Trim petunias on a regular basis by deadheading blooms as they wilt and by cutting growth back to at least half the length midway through the growing season. Trimming petunias ensures they continue blooming all season long and reduces the amount of nutrients and water they require.
Trimming approximately 1 inch from the branches of petunias when transplanting encourages the plant to form additional branches and more blooms as it grows. Deadheading petunias allows them to spend their energy forming more blossoms instead of forming seeds. In order to deadhead petunias, pinch the stem to the wilted flower just behind the point of attachment.
To perform the midseason trim, sterilize scissors or pruning shears with rubbing alcohol. Locate a growth node near the center of each branch while leaving the node attached to the plant. If necessary, it is acceptable to trim the branches to 3 inches. The plant recovers quickly by forming new branches and more flowers. A few days after this severe trimming, petunias benefit from the addition of liquid fertilizer to the soil.
Petunias prefer full sun, and they bloom most of the growing season with regular trimming. During periods of extreme heat, they stop forming new blooms temporarily, but they begin again as soon as temperatures begin to fall.