The temperature six to eight feet below the surface stays between 50 and 60F. Do you know your water table? An underground greenhouse should be set at least 5 feet above the water table. The inside can be cased in stone, mud brick or any dense natural material able to absorb large amounts of heat. I don't know if there is any ratio of sunlight to ground, but you are still going to need supplemental heat to grow orange trees if you need temps to be above 50 - 60 F. Granted the greenhouse will probably be a lot hotter during the day when the sun is shining, but on overcast days and at night, they will drop back down. A fossil fuel or solar heat storage system like the one I built for my greenhouse will be needed for these times. Let us know how you proceed. Nice to have you with us in the Greenhouse Forum!
