This provides them with several months to develop a good root system, which is crucial to their survival during the winter. Most mail-order sources ship in the spring, but few nurseries stock them then. In the North, mums need full sun. In the South, they need at least a half-day of sun. And everywhere, mums need well-drained soil.
More mums die from root rot than from freezing temperatures.
After choosing a well-drained location, dig a planting hole roughly twice as wide as the plant's rootball, and blend in a generous dose of compost. Then place the plant in the hole one inch deeper than it was growing in the nursery pot, gently spreading the roots out as you go. Trim the tops back to remove dead blossoms. Having done that, your plants should be fine through winter. When spring rolls around, you can dig and divide your mums, something you should do at least every other year.