You have several options for keeping a greenhouse warm in the wintertime—or adding extra warmth if you want to grow tropical plant species. Install an electric fan heater or heat pump designed for greenhouse use. You can also add a solar greenhouse heating system and increase the amount of insulation in your greenhouse.
Generate more heat by building a sustainable Victorian hot bed. Using a ratio of 3:1 for organic matter to growing medium, fill a raised bed with a layer of compostable material. Compress it, then add topsoil and compost. A plastic cover will increase the heat generated by the decomposing organic material.