Chrysanthemums, commonly known as mums, are perennial flowering plants celebrated for their vibrant blooms that arrive when most other garden flowers fade. They belong to the Asteraceae family and are a signature flower of the autumn season. Mums naturally begin blooming in late summer and continue until the first hard frost, typically lasting four to eight weeks. Their seasonal timing is not primarily dictated by temperature but by a precise biological response to the changing length of day and night.
The Role of Light in Flowering
The internal clock that governs flowering in chrysanthemums is photoperiodism, a process that senses the relative duration of light and darkness. Mums are classified as “short-day plants,” meaning they require a period of darkness longer than a certain threshold to initiate flower buds. For many cultivars, this floral induction begins when the daylight hours drop below 12 hours, which naturally occurs as summer transitions into autumn.
This response is regulated by a light-sensitive pigment called phytochrome, which exists in two forms: a red-light absorbing form (Pr) and a far-red light absorbing form (Pfr). During the day, sunlight converts Pr to Pfr; however, it is the continuous, uninterrupted period of darkness that slowly converts the Pfr form back to Pr. When the uninterrupted night is long enough, the Pfr levels drop sufficiently to signal the plant to stop vegetative growth and start developing flowers. An artificial interruption of the dark period by even a brief flash of light can reverse this chemical conversion, effectively convincing the plant that the night was short, thereby suppressing or delaying flowering.
Types of Mums and Their Bloom Windows
The wide variety of chrysanthemums available allows gardeners to choose plants for a continuous bloom from late summer through late fall. Varieties are categorized based on their inherent bloom timing. Early-season mums are the first to flower, often beginning their show in late July or early August, while mid-season varieties typically reach their peak bloom around September.
Late-season mums are the most cold-tolerant, often blooming from October and sometimes persisting into November, even surviving light frosts. This varietal difference allows growers to manipulate the bloom time for commercial purposes. Hardy garden mums are perennial and typically bloom naturally in the fall garden. In contrast, “florist mums” are greenhouse-grown plants often forced to bloom outside of the typical fall season. Selecting a mix of these early, mid, and late cultivars is the most effective way to guarantee a long, colorful display in the garden.
Extending and Managing the Bloom Period
Gardeners can influence the appearance and duration of mums through specific maintenance practices. The technique known as “pinching” is used in spring and early summer to manage the plant’s shape and delay flowering. This involves removing the top inch of new growth on the stems, which encourages the plant to branch out and become denser, yielding a greater number of flowers later in the season. Pinching should generally cease by mid-July or early August to allow enough time for the flower buds to form before the autumn bloom.
Once the plant begins to flower, the practice of “deadheading” can significantly extend the overall bloom duration. Deadheading involves removing spent blossoms, which redirects the plant’s energy away from seed production and into creating new flowers. Removing these spent blooms regularly, perhaps once or twice a week, keeps the plant looking fresh and encourages secondary flower buds to open. Consistent watering is important during the bloom period; mums are thirsty plants, and dry soil causes flowers to wilt prematurely. Applying a phosphorus-rich fertilizer during the blooming phase can further support the production and longevity of the flowers.