Found this video and thought some of you might like it.

Winter temperatures in Alliance, Nebraska can drop to -20°F (the record low is -40°F/C), but retired mailman Russ Finch grows oranges in his backyard greenhouse without paying for heat. Instead, he draws on the earth's stable temperature (around 52 degrees in his region) to grow warm weather produce- citrus, figs, pomegranates - in the snow.

Great video, I have watched this a couple of times. Would like to know other people's thoughts on his comment about not needing bees to pollinate the citrus. Any witnesses?

The lack of recorded performance data is disappointing but he certainly wouldn't have healthy citrus if it's didn't perform as he says.

I'm not a fan of moving air through the earth, for energy efficiency and air quality (mold, bacteria) issues, but this setup does show that direct geothermal heating for winter greenhouses is certainly viable with the classic solar deep winter greenhouse design.

Unless you own the backhoe, and you have rockless soil down to 10 foot deep, I think a l'd have a well driller place a series of trench connected bores for 1 inch pex, ala the ground source heat systems. If your water table isn't deep, a dual well system for extraction and re-deposit is also a great way of using direct geothermal energy. The water could heat exposed water tanks, or in floor pex, or a water to air heat exchanger could be used, and the air movement of the latter isn't necessarily a waste of energy.

Citrus are self pollinating. Mine have flowers in the winter and do set fruit, and no bees in the gh.

Low Tech Magazine had longish article a few years back.