Not all autumn-flowering chrysanthemums have to be lifted, but many do. The roots are potted into boxes, which means that you can start them off in warmth in spring.
Lift the roots after the plants have finished flowering and before severe frosts arrive. Shake surplus soil off the roots before removing from the garden. Trim the tops off and cut any long, straggly roots back to keep the root-ball compact.
Place a layer of soil or potting soil in a box or tray about 4 inches deep. Position the roots and cover them with about 1 inch of soil, firming lightly. Don't forget to label the plants. Keep the box in a cool, light place, such as a cool greenhouse, light garage windowsill, or a cold frame. For most types of chrysanthemum it does not matter if they receive a touch of frost. Keep the soil slightly damp but not wet.
To overwinter chrysanthemums, remember that it is only the autumn-flowering chrysanthemums that are likely to cause confusion regarding overwintering. In mild, frost-free climates they can all be left in the ground, and many of the species, like Chrysanthemum rubellum, are hardy even in cold areas.
In temperate climates, the highly bred early-flowering autumn chrysanthemums are best lifted and stored, and even those outdoor ones that flower later are best treated this way. Even those that tolerate some frost are more likely to survive if kept fairly dry. Wet and cold is the combination to avoid.