Chrysanthemums, often called “mums,” are popular perennial flowering plants that provide color long after summer blooms have faded. Mums are genetically designed to expand beyond their initial planting spot, which is how the plant naturally multiplies and sustains itself year after year. The degree of spread varies significantly, depending on the specific cultivar and its underlying root structure. As perennial plants, mums rely on their root system to survive the winter and produce new growth each spring.
Mechanisms of Horizontal Expansion
The physical process of a chrysanthemum plant expanding its footprint relies heavily on specialized subterranean structures. The primary mechanism involves the production of modified underground stems, which are a form of asexual reproduction. These horizontal stems grow outward from the main plant mass, effectively colonizing the surrounding soil.
New shoots, which are genetically identical clones of the parent plant, emerge from nodes along these underground stems. This growth pattern allows the plant to form dense patches over time, spreading outward in a continuous, connected mat. Hardy garden varieties, specifically bred for perennial survival, are particularly adept at this type of expansion.
Another fundamental way chrysanthemums spread is through natural crown division. As the plant matures, the central root mass, or crown, can become dense and woody, with the most vigorous growth occurring around the perimeter. The oldest, central part of the crown may eventually die out, leaving a ring of healthy, separate plant sections. This natural splitting results in the formation of multiple independent plants from the original single specimen.
Variation in Growth Habits
The intensity of a chrysanthemum’s spread is directly tied to its specific growth habit, which is determined by its cultivar. Garden mums are broadly categorized into two types: clumping and running varieties. Clumping varieties maintain a more compact shape, expanding slowly, primarily through the gradual division of the crown.
Running varieties are aggressive spreaders, producing numerous, fast-growing underground stems that can rapidly colonize a large area. These types require more spacing when initially planted to accommodate their expansion. The more cold-hardy “garden” or “landscape” mums typically have the strongest spreading potential, ensuring their return season after season.
“Florist” or greenhouse-grown mums are often non-hardy and have been bred for large, showy blooms rather than aggressive survival. These varieties produce few, if any, of the spreading underground stems necessary for perennial persistence. Consequently, they are less likely to survive winter conditions and do not exhibit the horizontal expansion seen in true garden-hardy types.
Strategies for Controlling Spread
Managing the horizontal expansion of chrysanthemums is necessary to maintain plant health and vigor. The most effective method for controlling spread is periodic division, which mimics the plant’s natural life cycle. Mums should be dug up and divided every two to five years in the early spring as new growth emerges.
During division, the woody, less productive center of the plant is discarded, and the vigorous new shoots and root sections from the outer edges are replanted. This technique controls the size of the clump and rejuvenates the plant, promoting better flowering and growth. When planting aggressive runners, physical barriers such as deep-set plastic or metal edging can be used to contain the underground stems.
Above-ground pruning, or “pinching,” plays a role in managing overall plant energy and shape, indirectly affecting spread. Removing the growing tips of the stems encourages the plant to branch out, resulting in a bushier, more compact dome of foliage. While this technique does not directly stop underground growth, it directs energy toward lateral shoot development rather than height, creating a denser plant that is easier to manage.