If this interests you, I recommend checking out this video of Russ Finch, a farmer who grows citruses (orange, lemons, etc), figs, pomegranates in his sunken greenhouse in the middle of Nebraska in the snow/20F, and uses geothermal heating (long tubes buried underground) to heat it (and his house) during the winter months, and cool in the summer months. Really interesting story/concept and I'm hoping to replicate on a small scale when I renovate my back yard in the coming years.