Our frost line can get down below 3 feet, depending on the winter. We have Styrofoam 4 inches thick that does down 6 feet all the way around our rock heat sink. That Styrofoam is inside of the poles that hold the arches upright. This is keeping the cold soil from outside the greenhouse from cooling off our soil inside the greenhouse. It helps to isolate the heat sink and keep it warm.
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That's a really good idea!