Chrysanthemums, commonly called mums, are popular flowering plants that signal the arrival of autumn. Gardeners often face the dilemma of protecting these vibrant blooms from the first signs of frost. While generally hardy, mums can suffer significant damage from freezing temperatures just as they reach their peak display. Determining the best action—whether to shelter them temporarily or prepare them for winter—depends on the severity of the expected cold.
Understanding Cold Tolerance in Mums
Mums possess a degree of natural cold tolerance, but this varies significantly depending on the temperature drop. A light frost occurs when temperatures briefly dip just below 32°F (0°C), typically hovering between 30°F and 32°F. Established, healthy chrysanthemum plants can often withstand these short periods of freezing without permanent damage.
A hard freeze is defined by temperatures remaining below 28°F (-2°C) for several hours. This sustained cold causes destructive ice crystals to form within the plant tissues, leading to cell rupture and the characteristic browning and wilting of the blooms and leaves.
Potted mums are considerably more susceptible to cold damage than those planted directly in the ground. The soil mass in a container freezes much faster and more thoroughly because the root system is exposed to freezing air temperatures on all sides. Ground-planted mums benefit from the insulating properties and residual warmth stored within the surrounding soil mass, offering a buffer against moderate cold.
Short-Term Protection to Extend Bloom
When frost is predicted, the goal is temporary preservation of the current floral display, not long-term survival. Protection should be implemented whenever the overnight temperature forecast falls below 32°F. This shielding allows the plant to continue blooming through milder daytime conditions for several additional weeks.
For potted chrysanthemums, the most effective short-term action is moving them to a sheltered location overnight. An unheated garage, covered porch, or shed provides sufficient insulation to keep the air temperature above freezing. This shelter prevents the radiative heat loss that causes frost to settle directly on the plant surfaces.
The pots must be moved back outside into the sunlight the following morning once temperatures rise above 35°F. Mums require significant light exposure to maintain bloom quality and prevent the plants from becoming weak. This process is the most reliable method for extending the life of container-grown mums.
Gardeners with mums planted directly in the soil must rely on covering techniques to prevent frost damage. Covering materials should be lightweight and breathable, such as specialized horticultural frost cloth or old sheets and blankets. Plastic sheeting should be avoided because it traps moisture and conducts cold directly onto the foliage.
The covering must be draped over the plant before dusk, ideally reaching the ground to trap the heat radiating from the soil. This trapped warmth creates a microclimate several degrees warmer than the ambient air. Remove the covering promptly in the morning after the threat of frost has passed, allowing the foliage to dry and preventing excessive moisture buildup.
If a light frost has already occurred, gently misting the foliage with water before the sun hits the plant can help reduce damage. This action helps melt the ice crystals slowly, preventing the rapid thawing that ruptures plant cells. However, this method is only marginally effective and is not a substitute for physical insulation.
Long-Term Overwintering Strategies
Once the continuous threat of hard freezes arrives, the focus shifts from preserving blooms to ensuring the plant survives dormancy for the next growing season. This long-term strategy requires different preparation depending on whether the mum is a hardy perennial or a non-hardy decorative variety.
Preparing Hardy Ground Mums
Hardy chrysanthemums planted in the ground should be prepared for winter by cutting back the spent, brown stems to about four to six inches above the soil line. This pruning reduces the plant’s surface area, which helps prevent disease and rodent damage during the winter months. Do not cut back the foliage until after it has been fully killed by hard frost.
After the ground has frozen solid, apply a substantial layer of insulating mulch over the plant’s root zone. A four to six-inch layer of loose material, such as straw, pine needles, or shredded leaves, acts as a protective blanket. Applying the mulch after freezing prevents cycles of freezing and thawing, which can heave the plant out of the soil and damage the root crown.
Overwintering Potted Mums
Potted mums intended for overwintering must be moved to a protected, dormant state indoors. The ideal storage location is a cool, dark area like an unheated basement, root cellar, or insulated garage, where temperatures consistently remain between 35°F and 45°F. Temperatures above this range may cause the plant to break dormancy prematurely.
Before storage, cut the stems back and ensure the pot has excellent drainage to prevent root rot. During the storage period, the plants require minimal care, typically only a light watering once a month. This small amount of moisture prevents the root ball from completely desiccating while maintaining the necessary state of dormancy.
It is important to acknowledge that many of the large, showy florist or decorative mums sold in the fall are not reliably cold-hardy perennials. These varieties are often treated as annuals because they lack the genetic resilience to survive typical winter conditions, making long-term overwintering efforts often unsuccessful.