Mums are ubiquitous in fall gardens. However for mums to be truly hardy, they are best planted in the spring. The mums for sale in fall nurseries have been coddled to set buds for September blooms and are putting an awful lot of energy into blooming, not growing roots. Planting these out in the garden in late summer / early fall doesn't guarantee sufficient time for the plants to become established. This is not a problem in warmer climates, where a bit of deadheading will satisfy most mums after bloom, but in areas with sub-zero winters, freezing and thawing of the soil will heave the plant out of the ground and kill the roots.
For a better chance of survival of fall planted mums in cold areas, leave the foliage on until spring and either mulch the plants heavily or dig, pot and move the plants to a more protected spot in the garden for the winter. If you choose to move the plants, do so before the first hard freeze. Established plants shouldn't be fed after July, so new growth isn't injured by frost.
Spring planted mums will have plenty of time for root growth. Many gardeners are surprised that their garden mums start to bloom in mid- to late summer. If you want fall flowers on your mums, you'll need to pinch the plants back periodically throughout the summer. Start when the plants are about 4-5" tall and repeat every 2-3 weeks until about mid-July. This will cause the plant to get stocky and bushier and by late summer, it should be covered with flower buds.
Of course, you can always grow mums as annuals. They do provide wonderful fall color and work great at filling in empty spots where summer bloomers have faded. Look for plants with lots of unopened buds, to have blooms well into the fall season.