The trumpet-shaped flowers of petunias are an old-fashioned garden favourite, but the plants can easily get leggy and overgrown if they're not properly maintained. Removing spent flowers, a process known as deadheading, can keep your petunias dense and compact and can also help ensure steady flower production throughout the growing season. Deadheading keeps the plant from producing seeds so that it can instead put more energy into growing more blooms and leaves. All petunia varieties can benefit from deadheading, with the exception of 'Wave' petunias, which do not require deadheading or cutting back.
Look for petunias with wilted or dead flowers. You may also want to cut back petunias that have become overgrown or that have small, sparse or weak leaves.
Cut back the stem with the dead flower at least to the nearest set of leaves, using a pair of scissors or pruning shears. If you desire, you may cut it back further to better shape the plant, but try to avoid cutting new flower buds.
Collect the cuttings in a basket or bucket to take to the compost pile.
Continue to monitor your petunias throughout the growing season. Depending on the weather and variety, petunias may need to be deadheaded either once a week or once a month.
Don't be afraid to severely cut back a petunia plant that has become seriously leggy. Petunias are pretty sturdy flowers, and so long as you leave some green leaves for photosynthesis, a healthy petunia should survive whatever kind of "haircut" you need to give it. Petunias are fairly undemanding flowers, but the right growing conditions will ensure stronger plants with lots of blossoms. According to the University of Minnesota Extension Office, petunias like at least five to six hours of daylight, well-drained soil and waterings about once a week. Petunias that are continuously deadheaded will not produce seeds. If you would like to save some petunia seeds to grow in your garden next year, stop deadheading a few weeks before the end of the growing season to allow seeds time to form and mature. You could also set aside a few petunias specifically for seed production, and not deadhead those at all.
Although the petals will come off easily in your hand, avoid simply pulling off the old flowers, as this will leave the seed capsule behind. In order to stimulate the petunia plant to produce more blooms, you must cut off the swollen growth at the base of the petals. New petunia buds can sometimes resemble dead flowers. Take a good look at what you're cutting before you snip.