Answer: Sounds like you did just fine with your mums last winter. You can repeat the process again this winter. Once the tops die down you can cut everything off at soil level. Water the containers once and they should remain reasonably moist all winter long. Since the roots are dormant they won't need water, but you will want to make sure the soil doesn't become bone dry during the winter. So, just check it once a month or so. If it is completely dry you can add just a little bit of water to help it rehydrate just a little.
You won't need to repot your mums for another year or two. Mums can be divided every 2-3 years and at that time you can pot them up in fresh potting soil. After taking them back out in the spring, plan on periodically shearing the plant lightly or pinching back the growing tips to encourage branching. Stop doing this in mid July to allow time for the flower buds to form and bloom before the cold weather sets in.
Hope this information is helpful.
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