I over winter in a unheated unattached garage. I live in central Illinois and can get brutally cold winters. I have mainly coniferous material but a few deciduous and broad leaf evergreens. I mulch around pots in Rubbermaid containers and put on benches to keep off concrete floor. I know I’ve heard Ryan answer this question but I can’t remember if it helps at all to put LED grow lights on them to help with the lack of light. It stays cold enough through the winter I’m not worried about them getting to warm and moving too much, so do you all think I would benefit from lights?
For your broadleaf evergreens it would definitely help, they still photosynthesise during the winter, albeit a bit slower. Not necessary for your deciduous until the days lengthen.
You might be better off on the concrete floor than of shelves. The floor temperature is apt to fluctuate less than the air temperature on a shelf. It might also be warmer on the cold nights due to the ground contact (ground down below the frost line (1-2 m / 3-6 ft) in most temperate areas is typically around (14C / 57F).