My experience is that the greenhouse will fall to the same temperature as outdoors unless the greenhouse is covered in snow. When not covered it will warm on sunny days, drop like a rock at night, and freeze like a brick, as you say, due to no snow protection inside. The warming and cooling off in a greenhouse is not favorable. Constant cold is better.

They'd be better off in a garage at 20-30F. If not that dig a trench outdoors and bury the pots for winter. Cover the top with leaves, hay, or anything that will insulate the roots.

Thanks. We have so many small critters I'm not comfortable covering them at all. I tried it with grapes and they got devoured. They were young and would have got cut back anyway. Good info. Thanks again