Here people store potted roses in their unheated garages all the time. But we normally never get below zero much and only to about 10 below. We did make it to about 17 below this year though! So I will find out if that is doable for my tree rose. The rest of my pots are outside under leaves and wrapped in burlap. I have no idea how they are going to do this winter because of the extreme cold. I've been doing pots like this for 6 years now with great results but this is by far the coldest winter they've had to endure. I'm hopeful but who knows!

Obviously, we are losing a lot of heat through the wall, but insulating walls isn't very high on our list of priorities (house was built in 1964)

I've kept rooted cuttings in the basement under lights with a fair amount of success. The basement is quite cold for living quarters, and that probably helps.

With respect to the garage temperature question, that will depend on a number of factors. Is the garage attached, or detached, does it have an attic, or insulation of any kind, etc. Mine is attached with 3 uninsulated walls, but an insulated appartment overhead. Outside temperatures have gone as low as -29 C / -20 F so far this season and the lowest tempeature indoors has been -6 C / 21 F.

If you have a detached uninsulated garage, then the ambient temperature inside will likely reach nearly the same temperature as the outside during extended cold spells. -20 F is almost certainly cold enough to kill a potted knock out rose. If you're garage is insulated the temperature swings should be less. You might try building an insulated box large enough to put you potted rose in, keep it in the garage and put a small 40 watt incadecent light bulb inside that you could turn on when the temperature is forecast to go below about -5 F.

Cheers, Rick