In a nutshell: Mulch the plants after the ground freezes and make a wish upon a star.
The whole story: Each autumn, gardeners pour into nurseries, beckoned by rows of mums bursting with blooms. But these plants, with their tender and succulent growth, are a bit like Jessica Simpson: They look good now, but they just won't last. "Mums that are forced to bloom in a greenhouse and planted in fall almost never overwinter, because they don't have time to become established and harden off," says Sandy Feather, a consumer horticulture educator for Pennsylvania State University cooperative extension.
Feather suggests planting mums in late spring next year. Amend your soil with compost, because poor drainage and excess moisture are mums' number-one enemy, and add a granulated organic fertilizer to the soil. Fertilize the plants with a foliar fertilizer, such as fish emulsion, every two weeks. Be sure to stop fertilizing after July 4, or you'll encourage topgrowth at the expense of the plants' root systems. For bushy plants with lots of blooms, pinch back new shoots, leaving two to three leaves per shoot, three times prior to July 4th.
You'll probably have to search your local nurseries to find mums in spring, but by the time fall rolls around, your well-established plants will put on a show that surpasses mums forced into bloom. You can encourage your established mums to overwinter successfully by cutting back the foliage to 3 inches after the first hard frost. When the ground freezes, mulch the plants with a double layer of evergreen boughs (hint: Cut off the limbs of a Christmas tree).