Chrysanthemums, affectionately known as mums, are a popular and vibrant addition to gardens and seasonal displays. They offer a spectacular splash of color just as many other flowers begin to fade. A frequent source of confusion for new gardeners is the timing of their floral display. Understanding the natural cycle of the chrysanthemum is the first step toward successfully growing these beautiful flowers.
The True Bloom Season for Chrysanthemums
Mums are botanically classified as autumn-blooming plants, with their natural flowering cycle beginning in late summer and continuing through the fall season. The colorful displays seen at garden centers in September and October represent the plant’s true nature. Mums do not naturally bloom in the spring, which is their primary season for vegetative growth.
Chrysanthemums are categorized as either florist mums or garden mums. Florist mums are bred for an abundance of flowers and are typically treated as annuals, often lacking the root system needed to survive winter cold. Garden mums are hardy perennial varieties that can return year after year, provided they are established in the ground during the spring to develop a robust root structure.
The large, dome-like plants found in nurseries are often the result of commercial growers manipulating the bloom time to coincide with the autumn retail season. These growers manage the plants throughout the spring and summer to ensure they are at their peak for purchase. Their natural tendency is to wait for a specific environmental cue before setting buds.
Photoperiodism: The Light Trigger
The science behind the chrysanthemum’s timing is photoperiodism, the organism’s response to the relative length of day and night. Chrysanthemums are classified as “short-day plants,” initiating flower buds when the day length shortens and the night length increases. This signal occurs naturally as the summer transitions into fall.
The plant measures the length of the continuous, uninterrupted dark period, not the duration of daylight. Flower bud formation is triggered when the night lasts for a minimum of about ten hours, typically achieved around mid-August. If this continuous dark period is interrupted, even by a brief flash of light, the plant may not set buds or the blooming process may be delayed.
This critical dark period must be maintained for several weeks for the plant to transition from vegetative growth to reproductive growth. The plant utilizes a photoreceptor protein called phytochrome to sense these seasonal changes in light duration. Once the signal is received, it takes six to ten weeks for the buds to develop fully and open into the dense autumn flowers.
Cultivating Fall Blooms: Timing and Pruning
Achieving a dense, vibrant fall display requires intervention throughout the spring and summer to manage the plant’s growth. The most important cultural practice is “pinching,” which involves removing the growing tip of the stem to encourage lateral branching. This action prevents the plant from becoming tall and “leggy,” promoting the full, mounding shape associated with healthy mums.
The initial pinch should occur when the stems reach about six inches in height, typically in late spring or early summer. Gardeners should repeat this process every two to four weeks, removing the top one or two inches of new growth whenever the plant has grown three to five inches. Each pinched stem will branch into two, effectively doubling the potential number of fall flowers.
Pinching must cease by early to mid-July, with some gardeners using the Fourth of July as an easy-to-remember deadline. Stopping the pinch allows the plant enough time to develop and mature the buds that form in response to the shortening dark period. If pinching continues past this point, the plant will not have sufficient time to set flowers before the cold weather arrives.
Proper care through the summer also supports the eventual bloom. Mums require consistent moisture and well-drained soil, along with at least six hours of direct sunlight each day. Overwintering established garden mums is possible by cutting the plant back to a few inches after the first hard frost, then applying a thick layer of mulch once the ground freezes to insulate the roots.