A greenhouse is a wonderful tool for extending your growing season later in the fall, or starting it earlier in the spring. When the sun is shining, your greenhouse can be 30 degrees F or more above the outside temperature. However, when the sun goes down, your greenhouse will likely fall to within a few degrees of the outside temperature. If the temperature goes below freezing, this can be detrimental to your plants. Add supplemental heat on freezing nights to keep your greenhouse adequately warm for your plants. A small electric heater works great for heating your homemade greenhouse on those chilly nights.
Calculate the area of the outside walls and roof of your greenhouse. Measure the length and width of each and multiply these two numbers together to get the area. Add the area for each wall and roof together to come up with the total area of the outside walls and roof of the greenhouse.
Determine how many BTU (British Thermal Units) will be needed to heat your greenhouse with a greenhouse heat calculator (see Resources). Enter the outside area (from step 1), the minimum outside temperature you expect, the inside temperature you wish to keep your greenhouse and the heat loss value of your outside walls and roof.
Convert BTU to watts to find out how big of an electric heater you will need to heat your greenhouse. Divide your BTU number by 3.41 to determine the conversion to watts.
Place the correct size heater into your greenhouse, plug it in and set the thermostat to your desired temperature.
Place a thermometer at the other end of your greenhouse so you can monitor the temperature of your greenhouse.
Turn the heater on and check your greenhouse to ensure that the heater is keeping it at the desired temperature.