Although petunias (Petunia spp., Petunia x hybrida) are perennials in U.S. Department of Agriculture plant hardiness zones 10 through 11, most gardeners grow them as annuals with expectations of spring-to-fall flowers. At their best, petunias cover themselves in fuzzy green leaves and bloom their hearts out. At their worst, they look as if mischievous garden pixies absconded with their flowers and foliage one midsummer night, leaving nothing behind but embarrassingly naked stems. Protecting your petunias from such a sorry state requires regular pruning, especially where summers are very hot.
The Point of Pruning
From a petunia's standpoint, flowers are a necessary step on the way to producing seeds . They put their energy into blooming only as long as it takes for seeds to form.
From your standpoint, flowers are a necessary step toward a beautiful garden. You and your seed-focused petunias are working at cross-purposes, and well-timed pruning gives you the upper hand.
The three petunia-suitable pruning methods are:
- Pinching back.
- Cutting back.
Each method is appropriate at a different stage of petunias' growth; all of the methods reward you with lush, flower-laden plants.
Pinching Back Method
Pinch back petunia seedlings as soon as they have their third set of true leaves. Between your thumb and forefinger, grasp a seedling's growing tip above the topmost leaves, and gently snap the tip free.
Pinched plants direct their growth hormones to the lower leaf nodes, which develop side stems. When the new stems have four or five sets of leaves, pinch them back to produce even bushier plants.
Pinch back nursery petunias at planting time.
Deadheading is the practice of removing old flowers before they set seed. To deadhead a petunia flower, hold it by its green base, where petals emerge, between your thumb and forefinger, and break if off.
Remove each flower's entire base when you deadhead; it's where the seeds form.
For tidier plants, pinch the flowers off where their individual stems join main branches. That way, the branches won't be lined with bare stems.
Cutting Back Method
The most drastic form of pruning, cutting back is reserved for excessively tall or leggy plants. By midsummer, petunias that haven't been pinched back or deadheaded regularly often have long, straggly, bare stems. Rejuvenation requires cutting them back by one-half their length.
Cut the plants back with clean, sharp stem cutters. To avoid spreading disease, rinse them between cuts in a solution that is 1 part household bleach to 9 parts water.
Cut the main branches just above leaf nodes, the slight swellings on stems where new leaves sprout. Choose nodes located about halfway between the stems' bases and growing tips. As you work your way around each petunia, periodically stand back and view the plant to ensure the cuts are symmetrical. Within a few weeks, your petunias will be blooming their hearts out once again.