How to prune perennial mums depends on whether they are new or established plants and whether you purchased and planted them in the spring or in the fall.
Spring-planted new mums or already established chrysanthemum plants should be cut back after the first hard freeze in the fall or winter. Use sharp scissors or shears to trim them back to 2 to 4 inches above ground level and then add 4 to 8 inches of mulch, depending upon the severity of your winters, to prevent the plants from heaving out of the ground and the roots from freezing.
If, throughout the winter, your nighttime temperatures stay above 40°F, you probably won't need to mulch, but you may have to stay weather-aware and cover them during any extremely cold, unexpected weather events. If you've mulched, you can remove or pull back the mulch once the nighttime temperatures are consistently above 40°F in early spring.
Newly planted, fall-blooming chrysanthemum plants have not had the time to develop a strong, well-established root system, as they've been devoting most of their energy to blooming. You should NOT cut back any new fall-planted mum plants in the first season. Wait until the following spring when the temperatures start to warm to cut back the foliage. Until then, your plants should either be heavily mulched, or potted and moved to a protected spot, in all but the most southern areas of the U.S. If you opt to pot and move them, do so before the first hard freeze.
If you'd like to prevent your chrysanthemums from blooming in mid- to late summer, preferring to have fall blossoms instead, pinch the new growth back periodically throughout the summer once your plants reach 4 to 5 inches tall, repeating every few weeks until about the middle of July. This will encourage bushiness, as well as a mass of flower buds ready to bloom in September and October. Then, clip the copious amounts of blossoms to make bouquets and live arrangements, which will also encourage extended fall blooming.
Happy Gardening from Garden Harvest Supply