I have some bulbs that maybe shouldn't make it through a 5a winter, so I put them on the south side near the granite foundation. On super cold days, the shining sun will warm the granite and there will be geothermal and whatever heat comes off the furnace from the other side of the granite block. I suppose that will not work in cold climates so far north you don't get much sun. Perhaps shoveling snow over the bulbs would insulate them from the air too.
