Warming the soil in your greenhouse has some serious advantages. The warmth will hasten the rooting of cuttings and speed up the germination process. The most convenient way to do this is by electricity. Alternatives are to use a low-voltage current that goes through bare wires or a full mains voltage that can be passed through soil warming cables. Either wire or cable should be buried at least 4-6 inches in the soil.
Before laying the wires, you first need to dig the soil to the required depth and put down a layer of sand at the bottom and rake level. Next, lay the wire in the required length in lines parallel from each other and as equi-distant from each other as possible. Bent, galvanized wire can be useful in holding the wire in place.
Low voltage wire doesn’t require any special precautions to protect the wire but mains cable can be hazardous if accidentally sliced by a shovel or other gardening tools. Lay down a netting of galvanized wire to avoid this problem. Add additional sand on top and then replace the soil. Be sure to plug the cable into a waterproof outlet. Ideally, the outlet will be above the soil and far enough away so as to avoid accidental splashing.
Warming cables with built in thermostats are available for your convenience. This way, the thermostats will turn off the current when the soil temperature rises above a certain point. This of course is convenient and economical but not essential because the cables will never get too high. The highest they will generally go is 60 degrees Fahrenheit.