Mums are quintessential autumn-blooming plants, providing color when most other flowers have faded. Their late-season display exposes them to the threat of cold weather and frost. While mums are relatively hardy compared to summer annuals, they are not impervious to freezing temperatures. Providing timely protection is necessary to extend the bloom time and ensure the plant’s survival through winter.
Distinguishing Between Frost Types
The gardener’s reaction to a cold forecast depends on the predicted temperature drop, which determines the type of frost expected. A “light frost” or “light freeze” occurs when air temperatures fall between 29°F and 32°F (around -2°C to 0°C) for a few hours. This cold typically causes minor damage to tender flower buds and foliage, such as slight wilting, but the plant’s core structure often remains intact.
More significant damage results from a “hard frost” or “killing freeze,” defined by temperatures dropping below 28°F (-2°C) for four or more consecutive hours. This prolonged deep freeze is destructive to the above-ground portions of mums. Cold damage involves ice crystals forming inside the plant’s cells, disrupting cell membranes and leading to tissue death that appears as blackened, mushy foliage. Tender new growth is particularly susceptible to this cellular rupture.
Immediate Covering Techniques
When a light overnight frost is predicted, immediate covering is the most effective temporary protection for saving current blooms. The goal is to trap the residual heat radiating from the ground and prevent frost from forming directly on the foliage. This technique is important for container mums, whose root systems lack the insulation of the surrounding earth and are more vulnerable to freezing.
The best materials for temporary covering are lightweight and breathable, such as old cotton sheets, blankets, or commercial frost cloths, which protect down to about 25°F. Drape the material completely over the plant, extending it to the ground to seal in the warmer air. Avoid plastic sheeting if possible; if used, it must not touch the foliage, as it can transfer cold and cause damaging condensation that freezes.
Place the cover over the mums in the late afternoon or early evening before the temperature drops. Removing the cover promptly the following morning is necessary once the temperature rises above freezing and the frost has melted. Leaving the cover on during the day can cause the plant to overheat and trap moisture, creating an environment conducive to disease. For small container mums, moving them into a cool, unheated space like a garage or shed before nightfall is the simplest protection method.
Preparing Mums for Winter Survival
Once the blooming season ends and sustained cold weather sets in, the focus shifts to ensuring the plant’s long-term survival. For perennial garden mums planted in the ground, proper winterization protects the crown and root system so the plant returns the following spring. The timing of pruning is a primary distinction between temporary frost protection and permanent winter survival.
Gardeners should resist cutting the stems back immediately after the foliage dies following a hard freeze. Leaving the dead stalks standing, cut back to about six to eight inches, provides natural insulation that protects the plant’s crown throughout the winter. Cutting the plant down too early exposes the crown to harsh conditions and encourages new, susceptible growth that will be killed by the cold.
A thick layer of organic mulch is then applied to insulate the roots from damaging freeze-thaw cycles. These cycles cause “frost heave,” which pushes the shallow root ball out of the soil, leading to dehydration and death. Apply a two to four-inch layer of light, airy material, such as straw, shredded leaves, or pine needles, around the base of the plant. This mulch should be applied after the ground has frozen but before the most severe winter weather begins, and it should remain until new growth appears in the spring.