Soil Heating Cable


Would you like to speed propagation of a favorite plant? Grow a few “hothouse” plants in a “cool” greenhouse? Accelerate growth on plants in outdoor frames? Get a head start on your seed grown annuals and vegetables, or hasten the sprouting of patio plants grown from bulbs or tubers? If you answer yes to any of these queries it will pay you to look into the possibilities of gardening with soil heating cables.

Soil heating cables come in a wide variety of sizes and prices. There’s the small one often sold as a unit with starter pots, flats and rooting medium. Then there are the larger types insulated and enclosed in lead or plastic which are about 40-foot cable in length with other sizes priced accordingly. A utility thermostat is also a good idea.

The larger sizes are made in several lengths but the most useful sizes seem to he 40, 60 or 80 feet, adapted to use with ordinary electric service of 110 volts.

A 60-foot cable will heat a 6- by 6-foot hotbed, or you may size the cable to suit your own space. Each 60-foot cable carries an electrical load of approximately 400 watts. The cost of operating such a cable on a continuous 24-hour basis would be only a few cents per hour. If you want to make sure how much it will cost you should consult someone at your power company.

soil heating cable

Once in use you will find that the soil heating cables run but a small part of most days. When the sun heats the hotbed or coldframe the thermostat shuts off the current. And as spring nears the outdoor temperature is heightened and the soil heating cable will be on for shorter periods of time.

Make Structure Tight

You can help conserve heat on outdoor beds by making the framework construction tight. Bank the sides with earth or other heat conserving material. Check the sash. It should fit tightly. If it doesn’t, “weatherstrip” the top of the frame with strips of felt. As with a greenhouse, make sure that the glazing is well puttied and laps are one-fourth inch at joints. Keep the glass clean to admit maximum light.

If the weather turns extremely cold and windy give the frame the added protection of extra sash laid over the top of the permanent sash. A covering of straw or mats is helpful during the evening hours.

Soil heating cables operated with thermostats keep the soil temperature at a constant level. If you run into warm weather where the sun heats the frame to a danger level you’ll have to control it manually through ventilating the frame. We just lift the sash on ours and slide wedges of wood between the frame and the sash. Some frames are equipped with permanent hinge or notched wood ventilators.

Installing the heating cable is easy. If you are working with an outdoor frame check to sec that there’s an ordinary light socket close to the frame. Place a layer of sand on the floor of the frame. Lay the cable back and forth in loops over the sand. (See photo). Cover with another layer of sand. As an added protection against breaking the cable when shoveling, place over the final layer of sand a hardware cloth (wire mesh). The growing medium is then shoveled into the frame, the cable and thermostat connected to the outlet, and you’re in business.

If you are using the cable in the greenhouse you may want to use it to heat an entire bench, any portion of the bench, or perhaps just a fiat or two. Here the procedure is the same as above. In most cases, however, you can eliminate the wire mesh as you probably won’t be using the shovel in the greenhouse benches. The exception to this might come when market crops are grown directly in the soil on the greenhouse “floor.”

Temperature to Use

Germinate the majority of seeds, sprout bulbs or root cuttings at a temperature about ten degrees higher than that recommended for growing mature plants. You’ll be amazed at the rapid growth on gloxinia tubers, tuberous begonias, amaryllids and caladiums when given some “bottom heat” Likewise with cuttings of African violets, gesneriads of all kinds and of course, cuttings of green plants, such as ivy and philodendron.

I have found a flat of equal parts of sand, peat moss and sphagnum moss heated with a small six-foot plastic covered cable an ideal greenhouse propagating unit. Into this propagator goes just about any cutting or tuber I want to use as propagation material. The temperature control is set at 75 degrees. This seems about right as the material I propagate grows best in a warm greenhouse.

Early in the season, use extreme care in watering outdoor hotbeds – excessive water may cause plants to damp off. As in the greenhouse, more water is required on bright days than on cloudy days. All plants grown in a hotbed should be hardened off before potting or planting into the outdoor garden.


The hotbed gives you a head start on vegetables such as tomatoes, peppers, eggplant, squash, celery or cucumbers. And with the use of a soil cable in the outdoor frame (or greenhouse if you have space) you can be certain of lavish amounts of annuals such as petunias, asters, salvia, bells of Ireland, verbena, snapdragon (these are slow when germinated naturally), ageratum, lobelia, cosmos, annual phlox, clatura, salpiglossis, scabiosa and hundreds of others. These are excellent annuals but when sown directly into garden soil they are slow in our short season.

For the flower arrangers who want to include celosia (coxcomb) in early summer flower arrangements start them in a hotbed. They’ll “jump” out of the ground when given extra heat.

Many gardeners start perennials from seeds in the hotbed. As the weather warms they can be set into garden nursery rows or left to grow until the following spring in the hotbed. When this procedure is followed, the glass frames arc removed during the summer months, the heating cable is disconnected and the plants are on their own.

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