In mild areas and with well drained soil, most Chrysanthemums should survive outdoors if covered with a mulch of organic material. However, in an ideal world, and certainly if your soil is not well drained, we would suggest following the information below.
Plants should be lifted in the autumn, after flowering, and any soil cleaned from the roots. Cut top growth down to around 15cm (6"). It does no harm to dip the root ball in a disinfectant, such as Jeyes Fluid, for a few minutes to maintain cleanliness. Some growers also use a hot water dip to control eelworm. The recommended treatment is five minutes in water at 46°C (114°F). After this, dry the stools thoroughly, trim the roots to about 15cm (6") in length and then box them in seed trays using an open compost. Store in an airy place (a cold greenhouse is fine) and keep the compost dry. Plants grown in pots can be moved into a cold greenhouse, or some other sheltered place and, as above, kept dry.
In the early spring, start watering to encourage growth. Varieties not covered by Plant Breeders Rights should be propagated from soft cuttings, to provide strong plants for the following season. Remember not to put Chrysanthemums outside, in the spring, until the danger of frost has passed.