Chrysanthemums (Chrysanthemum morifolium or Dendranthema x grandiflorum) naturally bloom in late summer or autumn, but that isn't the best time to plant them in cold areas if you want them to survive winter. Although they can be hardy in U.S. Department of Agriculture plant hardiness zones varying from 4 through 10, mums set out in autumn won’t have enough time to root adequately in their new location before freezing weather arrives. If you live where winter temperatures fall into the single digits or below, plant them in late spring instead.
Prepare Mums for Winter
In climates where freezing temperatures are uncommon, prune the mums' stems down to 8 inches once they have finished blooming, and they should require little other winter care. To help preserve hardy mums in more northern areas, plant them in well-drained soil in a protected location, such as near a building’s foundation. Refrain from fertilizing them after July ends so they won’t form tender new growth late in the year, and don’t cut back their frost-killed foliage in autumn. Instead, leave it on the plants over winter to help protect their crowns, which is where roots and stems join.
Protect Mums During Winter
After the surface of the ground freezes in late autumn or early winter, mulch the mums with 4 to 6 inches of a non-matting organic material such as straw, pine needles or evergreen boughs. This measure is not intended to provide warmth for the plants, but rather to keep them cold because intermittent freezing and thawing can uproot them. Remove the mulch and trim off the dead stems after the plants begin forming new growth in spring.
Take Tender Mums Indoors
If you have mums that aren’t hardy or that you didn’t get around to planting soon enough, then overwinter them under cover. Either plant them in an outdoor cold frame in late autumn or put them in pots so that you can take them indoors.
After frost has killed their top growth, cut the stems of potted plants back to 1 inch in height, and place their pots in a dark place -- such as an unheated garage, basement or closet -- where temperatures remain above 32 but below 50 degrees Fahrenheit. Water the soil in the pots about once each month, or as often as necessary to keep it barely damp.
In early spring, expose the plants gradually to sunlight again on a bright windowsill. Resume their regular watering and fertilizing, and they should leaf anew. To be on the safe side, wait until after the last hard frost to move them outdoors.