I have moved tender or not-ready-for-planting-yet plants into my south facing garage for years. In this climate it seldom will go below freezing out there because it is attached to the house, however if necessary I run an electric oil radiator on the coldest nights. (I did a bit of weather sealing too, and I don't put my car in there so I never open the door between Thanksgiving and late March)

The typical shade evergreens like rhododendrons will survive fine in there with the big south and west facing windows...however a couple years ago I experimented with running an old hydrofarm 250W MH light I had from back in my apartment days during the winter. It wasn't a huge difference but I could tell the plants were growing rather than just surviving...growing roots probably. They seemed to pop into growth faster in the spring.

What I have realized is I only have 1/2 the garage covered in potted BLE liners, one car-unit. I'm thinking a lot of the light is getting wasted hitting the other side of the garage. What's the best option for sealing that off? Does someone make a reflector material just for this purpose that isn't super expensive? I'm thinking I might just buy some of the super-cheap thin plastic tarp material at HD, used for floor covering during painting etc., and spray paint it white or silver-metallic?