Greenhouses can expand your growing horizons from seed starting to growing tender plants. Heating and cooling in your greenhouse are critical for getting the most from it. The common heating methods are oil, gas, wood, electric, compost, and hot water. It's important to do your homework on the amount of heat you need and the price for maintaining the desired temperature.
For simple frost protection you want to keep the temperature at 40°F. If you want your plants actively growing throughout the year plants like it at 60°F.
Fortunately, we have a handy-dandy calculator to help you on your quest for greenhouse heating. Ok, have your BTU number? Look for a heater that will provide the BTU's you need and you're set to grow through the winter months.
If you really want to dive into greenhouse heating check out this in-depth article from the University of Georgia.