There are many varieties of mums, not all particularly hardy. The plants sold in the fall as 'Hardy Mums' should have been sold to us in the spring, to be reliably hardy in the north. However we wouldn't have had the patience to plant them and wait. Mums and pumpkins are the flag bearers of fall. Try and get your potted mums in the ground ASAP. Keep them well watered and mulch once the ground freezes and you'll stand your best chance of having truly hardy mums.