• The purpose of a greenhouse is to retain heat.Metal greenhouse in summer with open door image by Scott Latham from Fotolia.com
  
  The purpose of a greenhouse is to raise the ambient temperature for the plants inside of it, usually by retaining the rays of the sun as they enter through the glass roof and walls. For colder climates, and in early spring and late fall, auxiliary methods of maintaining heat in the greenhouse are necessary.
  
  

Wood

• Temperatures are kept up during cold nights in a greenhouse by installing a woodstove. For greenhouse owners who also own a woodlot, this is an inexpensive alternative. The stove requires proper installation, a safe distance from any plants, and must vent to the outside with a masonry chimney or insulated stove pipe. A drawback of a woodstove is that you can't leave it for extended periods since it needs stoking with fresh wood to continue heating. As with other forms of heating, the heat from a woodstove is more efficiently used if the outside of the greenhouse is covered with an insulating blanket overnight.
  
  

Geothermal

• The expense of geothermal development certainly makes it impractical for a small or hobby greenhouse. For a large commercial operation, or in conjunction with a home that is having geothermal heating installed anyway, it might make sense. Geothermal systems use wells or extensive underground piping systems to draw heat out of the earth and use it to heat buildings. The pumps required in geothermal systems can use a significant amount of electricity, but these systems are otherwise nonpolluting and will heat greenhouses without exposing plants to potentially lethal gases and fumes as is the case with kerosene or gas heaters.
  
  

Passive Solar Storage

• A method of heating that is very compatible with the structure of a greenhouse is passive solar storage, which uses large amounts of thermal mass to capture heat from the sun during the day and release it at night. In a greenhouse setting, this is accomplished by stacking large barrels against a back wall, filling them with water and painting them flat black. The dark color will absorb the sun's heat, the water (thermal mass) will store the heat, and, at night, when the exterior temperature drops, the water will begin releasing its stored heat into the greenhouse. This method is unlikely to provide sufficient heat in cold climates but will assist in regulating the ambient temperature inside the greenhouse.