylinder of heat-sensitive liquid that expands when it gets warm. To find sources, search on “auto vent opener”.
In cold weather, insulate the outer sides of the box with banked soil or sawdust. Cover the top with sacks stuffed with straw or excelsior, wood planks, or other insulation during cold nights or chilly overcast days.
Quick or temporary cold frames can be made of hay bales, children’s swimming pools, or other containers. Cover the top with old windows or polyethylene sheeting.
Hotbeds
By adding a source of heat, you can convert a cold frame to a hotbed.
In earlier times a layer of fresh horse manure generated the heat, but today the usual source is an electric soil-heating cable. Incandescent lamp bulbs under the glass can also warm the box, though not as well as the cable.
To use manure, dig a pit under the cold frame from 2 to 4 feet deep. Pack fresh manure into the pit and let it heat up before placing plants inside. It will begin to heat in just hours, and will become quite hot in a day or so. Watch the temperature, and let it begin to drop before placing plants inside. After its first surge of heat, it will continue to generate just the right amount of heat for weeks.
Heating cables are simpler to use, but require a source of electricity. As always, be careful of electrical outlets outdoors; they can be dangerous when exposed to water and damp soil.
Heating cables are available in a variety of sizes, described according to length and wattage. A 30-foot 200-watt cable is typical. In an average area you should use 12 watts of heating power per square foot of hotbed. If you live in a very cold area you may want to double the watts per square foot. Use small pegs to hold the cable in place while you wind it back and forth across the bed to provide an even distribution of the heat. Cover the cable with a piece of hardware cloth to protect it, then cover the hardware cloth with 2 inches of sand.
Commercial Cold Frames and Hotbeds
Portable cold frames and hotbeds are available as commercial products. They come in many sizes and types; some can be placed on tables, making it easier to tend the plants inside. Many have self-ventilating lids, freeing the gardener from watching the thermometer.
Using Cold Frames and Hotbeds
Starting seeds in a hotbed, or in a cold frame in milder climates, has several advantages over indoor germination. When indoors, plants are in an environment that is always warm. A seedling that gets warm days and cool nights will be sturdier. The low humidity inside a house is also detrimental to good seedling development.
In cold climates a cold frame can be used to harden seedlings that have been germinated indoors. Hardening plants is accomplished by placing them for a few days in a location that is cooler and perhaps sunnier than the germination area. Most seedlings that have developed indoors need to be toughened before planting outdoors or they will be shocked by the abrupt temperature change.
Watch the thermometer in the hotbed, ventilating it when necessary. During hot weather, shade the plants inside with shade cloth or lath.