Geothermal systems for greenhouses can range in depth, as long as there is enough room to collect heat, and it is far enough below effects of surface freezing. In most moderate climates a constant temperature of 55 to 60 degrees F can be found at about 6 ft below the surface. In more extreme climates, the required depth could be 12 ft, also taking into account any permafrost layer.
Geothermal greenhouses are heated by pumping air through underground pipes where the soil heats the air to the ambient soil temperature before it's pumped back into the greenhouse. If warmer temperatures are needed, a regular household heat pump can be added. In extremely cold weather, heat pumps become ineffective but in concert with a geothermal system, they are able to extract heat from air already above 50 degrees, a range where they operate at maximum efficiency.
