If you keep the ground a shade warmer in deep freezes can you raise your zone a notch or two with geothermal in the location? Or does the top of the tree die no matter if the roots are kept warmer? I was thinking some of the warmer zone persimmons in zone 6.
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I’d guess it would depend on what you do with the geothermal. There is a million square foot greenhouse west of here that is only here because there was geothermal heat available.
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Lets say you wanted to gain a full zone or two of warmth without a greenhouse there are options. I put a hill of dirt on the north side of my pawpaws and planted them beside a pond for a slighter warmer lake effect off the water. Could i put a 40 foot hill on 4 sides with a drain pipe as an entrance? I think the answer is yes. @alan has mentioned planting in a hole many times. Here is a cheat sheet to zone stretching for pawpaw.
I would think " no" as to keeping the top from having weather issue.
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You definitely can improve the ground zone by a zone at least. “Back to Eden” approach is basically a foot deep mulch level which insulates the ground, keeping it cooler during summer and as beneficially warmer during the winter. This is my method in zone pushing successfully and don’t think I could have the success I do without it. Now above ground there are other methods that take much more effort. One way is a water feature, this can be a small fishpond with some aeration or large water barrels near plants. Planting next to structures, or chicken coops. I have even heard of people roosting their chickens in their greenhouses which work as a heater during winter as they put off a good amount of passive heat. Plenty of ways to do it, but I think it’s reasonably easy, if not a bit of hard work to set up, it makes upkeep rather minimal