Chrysanthemums, often called “mums,” are a signature flower of the autumn landscape, filling gardens with vibrant color when most other plants are fading. Their sudden appearance in garden centers each fall prompts questions about their bloom timing and what controls it. The secret to their seasonal display lies in their biological response to the changing length of the day and night. Understanding this mechanism allows gardeners and commercial growers to manipulate the exact moment these flowers will open.
The Standard Blooming Window
The natural blooming period for chrysanthemums spans from late summer through the first hard frost. Their peak display occurs between September and November in many temperate climates. The exact timing is highly dependent on the specific variety planted and local climate conditions.
Horticulturalists classify varieties into three groups: early, mid-season, and late bloomers. Early-blooming mums show color in late July or August, while mid-season types are the traditional September flowers seen at the start of fall. Late-blooming varieties offer a final burst of color, often flowering from mid-October into November, sometimes surviving initial light frosts. Selecting a mix of these classifications allows a gardener to maintain continuous color throughout the autumn season.
How Light Triggers Flowering
A chrysanthemum’s bloom time is precise because it is classified as an obligate short-day plant. This means the plant requires a period of continuous darkness, longer than a specific duration known as the critical night length, to initiate flower bud formation. In the northern hemisphere, this critical dark period is naturally met as the days shorten after the summer solstice.
The plant perceives this change using specialized light-sensitive proteins called phytochromes, which act as a biological timer. When the continuous dark period exceeds a threshold, often 12 to 13 hours, the plant’s internal chemistry signals the development of flower buds. If this required dark period is interrupted by even a brief flash of light, the phytochrome is reset, and flowering is inhibited, keeping the plant in a vegetative state.
Practical Steps for Controlling Bloom Time
Gardeners can influence when their mums bloom through “pinching,” a process that encourages a fuller plant and slightly delays flowering. Pinching involves removing the top inch or two of new growth from the stem. This technique removes the plant’s main growing tip, overriding apical dominance, which normally encourages upward growth.
Removing the tip redirects the plant’s energy to produce lateral branches, resulting in a bushier structure with more potential flower sites. This pruning should begin when the plant is six to eight inches tall in late spring and be repeated every four to six weeks. Gardeners must stop pinching by mid-summer, typically around the Fourth of July, to allow flower buds to form without interruption. Pinching too late removes newly forming buds, which can significantly delay or eliminate the fall bloom.
Commercial growers often employ artificial light control to precisely time their blooms. Because flowering is inhibited by a break in the dark cycle, growers use low-intensity lighting during the middle of the night to simulate a long-day period. This “night-break lighting,” applied for a few hours, prevents the formation of flower buds, keeping the plant vegetative. Conversely, growers can use opaque black cloth to cover plants in the late afternoon, artificially creating the required long night period earlier to force an earlier bloom.