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.
