[header] How to keep perennials healthy [title] Keep them groomed. [step] Cut away dead and decaying foliage regularly. [substeps] Early summer: dead-head flowers and remove any stringy bits at the end of flowering season. 
Dead-heading should be done weekly or fortnightly during summer unless you are looking for seed stock. Removing flowers promotes the growth of fresh leaves and often more flowers in many species of perennials.