Small greenhouses may be easier to heat than large ones, and there are several ways to heat your greenhouse that will not break the bank. Simple greenhouse heat solutions include passive solar options as well as conventional heating methods.
Electric Space Heaters
Electric space heaters on a thermostat are an ideal way to heat a greenhouse without overheating the plants. Depending on the expected inclement weather, you may need more than one heater to keep your greenhouse at an ambient temperature. Experiment by placing the heaters in different spots, waiting until they cycle off and then testing the temperature nearest your plants.
Heating a greenhouse with a wood stove is an economical way of keeping your plants from freezing. The stove will need regular stoking to keep its ideal heat both day and night. A wood stove with a blower attached to a thermostat keeps the greenhouse at an even heat. To ensure that you have plenty of fuel on hand for your wood stove, stack dry logs inside the greenhouse away from the wood stove.
Place barrels filled with water in a sunny disposition in your greenhouse, where they will act as passive solar collectors. Ensure the barrels are away from exterior walls that will quickly sap heat during the night. Passive solar energy heats up the water in the barrels during the day and slowly releases warmth throughout the night. The water in the barrels may also be used as a handy watering source for your greenhouse plants.