If you intend to start bedding plants early or overwinter tender plants, supplemental
heat will be needed. For the smaller greenhouses, a fan-forced electric heater
with an output of approx. 1750 watts (or about 5000 BTU) should be adequate.
Be sure it has a grounded (3-prong) plug. The larger models will need a correspondingly
larger heater--15,000-20,000 BTU. Electric heaters this size are expensive to
run--you will save on heating costs by using a propane or natural gas heater
instead. (If you live in an unusually cold area, you may need a correspondingly
larger heater.) Be sure you get a heater with a built-in thermostat, or get
a separate thermostat to plug the heater cord into. Remember to have a GFI (ground-fault
interrupter) circuit installed on the outlet in the greenhouse by a qualified
A suitable heater can be obtained from many hardware/ electrical supply stores, or a natural gas company.