Petunias are a garden mainstay all over the world, prized for its ease to grow and non-stop blooms from early spring to mid-summer. Variegated varieties such as the red-tip petunia are trailing varieties that can get leggy and straggly in late summer unless properly pruned to allow for new growth and a second shot at blooming.
Deadhead petunias on a regular basis from the time of planting. Pinch spent blooms just below the bloom or trim with pruning shears or clean, sharp scissors as they begin to die. Annuals expel their energy on three parts of the growing cycle--producing the plant, flowering, and going to seed. By deadheading at the flowering stage, you will keep the plant from going to seed, thus resulting in more blooms because it's trying to get to the final part of its life cycle.
Prune in mid-July as stems begin to get leggy and leaves are small. Start with just a few stems at a time, allowing them a few days to a week to recover before pruning more.
Cut back a couple stems at a time, four to five inches back. This allows for the plant to have the time to recover from the stress of pruning and gives the new growth a fuller, staggered look.