Petunias are classified as annual plants because they complete a life cycle in less than three years. When grown in temperate climes with good care, petunias can act as perennials, with the plants surviving multiple seasons and years without dying back entirely. According to Texas A&M University, local climate and the heat and cold tolerance of the particular plant help determine if it performs as an annual or perennial.
Excavate a hole for your petunia plants that is twice the width of the pony pack or nursery pots in which they are currently growing. Dig down with a trowel to make the hole just as deep at the root ball.
Slide the petunias from their pots and break up and separate the roots gently if it seems to be circling the root mass. Set the plant into the prepared hole. Add or subtract soil from beneath the root ball so that the top of the root mass is level with the surrounding soil.
Firm the soil around the roots, pressing down gently. Scatter a label-recommended dose of slow-release 10-10-10 granular fertilizer over the soil surrounding your petunias. Nestle it into the top inch of soil with your hands; there's no need to bury it entirely.
Water the soil surrounding your transplanted petunia well until it is drenched to a depth of 3 to 4 inches, but not so much that the soil becomes soupy. Keep the soil moist with regular watering until the roots establish themselves in the new location.