I see that you are in zone 6b and I am in supposedly zone 4 but it is actually 3 most of the time. What about are the temps in your garage and shed? Here the outside temps can drop to -20's and once a few years back it went to -35.I am experimenting with overwintering a K.O rose but in the house.I was toying with the idea of putting it in the shed and doing the snow as h20 thing and let it go dormant.My garage has the capability of being heated but not about to fill that oil tank-rather spend it on the house heating.
What temps would you feel safe keeping a potted rose at
in a garage or shed?

I don't really have any experience with anything that cold. Hopefully someone who is in a colder zone than I am will chime in with some answers for you. Part of the protection the garage or shed offers is keeping them out of the very cold drying winds. Dehydration is a real problem and can kill them just as easily as the cold can.

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!

For some perspective, I've never had any trouble in my garage with overwintering roses. However, the min/max thermometer in there has never gone below 20, even when it was -30 outside. Low 20's is about as cold as I am comfortable with, but a dormant rose may be able to handle a bit more.

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.

Thank you, Seil for the reminder. You are the best! I think you just might have saved some plants from dehydration. Thanks again.

Thank you for the timely reminder, I went out and gave mine all a good watering yesterday. They were definitely ready for it.

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