I live in an entirely different zone than you but my Mom grows a clematis in a pot and she leaves it out all winter and it comes back every year. I think to be safe in your zone I would bring it into the garage. But, I don't of course know that from experience and I could be totally wrong.

Carol

I've done both methods (unheated garage and packed in leaves in protected spot) and they both work pretty well. I've lost a couple here and there over the years with either method, but I would guess at least a 90% success rate overall.

I too have not had any problems growing clematis in pots and leaving them outside. Once again, growing zones do not give enough info to use that as the ultimate decider of whether clematis will survive outside in pots since the zones only give you average highs and lows. What would be more important is the overall size of the container you are growing them in, the exposure the pot is in (direct sun or sitting in partial to full shade all day during the coldest days) and how long on average the plants would be exposed to frozen conditions in your pots.

I agree there are lots of variables in play too besides general temps. One thing I often do in the fall is to check if the plant is rootbound. If so, I pot it up to a bit larger size (not too large which would hold too much moisture). The extra soil is good insulation for the roots.