Cut back the stalks after the flowers have faded. Tidy up old dead leaves in March. Apply a generous 5-7cm mulch of well-rotted garden compost or manure around the base of the plant in spring. Avoid moving the plant since it resents disturbance. Where necessary lift and divide congested clumps in early spring.
Keep in mind when planting that September Charm is thought of as hardy, so September Charm will tend to go dormant or grow slowly over the winter months.