Chrysanthemums, commonly known as mums, are a signature plant of the autumn season, delivering a burst of color when most other flowers have faded. They are seasonal bloomers that time their spectacular display for late summer and fall. Mums are naturally programmed to flower in response to the changing seasons, specifically the decreasing length of daylight hours, offering a reliable show of color before winter arrives.
How Light Dictates Flowering
The precise timing of a chrysanthemum’s bloom is governed by photoperiodism. Mums are classified as “short-day plants,” meaning they require a sustained period of uninterrupted darkness to initiate the shift from vegetative to reproductive growth.
The plant perceives the changing seasons through a pigment called phytochrome, which acts as an internal clock. When the hours of darkness exceed a specific, or “critical,” dark period, a hormonal signal called florigen is produced in the leaves. This signal travels to the plant’s tips, triggering the formation of flower buds instead of new leaves and stems.
During the long days of spring and early summer, the short nights prevent this process, keeping the plant focused on growing large, bushy foliage. As the summer solstice passes and the nights lengthen, the critical dark period is finally met, which sets the entire flowering process in motion. This explains why mums naturally begin blooming in late August or September, continuing their display until a hard frost ends the season.
Maximizing the Current Bloom Period
Gardeners can manipulate the plant’s growth habit to ensure a dense, flower-laden display when the natural bloom time arrives. Pinching is used throughout the spring and early summer to encourage a fuller, more compact plant structure. It involves removing the top few inches of new stem growth, which forces the plant to create two new side branches in place of the single stem.
This process should be repeated every few weeks to promote lateral branching and increase the number of potential flower sites. The final pinch must occur by early to mid-July. Stopping the pinching process at this time allows the plant sufficient weeks to develop the necessary flower buds before the long nights trigger the blooming cycle.
Once the flowers appear, deadheading helps prolong the display. Deadheading is the removal of spent, faded blooms, which prevents the plant from diverting energy into seed production. The plant is encouraged to put its resources toward opening secondary buds and maintaining a fresh appearance. This simple act can extend the aesthetic appeal of the fall bloom by several weeks, maximizing the limited flowering window.
Ensuring Next Season’s Return
There are two main types of chrysanthemums: hardy “garden mums” and tender “florist mums.” Hardy garden mums are perennial in USDA Hardiness Zones 5 to 9 and are bred to survive winter when properly established. Conversely, florist mums, often sold in pots as annuals, are not bred for cold tolerance and are less likely to survive freezing temperatures.
To prepare a hardy garden mum for dormancy, leave the dead foliage and stems intact after the first frost, rather than cutting them back immediately. This remaining foliage helps to insulate the plant crown and trap protective winter snow. Once the ground has frozen, applying a thick layer of mulch around the plant base helps prevent the damaging cycle of freezing and thawing that can push the plant out of the soil.
If planting a new mum in the fall, it should be established in the ground at least six weeks before the first hard frost to allow root development. For the most reliable return, hardy mums are ideally planted in the spring. This gives the plant an entire growing season to establish a robust, deep root system.