Answer: In general, hardy means it will withstand freezing temperatures. You would need to know which hardiness zone it was expected to be hardy through in order to judge whether or not to hope it to overwinter for you. (Your zip code places you in USDA winter hardiness zone 5. This means you would need to have plants rated hardy to zone 5 or a lower number.)
Mums are special in that they really need a well drained location or they will not survive the winter. If planted in a low spot or in heavy clay soil, they often don't make it even if rated hardy for your given zone.
Also, mums planted in the spring have a longer time to become established and rooted and thus tend to survive better than mums planted in the fall. I'm sorry you lost so many but I hope this helps you trouble shoot.
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