Ceci, we also overwinter in our garage. We don't have the extreme cold you have or if we do, not for very long. Gorden in right about the concrete, tho. The cold concrete will affect the potted plants, so, I use this rigid waterproof foam insulation but several layers of cardboard, with a moisture barrier like plastic if you don't want a soggy mess come Spring, should do the trick for your brugs. Mine are pretty much in the dark all winter. I have had them bloom right after I bring them in in late Oct, early November------ a little extension of the season is welcome!
