I usually buy them in the fall and when it gets cold, I cut them back and put them in the basement for the winter, watering them occasionally. When Spring comes, I take them out, cut back the new growth and plant them in the ground after the danger of frost has past. I prune them mid-summer and then they flower again in the fall. This time, I cover them with leaves/mulch for the winter.
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Many new mums just require adequate rooting time before freezing water hurts their roots. I would plant them a bit deep and mulch around them with lots of water, until it freezes.
Mums aren't that hard to grow but it all depends on your area and the type of mum. I live in southern Virginia and can leave button mums in the ground over the winter, I just have to mulch around them. Know your growing zone for temperatures as well as your soil type then check the mums to see if they fall within your area. You can look up on the internet to see if your chosen mums will grow/winter through for you. If not then you do have to dig them up, pot them and store them like described above. Good luck and happy gardening.