They're supposed to be hardy to zone 7 or 8, so it's a bit of a stretch here in zone 6, and there's always a chance of getting a colder than usual winter that will do them in, but if they're well mulched and in a protected area, they usually don't have a problem. I know of some folks in zone 5 who successfully overwinter Callas in the ground, close to the foundation of the house.
