The best time to plant mums for winter hardiness is spring to early summer, but most mums are not available until fall.
Some things that to do to give mums the best chance to overwinter are:
1. Plant them as soon as possible in the ground – water and maintain normally
2. After the foliage has died from the cold, place mulch in amongst the stems to insulate the base. I usually can get a foot or so of mulch placed in the mum.
3. During spring bed cleanup, remove the mulch from the center and spread it around the mum. Then cut back the stems to an inch or so from the ground.
4. Fertilize while watering during the season.
5. Cut an inch or two off the new growth before July 4th for a bushier plant with more blooms in the fall.
Mums are usually ok once they have survived the first winter.