Chrysanthemums, commonly known as mums, are a favorite for bringing vibrant color to the autumn landscape. While they are technically perennials, keeping them alive in a pot through the winter presents a distinct challenge. The soil mass in a container freezes faster and deeper than the ground, offering the roots little protection from cold temperatures. Successful overwintering requires providing adequate insulation to the root zone, allowing the plant to enter and maintain a state of healthy dormancy until spring.
Understanding Mum Hardiness
The likelihood of a mum surviving the winter is determined by the specific plant type and the local climate. Mums are broadly categorized into two groups. Hardy “garden mums” are perennial and bred to survive the winter in the landscape, typically thriving in USDA Hardiness Zones 5 through 9.
The other type, “florist mums,” are the dense, heavily flowered plants often seen at grocery stores and are usually treated as annuals. These varieties are less cold-tolerant, possess a more shallow root system, and are not bred for long-term survival. Confirming the variety is important before attempting overwintering. If the plant’s rated hardiness zone is significantly warmer than your local climate, container survival outdoors is improbable, regardless of the variety.
Outdoor Protection for Potted Mums
For those in milder climates or with true hardy garden mums, outdoor protection focuses on preventing the root ball from freezing solid. The most effective strategy involves mimicking the insulation of the earth by “heeling in” the pot. This process requires sinking the entire container into the ground or a raised garden bed, ensuring the pot’s rim is level with or slightly below the surrounding soil. This surrounding mass of earth offers a buffer against temperature fluctuations, which is the primary cause of root damage.
If burying the pot is not feasible, creating a protective barrier around the container is the next best option. You can wrap the pot with insulating materials like bubble wrap, burlap, or straw to slow the transfer of cold. Grouping several potted mums tightly together against a protected area, such as a south-facing wall, also offers mutual insulation and blocks harsh winter winds. Before the ground freezes, prune the dead foliage back to a few inches above the soil line and ensure the soil is slightly moist, as a completely dry root ball is more susceptible to cold damage.
Indoor Dormancy as an Alternative
Moving plants into an unheated structure is an effective method for overwintering. The goal is to force the mum into a state of dormancy, which requires a dark, cool environment. An ideal storage location, such as an unheated garage, shed, or cool basement, should maintain a consistent temperature range between 32°F and 50°F.
Before moving the plant indoors, cut the stem structure back severely, leaving only about four to six inches of growth above the soil. This reduction signals to the plant that it is time to rest and minimizes the chance of pests or disease spreading indoors. Allow the potting mix to dry out slightly before storage. Check the soil moisture every few weeks and provide a small amount of water only if the soil is bone dry, ensuring the roots do not rot from overwatering.
Spring Care and Replanting
When the threat of hard frost has passed, typically in early spring, the overwintered mum should be gradually brought out of dormancy. Begin this transition by moving the pot to a location that receives increasing light and warmer temperatures over a few weeks, a process known as hardening off. This slow reintroduction prevents shock to the plant after its period of cold storage.
Once new green shoots begin to emerge from the base, prune away any remaining dead stems. This is the best time to repot the mum, as container plants quickly deplete soil nutrients and become root-bound. Transplant the mum into a slightly larger container with fresh potting mix and begin a light fertilization schedule to support the new growth.