From what I remember others saying on this subject it depends on the kind of winter cold: wet with lots of snow that stays around; dry windy cold; wet then dry then wet again, etc.; heaving can be a problem if you get wet then frozen.
Insulating the pots with mulch or something is a good idea. I have also heard of people planting in pots, then "planting" the pot in a hole that you can remove later.
Good luck. Also be aware that just because it worked last winter you cannot always count on it working the next. I had many plants in pots that had overwintered for several years in a row. Last winter was very cold for our zone 7b in Huntsville and I lost over 1/2 that were in pots and even some that had been in the ground for several years (not newly planted ones.)
