Although gardeners typically raise petunias as annuals and constantly rotate them in and out of garden beds, the tropical flowering plants are technically perennials except for some hybrid varieties. Provide your perennial petunias with the water they need to stay lush and hydrated, adding green foliage and vibrantly colored blossoms to any backyard landscape.
Spread 3 to 4 inches of mulch around your perennial petunias. Mulch helps limit moisture evaporation to keep your petunias watered longer, according to the University of Rhode Island. Potential mulching materials include grass clippings, shredded leaves and wood chips.
Apply water to the perennial petunias, using enough water to moisten the soil to a depth of approximately 8 inches, according to the University of Minnesota.
Repeat watering every seven days. Petunias are very drought tolerant and thrive in low-water conditions. Observe the plant for signs of drought stress, such as wilting leaves, and increase watering if needed. The University of Minnesota says petunias grown in pots may need more frequent watering due to the smaller space in which they're grown.