Deadhead petunias does not refer to a heavy-metal rock band. It describes a pruning method to extend the time your flowers produce blooms, increase your number of blossoms and make your plant look healthier. Cutting off the dead and dying petunia blossoms prevents seed maturation that occurs in the green portion just under the bloom. Though the flower dies, the green portion left behind will produce seeds unless you cut it off with the petunia blossom. This process could take minutes -- or longer if you have many petunias. You'll notice more dramatic results on smaller container-grown petunias when you deadhead them than on large garden plots of these perennials.
Inspect the petunias for dead, fading or drooping blooms.
Snip off the blossoms below the wide green portion that holds the flower to the stem.
Repeat deadheading the petunia until you remove all of the dead and decaying blossoms.