Depends what you want to grow. I heat mine with a wood boiler. How is your water table?50- 55 is just a slightly warm source not really a heater. Mr lemons has a very special system it’s not just a hole in the ground.6b 4 feet in the hillside.
Heated with a woodstove made from 55 gallon, metal drums, wood fed from the outside, zone 7b. I don’t need to build fires unless overnight temps get close to 20°F.