Greenhouses are a great place to grow plants year round in the United States. They allow ample sunlight to filter in to your plants. Because nearly all of their walls are transparent, solar energy that enters a greenhouse is trapped and cannot escape easily. This solar energy substantially warms up your greenhouse and saves money on energy bills. However, during nights and cloudy days you need to provide supplemental heat to your greenhouse. Otherwise your greenhouse can rapidly fall to below freezing and your plants may perish. To heat greenhouses properly, a few careful steps need to be followed.
Caulk all seams in your greenhouse with a high quality silicone caulk. Insulate north facing walls in your greenhouse since sunlight won't penetrate from this direction. "Reflectix" is a foil-backed bubble wrap material that insulates well.
Fill large 55-gallon drums with water and place throughout your greenhouse wherever you have free floor space. Ideally you want 2 1/2 gallons of water per square foot of wall and roof space. Paint the drums black. They will absorb the solar energy during the day and release it at night.
Purchase an electric greenhouse heater that will sufficiently heat your greenhouse. Calculate the amount of BTUs your greenhouse needs using a greenhouse heater size calculator. See resources for an online calculator.
Plug the greenhouse heater in and set the thermostat to your desired temperature. The heater will run diligently when needed and most likely shut off during the day as the sun warms the greenhouse.