I cannot advise anything because my experience is so different from all what I read about overwintering peonies in pots. After a big disaster last winter I come back to my original method - no freezing at all, just cold room with temperatures slightly above freezing. Once I see pips which are about to open (sometime in March) I place the peonies in a sunny place indoors. Not ideal since they get confused when I place them outside in May where the temperatures are lower than indoors but at least peonies can survive and even grow.
In much warmer climate like zone 6a people can have very different method.
