You can grow colorful, easy-to-maintain petunias (Petunia x hybrid) as perennials in U.S. Department of Agriculture plant hardiness zones 9 through 10 or as annuals in cooler climates. Petunias have four classifications based on habit of growth and size of blooms. Grandiflora types produce large plants and flowers, multifloras are more compact and the flowers are smaller, milifloras are considered miniatures producing an abundance of small flowers and groundcover or spreading types grow only 6 inches in height and produce large amounts of flowers.
Soil, Sun and Water
Petunias grow best in light, sandy soil that is fertile, but will grow in other soils that are from 6 to 7 pH and drain well. They bloom best in full sun but will also bloom in bright, indirect light. Water 6 to 8 inches deep once a week with a sprinkler. Do not let the soil get soggy. Petunias growing in containers and hanging baskets need more frequent water and prefer a fertile and well-drained potting mix. Depending on the size of the petunias and the volume of soil in the containers, you may have to water them daily to keep the soil from drying out.
Soil Amendment and Fertilizing
If your soil is sandy or heavy clay, incorporate 6 cubic feet of compost 6 to 8 inches deep per 100 square feet before planting. This will help clay soil drain better and sandy soil hold more nutrients and moisture. Before planting petunias, work 8-8-8, 10-10-10 or 12-12-12 fertilizer into the top 6 to 8 inches of soil at the rate of 2 pounds per 100 square feet. For petunias grown in containers, incorporate a 19-6-12 slow-release fertilizer into the soil before planting. Use 2 tsp. for each 8 inch container. Once outdoor and container plants begin growing, use a 10-10-10 liquid fertilizer. Use 1 tsp. per gallon of water and fertilize every two weeks. Groundcover types require weekly applications.
Disease and Pests
Yellowing petunia foliage suggests aphid infestation, which cause viruses such as ring spot, spotted wilt, mosaic and curly top. The best defense against plant viruses, which are incurable, is to control aphids. To do that spray with a solution of 1 1/4 to 2 1/2 ounces of insecticidal soap to one gallon water. Wet all surfaces that contain aphids, including the bottom of leaves. Caterpillars may feed on petunia foliage leaving nothing but veins behind. Pick off caterpillars and drop them into a bucket of soapy water. If you desire butterflies, pinch off the damaged parts when the pupae turn into butterflies and then water and fertilize them, petunias should recover. If you are willing to give up the butterflies and have heavy caterpillar infestations, you can spray larvae with ready-to-use Bacillus thuringiensis, a naturally occurring bacteria that wreaks havoc with their digestive systems. Spray the caterpillars until saturated and repeat weekly or as needed. Application rates vary by product.
Spread a 2-to-3-inch layer of dried grass clippings, wood chips or shredded bark around petunias to keep the soil cool and moist. Pinch spent petunia flowers as they fade. Petunias get spindly and leggy by midsummer, producing smaller and fewer flowers. To revitalize the plants, cut each stem back by half. Disinfect any tools that come in contact with petunias by soaking them from two to five minutes in a 70 percent solution of isopropyl alchohol -- available at most garden supply centers -- then air dry.