How to Winterize Mums

Commonly called mums, garden chrysanthemums (Dendranthema morifolium) erupt into bloom in early autumn, adding a splash of much-needed color to otherwise drab late-season beds. Although typically grown as annuals, mums will grow perennially within U.S. Department of Agriculture plant hardiness zones 5 to 9 if nursed through the winter months. The winterizing needs of mums varies according to the local climate, but they will survive outdoors in most areas with a little foresight and extra attention.

Mums require weekly applications of fertilizer during the blooming season to sustain their abundant blooming. However, regular fertilizer applications also help produce a proliferation of young, succulent leaves that may not fully harden before the first killing frost. It is best to decrease feeding as the blooming season progresses, reducing the fertilizer amount by half every two weeks toward the end of summer. All feeding should be suspended at least one month before the first anticipated frost so the newer leaves have time to acclimate to colder temperatures before the first hard frost strikes.

Mulching serves two purposes when overwintering mums: it regulates the soil temperature around the roots and helps hold in much-needed moisture. However, mulch can also cause serious problems if the wrong type is used. Heavy, easily compacted mulches such as dead leaves should be avoided because they prevent excess moisture from escaping the soil, which can cause rot. Lofty, lightweight mulch such as pine boughs, straw or shredded bark are best because they create a permeable and more insulating layer of protection. The mulch should be added just after the first light frost and left on until after the last frost in spring.

Pruning needs vary according to the local climate. Mums grown in colder northern climates perform best and will survive the cold more easily if they are not pruned. However, mums in milder climates and coastal areas will look tidier in the winter and more lush in the spring if pruned back after the first frost. Prune back the entire plant by half using very sharp pruning shears, and be sure to wipe them clean with rubbing alcohol before use to prevent the spread of pathogens. Also, rake up and discard the pruned stems to keep insect pests at bay.

Once spring rolls around, mums may need a little extra help to regain their vigor. The first step to welcoming spring is to remove the old, degraded mulch layer from around the plant roughly two weeks after the last frost. Water the plants to a 3-inch depth, then apply granulated 5-10-5 ratio fertilizer to the moistened soil at a rate of 2 to 3 pounds per 100 square feet of area. Any dead or damaged growth should be pruned off once new growth emerges. Older mums may need to be divided in spring every three to four years to improve their blooming; simply lift the old plant, cut it into equal sections and discard the old, dead center. After replanting the divisions, water deeply and apply fertilizer as you normally would.