Answer: Petunias will bloom more prolifically if the spent flowers are removed from the plant instead of left to develop seeds. Since petunias can get rangy or leggy if not pinched back, you can remove some of the stem along with the spent flowers, to encourage bushy plants. Cut back to the main stem where the flowering stem is attached, and you'll encourage the plant to develop more flowering stems.
Some annuals can be sheared back by one-half after they've finished blooming to get a second flush of bloom. Shearing back works on plants like alyssum that usually bloom once in the early spring. With your petunias, regular deadheading (removing spent flowers) is preferable to shearing the entire plant back. Hope this clarifies things for you!
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