Properly maintained greenhouses can be used year-round, but they must be insulated and kept warm during the cold months. Some gardeners prefer methods that do not require energy use. For those who want an energy-regulated system, there are both propane and electric heaters of various sizes made specifically for use in a greenhouse.
Insulate the greenhouse. The first step in keeping a greenhouse warm is making sure it is well-insulated.
Make sure all window panels are caulked and sealed in a solar greenhouse. Check the seal around the greenhouse door when it is closed, and make sure the windows sit correctly when closed.
Use silicone caulk to plug up any holes in corners.
Build a water wall. Collect plastic milk jugs or soda bottles and keep the caps. Rinse them out well, then fill them with water with a little black dye mixed in.
Stack these jugs against the south wall inside of the greenhouse. Stack them between 2/3 of the way and all the way up to the ceiling. Arrange greenhouse shelving to hold the jugs to create this water wall.
The black water will collect heat during the day and provide heat in the greenhouse at night.
Purchase a small greenhouse heater. Know the dimensions of your own greenhouse, and check with manufacturers to find out what size heater is appropriate. Any random small heater will not do---a heater designed specifically for greenhouses is necessary. Greenhouse heaters are made to operate in a humid environment.
Choose between propane and electric heaters. This is a matter of personal preference. One feature to consider with either type is automatic shut-off, to prevent overheating.
Maintain heat for seedlings in the greenhouse with heating mats. Heating mats are similar to electric blankets for people. Arrange heating mats on your greenhouse shelves, then place seedling trays on top of the mats to keep the soil warm while the seeds are germinating.