Some greenhouse farmers use geothermal energy, but this is far from a common energy source. It's been suggested that this might be our way to get off of natural gas, though. (After small earthquakes in the province of Groningen, the extraction of gas has been greatly reduced. Seemingly of less concern politically in NL, gas isn't sustainable.)

Wow, today I learned: http://www.thermogis.nl/general_info.html

Not as hot but definitely useful for greenhouse direct heating.