Perennial flowers offer good value in the garden if properly cared for, as they return year after year without the need for replanting. Cutting back perennials encourages them to either bloom a second time or encourages further foliage growth and fuller plants. In fall, cutting back removes dead stems and leaves which alleviates disease concerns in spring. The specifics for when to cut back differ between each variety of plant, but there are some basic techniques to keep in mind regardless of variety.
Cut off the top 2/3 of each plant in fall once they die back naturally, usually after the first frost. Peonies are an example of a plant that responds well to this method. Use clean, sharp shears and dispose of all removed plant matter to prevent the spread of disease.
Cut back plants that provide winter interest in late winter or early spring before they begin developing new flower buds. For example, the dead seed heads of coneflower provide winter interest and attract birds so are best left in place until just prior to spring bud setting.
Pinch off the growing tips of some perennials, such as Russian sage, once they are actively growing to encourage fuller, bushy plants. Pinch off the top 1/2 inch of each branch.
Cut back continual flowering perennials, including geraniums and other varieties that bloom throughout summer, if they become overgrown or begin to flower less in midsummer. Cut off 1/3 to 2/3 of their existing height. Most plants will regrow and be blooming again within two to three weeks after a midsummer trim.