Scraggly petunias should be pinched back to about 4 inches above the soil once the plant has bloomed for the first time, advises the Oregon State University Extension Service. This encourages branching and results in a fuller plant. Applying a fertilizer high in phosphorus twice a season encourages full blooms.
In addition to the benefits of pinching back, petunias perform better when their spent blooms are removed, a process known as deadheading. Overhead watering causes petunias to close, so watering at the soil level is most effective. Petunias usually perform best in full sun, but partial shade also works in the hottest climates.