Chrysanthemums, commonly known as mums, are signature flowers of the fall season, celebrated for their vibrant, late-season color. Many gardeners purchase these plants without a clear understanding of their perennial nature and seasonal decline. The plant’s “die-off” in late autumn is not a sign of death, but a natural transition into a dormant state that allows it to survive the winter and return the following spring. Understanding this biological process is the first step in successfully overwintering these hardy garden staples.
The Biological Triggers of Dieback
The chrysanthemum’s cycle is governed by photoperiodism, the plant’s response to the changing length of daylight and darkness. Mums are classified as “short-day” plants, meaning they require extended darkness, typically 12 or more hours, to initiate flower bud formation. As the days shorten after the summer solstice, this extended night period triggers the shift from vegetative growth to blooming.
Once the flowering cycle is complete, the plant’s energy is redirected away from the blooms. Decreasing temperatures signal the end of the growing season. The combination of shorter days and colder weather causes the above-ground foliage to stop photosynthesizing, leading to the visual decline of the stems and leaves as the plant stores energy in its crown and root system.
Seasonal Timing and Visual Cues
The time mums die back depends on local climate, but the process generally begins in late autumn and is accelerated by the first hard frost. A hard frost, where temperatures drop below 28°F (-2°C) for several hours, is the definitive cue. This severe cold causes the water inside the plant’s cells to freeze, leading to cell wall rupture and the rapid collapse of the stems and leaves.
The visual signs of true dormancy include vibrant green foliage turning shriveled, dark brown, or black. The flowers fade, dry out, and lose their color. This complete dieback confirms that the plant has successfully moved its energy reserves to the protected crown beneath the soil surface, signaling gardeners to begin post-season care.
Overwintering and Post-Season Care
Care after the plant has died back is essential for its survival through the winter months, especially for perennial garden mums. The initial step is to cut back the dead stems to a height of two to four inches above the soil line. This pruning removes decaying material that could harbor pests and diseases while leaving enough stem to mark the plant’s location.
After cutting back, apply a thick layer of protective mulch, such as straw, shredded leaves, or pine needles, three to four inches deep around the plant’s base. This insulating layer helps regulate soil temperature, preventing the harmful cycle of freezing and thawing that causes “frost heaving.” The mulch should be placed around the perimeter of the crown, not directly over the remaining stem stubs.
Even in winter, the dormant crown requires some moisture, so the soil should be kept slightly damp until the ground freezes solid. Once the soil has frozen, cease watering until early spring. Container-grown mums should be moved to an unheated, protected area, such as a cool garage or shed, to prevent the entire root ball from freezing solid and cracking the container.
How to Determine Dormancy Versus Death
The appearance of a dead, brown top can lead a gardener to believe the plant has perished. To determine if a seemingly dead mum is merely dormant, check the viability of the plant’s crown and roots. In late winter or early spring, a simple “scratch test” can be performed on the remaining stem stubs.
Using a fingernail or a small knife, gently scrape away the outermost layer of bark near the soil line. If the tissue immediately beneath is bright green, moist, and pliable, the stem is alive and the plant is dormant. If the underlying tissue is brown, dry, or brittle, that part of the stem is dead. Continue checking lower down the stem until green tissue is found or the crown is reached.
If the stems are entirely brown, gently probe the base of the plant to check the crown. A healthy, dormant crown will be firm to the touch, often showing small, light-colored foot buds forming around the base. A dead plant’s crown and roots will feel mushy, dark, and potentially smell of decay, indicating irreversible tissue damage from rot or extreme cold.