In springtime, dig down 8 inches around each chrysanthemum plant with a garden spade, lift it out, and brush off any soil around the roots. Separate the younger shoots into three or four divisions, discarding older, woody portions. For each division, dig a hole twice as wide as the roots, and set a plant into it with its crown at soil level and its roots spread out. Gently pack the soil back around the roots, and water the plants thoroughly.
Potted chrysanthemums appear in many garden centers in late summer and early fall, but if they are transplanted into the ground at that time, they will not survive in most climates. Although they are perennials, they have no opportunity to establish adequate root growth to carry them through the winter months because their energy is centered on flower production in late summer.
To overwinter chrysanthemums purchased in autumn, pot up the plants after the first frost, including as much of the root systems as possible. Water them well, then place them in a basement, closet or other location of total darkness with a temperature range between 32 degrees and 50 degrees Fahrenheit. The mums enter into winter hibernation, needing only minimal watering to keep their roots dampened. In spring, after the last killing frost, gradually acclimatized them to light, and transplant them into the garden.