During the spring to autumn growing season, branches of chrysanthemums, or mums (Chrysanthemum spp.), gradually evolve into their flowering phase. Pruning the plants properly encourages full foliage and prevents them from becoming tall. Mums are also pruned to encourage the growth of large blossoms or sprays, each of which consists of a large blossom surrounded by smaller flowers.
Garden mums (Chrysanthemum x morifolium and Dendranthema grandiflorum) are perennial, short-day flowering plants that are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9. In the coldest parts of their growing range, sometimes they are grown as annuals. The term "short-day flowering plants" means the shorter daylight hours of late summer trigger their blossoming. Their pruning requirements are tailored to fit that growing cycle. Mums naturally grow 4 inches to 5 feet high, depending on the cultivar.
General pruning of mums begins with using your thumb and forefinger to remove the tops of new shoots that are 3 to 4 inches long, leaving two or three leaves on each shoot. That task needs be done roughly one to four times each month, but don't prune the shoots after July 15 or Aug. 1 at the latest. If you prune them after that time, you may eliminate some flower buds and delay the blooming of those that remain. Delaying blooming in climates with cold winters can take mums past the first autumn frost without yielding flowers.
Old mum stems and leaves may look untidy when the plants are dormant in winter. You can prune to remove those stems and leaves if you wish, but leaving them in place helps to protect the plants from cold and to conserve moisture. Pruning them in fall can cause crown rot from rain sliding down exposed stems. Purdue Extension recommends removing dead chrysanthemum tops in early spring. Pruning shears are handy for that chore. Before using them, sterilize their cutting blades by soaking them for five minutes in a 70 percent solution of isopropyl alcohol, available at many garden supply centers and most pharmacies, and let the blades air-dry for five minutes.
For Plant Shape
Pinching the tips of new shoots on chrysanthemum cultivars commonly found in home gardens encourages full, bushy growth in the plants. Shoots that are not pruned develop straplike leaves and the plants grow tall and spindly. Flowers growing on spindly mums tend to make the plants top-heavy, often causing them to topple from the weight. Also, tall mums block sunlight from their lower parts, causing fewer leaves and dead leaves.
For Flower Growth
If you don't prune chrysanthemum shoot tips, the end of each shoot will produce a flower bud. Pruning shoots tips encourages side shoots to grow below the top bud. Each side shoot yields clusters of flower buds. If you want your mums to develop flower sprays, then let the flower bud clusters develop. If you want your mums to have large blossoms, then pinch off all flower buds except the top flower bud of each side shoot. Encouraging mums to grow more flowers, extending their bloom season, requires removing their old, spent flowers, a practice called deadheading. Deadhead by using your thumb and forefinger to pinch off a stem roughly 5 inches below its spent flower. Using that technique prevents the plants from displaying a thicket of stems that resemble sticks.