Keep track of the temperatures and make sure that they stay below 35/40 degrees F. If it does start to warm up, are there windows or doors that you're able to open to bring in outside air? Also, you can cover a greenhouse with white plastic, which can help keep temperatures down.
Each tree species has a dormancy range, a period of time during which it needs to stay dormant. Once it's dormancy requirements have been met, the tree will react to temperatures that are above 50F. Going into the fall with the shorter days and cold months, you shouldn't see any issues with the trees breaking dormancy, since they have just started their winter slumbers. Starting in Jan. / Feb. is when you need to make sure that the temps in the greenhouse stay below 40F since that is when certain species will have completed their necessary dormant period and will just be sitting and waiting for temperatures to raise to being growing.