Most landscape and garden plants require a period of dormancy in order to rest. This dormancy is gradual, and usually brought on by cooler weather. It is very important for garden plants such as roses and perennials to enter a resting phase with controlled moisture and no fertilization in order to be long-lived. Several light frosts -- those that nip the tops of the foliage of perennials deemed tender in your area-- generally precede what is termed the first killing frost of the season. As light frosts begin in many areas, it's time to prepare the perennial bed for winter.
As a rule, remove dead and diseased stalks, stems and leaves first; then trim the remaining foliage to 4 inches. Pull weeds and discard trimmings to prevent overwintering pests; otherwise they'll reemerge in greater numbers come spring.
A winter mulch can be applied after the first freeze, carefully avoiding the rosettes of perennials such as gerbera daisies that will rot if deeply mulched while dormant.