Many of those hardy Mums are, in fact, perennial. It really depends on when they get planted and where. Some would call them a 'tender perennial' though the patch I once had, which originated from a typical pot of them sold here ever late summer/early fall, could not be killed till I ripped out every little piece of root. Most of the ones planted in Fall don't have enough in ground time to survive the winter...but plant those same things earlier in the year and you may be surprised. Personally, I won't plant them in ground again...they became a bit of a bully in my garden and got into spaces I did not want them in.
