Because the hotbed has a source of heat, you can use for a longer period of time, than you can a cold frame greenhouse.

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During the spring, you can use the hotbed earlier than you could a cold frame greenhouse. You can also keep using the hotbed later in the fall than what you could use a cold frame greenhouse.

The construction of the hotbed is much the same as the construction of a cold frame in that it comes in a ready-to-assemble kit. There is a sloping surface on the south of the greenhouse. The sloping surface is there to help any accumulated snow or water to run off of. The sloped surface also allows more of the sun's heat to transfer into the greenhouse. 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 thaw approaches and the temperatures begin to rise, you should begin to ventilate the greenhouse. You need to lift up the ventilation sash a tiny bit. This goes for hotbed greenhouses as well as the cold frame greenhouses.

Another important factor to remember is that you need to keep the greenhouse ventilated during the warmer spring days. On the warm spring days, you'll have to make sure there is sufficient ventilation. The times your plants are most vulnerable to the heat is midday up to mid-afternoon, when the sun is the hottest. Whatever you do, don't forget to close the ventilation sash before the evening temperatures dip down at night. Just as much as you don't want to bake your plants, you don't want to freeze them either.