Chrysanthemums, commonly known as mums, signal the transition into the autumn season with their vibrant, late-blooming colors. These popular ornamentals provide a welcome burst of color when much of the garden has faded. Exactly how much cold can these flowers withstand before succumbing to winter temperatures? Understanding the specific temperature thresholds and the differences between mum varieties allows for practical care decisions that can extend their beauty and ensure their return next year.
Defining Cold Tolerance: Temperature Thresholds
The immediate tolerance of a chrysanthemum’s blooms and foliage differs significantly from the survival threshold of its root system. A light frost, typically occurring when temperatures dip between 30°F and 32°F, will often cause minor damage to the outer petals and leaves, but the plant can generally cope and continue flowering for a period after the temperature rises again.
However, the plant is at risk once a hard frost sets in, defined as temperatures consistently falling below 28°F. At this point, the entire above-ground structure—flowers, foliage, and stems—may turn black and die back quickly. For established garden mums planted directly in the ground, the insulated soil offers protection, allowing the root crown to survive temperatures that may temporarily fall to around 20°F.
The true survival of the plant hinges on the root system’s ability to avoid sustained deep freezing. While the tops may die back, the roots of hardy varieties can withstand conditions approaching -20°F if they are well-established and properly protected. Factors such as the duration of the cold exposure and the plant’s hydration level before the freeze influence the actual extent of the damage.
Garden Mums Versus Florist Mums
The cold tolerance of a chrysanthemum depends entirely on the variety, which typically falls into one of two major categories. Garden mums, often labeled as “hardy mums,” are specifically bred to survive winter and return year after year. These perennial varieties are generally hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9, featuring a robust root system that develops underground shoots, or stolons, necessary for perennial survival.
For garden mums to overwinter successfully, they must be planted early in the spring, allowing their root systems ample time to establish deep into the soil before the winter freeze. Fall planting does not allow for this crucial root development, making them much less likely to survive the first winter. Garden mums tend to have a more open growth habit, with more visible foliage and smaller blooms compared to their decorative counterparts.
Florist mums, also known as decorative or potted mums, are the compact, heavily flowered plants commonly sold in stores in the fall. These varieties are bred for maximum visual impact, often sacrificing root hardiness for showy, abundant blooms. They are typically grown in small pots, which restricts the development of the deep root system needed for insulation against freezing temperatures.
Although they are technically the same species as hardy mums, florist varieties are usually treated as annuals because their shallow roots make them ill-equipped for winter survival. Their fate is determined by their late planting time and inadequate root structure, which is why a florist mum is unlikely to return the following year.
Preparing Mums for Winter Dormancy
Proper preparation is essential for maximizing the survival of garden mums once the temperature consistently drops below freezing. For hardy varieties planted in the ground, the first step is to wait until the foliage has completely died back, which usually occurs after the first few hard frosts. The dead stems should then be cut back, leaving a short stubble of about three to six inches above the soil line.
Applying a thick layer of mulch is the most effective method for protecting the root crown of in-ground mums. A three- to four-inch layer of loose, airy material like straw, pine needles, or shredded bark should be placed over the plant’s base. This mulch acts as an insulator to prevent the soil from repeatedly freezing and thawing, which can cause the plant to heave out of the ground.
For florist mums or any potted mum a gardener wishes to save, preparation involves moving the plant to a protected environment. Once the top growth has been cut back to a few inches, the potted plant should be stored in a cool, dark location, such as an unheated basement or insulated garage. The ideal temperature range for this dormant period is between 32°F and 50°F, which keeps the plant resting without allowing the soil to freeze completely.
During this indoor dormancy, the plant requires minimal water, only enough to prevent the soil from becoming completely dry. The soil should be checked every few weeks, and a light application of water should be provided only when the growing medium feels dry to the touch. Too much moisture will encourage root rot in the cool, dark conditions.