For best results, petunias should be pinched back at the tip of the main stem. This can be done using either scissors or the forefinger and thumb. Pinching petunias back ensures bushy growth and prevents plants from becoming scraggly.
Some modern varieties of petunias are not supposed to require pinching in order to thrive, but a little pinching back can't hurt these varieties. Petunias of all varieties are profuse bloomers, and pinching them back encourages lateral growth that results in more flower buds. Gardeners should only pinch back about one-third of an inch, though.
Old blooms should be pinched off when they begin to fade. Pinching spent flowers just below the blossom will encourage the growth of new flower buds. Petunias provide floral displays up until the first hard frost if cared for properly.