Garden chrysanthemums (aka, hardy mums) are those cultivars of chrysanthemum that have been developed for winter hardiness. They have become very popular for adding color to the landscape in early fall, or for use as autumn décor on porches, decks, and patios around the home. The proliferation of cultivars with different flower forms and colors have helped to increase their desirability. In short, there are few other perennial garden flowers that put on the spectacular show of fall color as well as hardy mums.
Depending on the severity of fall weather, most blooms of hardy mums have faded (frozen) and the spectacle of color is over for the year in early to mid-November. The question arises, "What should be done with these plants now?" While composting is an option, the frugal gardener might want to get added years of pleasure from their mums by overwintering them. The best method of achieving this depends on whether the plants were planted directly into the landscape after being purchased or kept in containers for autumn décor.
The common name "hardy chrysanthemum" is somewhat misleading and might imply they are able to endure the rigors of Midwestern winters in a manner similar to peony, daylily, and iris. Unfortunately, such is not the case. There is considerable variability in the cold hardiness between cultivars of hardy mums because of their complex genetic makeup. Winter survival may also vary with location, culture, site, or winter severity.
For gardeners who want a newly planted mum to survive for another season of bloom, it is best to look at a plant's care tag to determine its zone of hardiness rating. Even though many garden mums are hardy to zone 5, it is safe to assume the new plant will require some winter protection, especially its first year. This is true for spring-planted mums as well as those planted in the fall.
For any mum in the garden, mulching can help it to survive the first winter as well as succeeding winters. Cut back the tops of plants after leaves have turned brown each fall. Use several inches of a loose, airy mulch that will allow light to get to the small side shoots throughout the winter. Loose straw, pine needles, or evergreen branches anchored in place so wind will not blow them away normally provides sufficient protection.
The intent of these materials is not to keep plants warm. Chrysanthemums have shallow root systems. Repeated freezing and thawing of the soil during the winter months can "lift" plants out of the ground and cause severe damage known as "frost heaving." In addition to providing some protection from winter winds, mulches keep the soil in which the plants are growing shaded. This discourages alternate freezing and thawing of the soil. These protective mulches may be removed or pulled away from the crown of the plants by early to mid-April, after the danger of severely cold weather has passed.
Once growth resumes the following spring, water and fertilize regularly. Mums are heavy feeders and will need adequate water and fertilizer throughout the summer months. Remove (pinch) the tips of new growth when it achieves a length of six inches to promote bushiness and more abundant flowering. This procedure can be repeated, if necessary, but should not be done after the Fourth of July.
Most hardy mum cultivars initiate flower buds in mid to late July, as they respond to the shorter days of the season. After initiation, flower development requires between six and ten weeks, depending on the cultivar's "response group." Many of the newer hardy mum cultivars have relatively short response times thus causing them to bloom earlier in the fall.
Overwintering mums still in the containers in which they were purchased represents a bit more of a challenge. At our latitude, such plants have a limited chance of survival if planted into the garden after they have flowered. Because of the lateness of the year, they will not have sufficient time to develop the type of root system needed to survive cold winter temperatures, even if mulches are used. Therefore, the entire plant somehow will need to be protected while still in its original container.
Gardeners who have cold frames or hobby greenhouses have the option of moving them into such protective structures while still in the pots. Alternatively, if space permits, they can be moved indoors and placed in a bright window in a cool room (about 40 to 45 degrees F.) for the duration of the winter.
Another possibility for overwintering mums still in their container is to force the plant into dormancy while not allowing it to freeze. To accomplish this, first cut back the top of the plant to several inches above the growing medium in the container. Then store the plant and container in the corner of a basement of well insulated garage. Keep the growing medium in the container barely moist throughout the winter. Only when the medium is dry to the touch should a light application of water be made. Do not saturate it or allow it to totally dry out.
In the spring, after the soil begins to warm, plant the mum in a location in the garden where fall color is desired. Hardy mums require about six hours of direct sunlight to thrive. Water carefully until the plant establishes a vigorous root system. When new growth appears, follow the steps outlined above for summer care.