Ask Mr. Smarty Plants

Petunias are highly hybridized for color, shape and height, so they're not in our Native Plant Database. However, we can tell you that deadheading an annual plant such as petunias will help to encourage more bloom. The whole point to having an annual is to have a long blooming season, and then the plant dies and is discarded. A plant's goal in life is to reproduce itself. In order to do that, it must produce seeds. In order to do that, it must flower. If, as soon as a flower has faded, you snip it off, the plant will immediately try to flower again. There is no point in trying to gather seed from a hybridized plant such as a petunia, because it will rarely breed true from seed. Besides, petunia seeds are like dust, and very difficult to gather, store and germinate. You can, of course, just pinch the spent blossom off with your fingers, but we always preferred some sharp snips or garden scissors, and snipped off the stem above the next leaf.

For more general information on petunias, read this University of Minnesota Extension article by Deborah Brown on Growing Petunias.