Deadheading is a non-favorite as far as garden chores, but it is an important task and not for appearance alone. Some plants may stop blooming if they are not attended to promptly and hybrid perennials allowed to go to seed may in time be crowded out by less desirable offspring. Some of your favorites may repeat blooming when the spent flowers are clipped off. Cut off old blooms throughout the season, stopping several weeks before the first frost. Cut perennials with flowers at the tip of leafy stem, just below the fading flowers. Trim others to just above the topmost unopened bud. A good pruning shears is worth the investment. Don’t forget to smell the flowers while you are enjoying this newly acclaimed garden time.
from: The Encyclopedia of Flower Gardening and Landscaping