As chrysanthemum season winds down and the flowers change from beautiful yellows, oranges, and reds to dull, dark brown, gardeners need to trim these plants. In my view, when a plant has become unsightly, it is time to make changes for the greater good of the landscape.
If these mums are in your garden bed, the best thing to do is to cut back the stems to 4 inches. You can cover them with 4 inches of mulch, such as pine boughs or straw. Although mums are perennials, they are subject to soil heaving during warm winter days which can damage or kill the roots. Keeping the plants covered insulates and protects them from the fluctuations in soil temperature. When spring arrives, remove the mulch.
If you have purchased a mum this fall for its beautiful flowers and it is still in the container, you have two options. The first is to treat it as an annual. Take the plant out of the plastic container and put the plant in the compost pile. By now it is too late to plant it in the garden bed; the roots will not have time to establish themselves. Decorative mums that are forced to bloom for the fall are usually thought of as fall décor, much like the gourds, pumpkins, and corn.
The second option is to try to save the plant by keeping it in the plastic container and cutting back the stems to 4 inches. Cover the plant and pot with mulch. For extra insulation, put the plant and container on its side so water will run off and place under a deck. There is a fifty-fifty chance the plant will survive the winter but at least that is better than zero chance if you throw the plant in the compost pile now.
The best time to plant mums in your garden is in the spring after the last frost. This will give them all summer long to get established.