You may provide heat to your greenhouse and protect your plants in extremely cold weather using a homemade heating system. There is no need to purchase expensive greenhouse heaters. Use heaters you have on hand in your home---portable utility heaters work best. Calculate the heat needs for your greenhouse first, using an online greenhouse heat calculator, then set up the appropriate number of heaters. Heated greenhouses offer a way to grow tropical or Mediterranean plants outside their natural, temperate habitat and to protect them from harsh winters. Use a few household fans to help circulate the heat around your plants and keep them happy.
Measure your greenhouse and enter that information into an online greenhouse heater calculator in order to determine the correct size and number of heaters to use.
Locate a power outlet and use a power extension cord, if necessary, to plug in your heaters. You may also use battery-powered heaters. Space the heaters evenly apart in your greenhouse and keep them at least two feet away from any objects or the sides of the greenhouse.
Set up fans---one fan per heater---throughout the greenhouse to circulate the warm air over and around your plants.
Place a thermometer in an area away from the heaters and out of direct sunlight so that you may monitor the temperature of the greenhouse.
Research your plants' needs to determine the appropriate temperature range.