Constructing Hot beds

A hotbed, obvious as it may sound, is basically a cold frame with heat. While cold frames receive all of their heat directly from the sun, hotbeds are heated with electric soil cables, stable manure or steam, or hot water heated with flues. The hotbed can be used earlier in the spring and later in fall and early winter than the cold frame.

Hotbeds are constructed just the same as cold frames, with a slope to the south to admit heat from the sun and to allow water or snow to run off. Plants growing in these frames are protected on cold spring nights with the same kind of mats suggested for cold frames. Hotbeds are usually built to be permanent structures, with the frame of wood, concrete, or brick extending into the ground below the frost line. As with the cold frame, you can build it yourself, purchase a kit of materials for building it, buy a ready-built one, or have someone construct the entire thing for you.

A soil-heating cable furnishes the simplest kind of heat for the hotbed and these cables come in a variety of sizes and prices. The type used for hotbeds is insulated and enclosed in lead or plastic sheathing. The cables are made in several lengths but the most useful sizes are 40, 60, or 80 feet, all adapted for use with an ordinary electric service of 110 volts.

A 60-foot cable will heat a 6- by 6-foot hotbed. You should reckon your cable to suit your space. Each 60-foot cable carries an electrical load of approximately 400 watts. In our area the cost of operating such a cable on a continuous 24-hour basis is about 1 cent per hour. You should have a thermostat to regulate air temperature and another to regulate soil temperature. However, you will find that during many hours of the day the sun will heat the hotbed enough so the thermostat shuts off the current. As spring nears, the outdoor temperature rises and the artificial heat will be on for shorter periods of time.

You can conserve heat by making certain that all construction is tight. Bank the sides of the hotbed with earth and check the sash-it should fit tightly. If it doesn’t, weather-strip the top of the frame. Make sure that all glazing is well puttied and that it laps J4 inch at joinings. Keep the glass clean to admit maximum light. In my area it is not practical to use a hotbed before March first. As the spring temperature increases, start ventilating the hotbed by raising the sash a crack. This applies equally to cold frames. From midday until mid-afternoon on warm spring days, you will have to ventilate more. Be sure to close the frame before the temperature falls at night.

Installing the Heating Cable

You will need an electrical outlet close to the frame, so you can connect the soil-heating cable. Place an inch of sand on the floor of the frame. Loop the cable back and forth over the sand. Cover with 2 inches of sand and a layer of hardware cloth (coarse wire mesh). If you grow plants in flats, place them directly over the hardware cloth. If you plant seeds or cuttings in soil-without flats-add about 4 or 5 inches of good soil.

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