Selecting a Site. Petunias flourish in full sun, but they will grow adequately in part shade. In part shade, the stems will stretch more, and the plants will flower less, but they will still add color to your landscape.
Full sun translates into 6 or more hours of direct sun daily. That may actually be too much for petunias grown in the deep south and southwest, however; in those areas, try to pick a site that has some midday shade to protect the plants from the hottest rays.
If you're planting window boxes, remember that overhanging eaves will produce some shade and will also shelter the plants from rain--beneficial to the blooms, but detrimental to the plants if you forget to water them before they wilt!
Preparing the Soil. Even though petunias are very adaptable and will grow in almost any kind of soil--rocky, sandy or clay--they do best in a light but rich soil that has good drainage. When you have selected a site, amend the soil by digging in compost or peat moss (or Bumper Crop soil ammendment) before planting: a 1- to 2-inch (2.5 to 5 cm) layer should do.
Plant the petunias outdoors when the weather and soil have warmed up, about the time you plant impatiens or peppers.
Planting Petunias. If you buy plants at the garden center, they will undoubtedly be in bloom, so you can see the colors. The plant should have buds as well as flowers. For planting in window boxes and containers, fill with a commercial soilless mix because it's lighter in weight.
The best time to plant petunias is on a cloudy, breezeless day. Set them in the ground or in a container at the same level they were growing in the nursery pot. Space plants in the ground as follows: Multifloras, grandifloras and floribundas, 10 inches (25.5 cm) apart; spreading petunias, 15 to 20 inches (38 to 51 cm) apart, because they can spread as much as 3 to 4 feet (.9 to 1.2 m); and millifloras, 6 to 10 inches (15 to 25.5 cm) apart. Plants in containers are usually spaced closer together, so that the pots overflow with color.
Consider the heights of the plants when you're deciding where to put them. Most petunias grow 12 to 14 inches (30.5 to 35.5 cm) high, but floribundas may reach 16 inches (41 cm), millifloras are more diminutive at 10 to 12 inches (25.5 to 30.5 cm) and spreading petunias are the true low-growers, reaching only 6 inches (15 cm).
You may want to spread a layer of mulch around the plants, especially the double-flowered ones, to prevent mud from spattering up on the blooms. The mulch helps retain soil moisture and discourage weeds.
Petunia Maintenance. Petunias don't require a lot of care, but they do benefit from some attention.
Fertilize the plants monthly with a balanced fertilizer; double-flowered cultivars appreciate a bit more, perhaps once every two to three weeks. Wave petunias benefit from weekly feeding.
Because they're quite drought-tolerant, petunias seldom need daily watering other than what they receive with rain; in prolonged periods of drought, however, watch that the soil doesn't get too dry. And, if you're growing the plants in window boxes or other containers--where soil can dry out quickly--check the soil daily in very hot weather and water as needed.
The stems of most petunias have a tendency to stretch out by midsummer and bear fewer flowers, since blooms are formed at the ends of the stems. Prune them back quite severely so they will produce new shoots and more flowers. The exceptions are milliflora and trailing petunias--they don't need to be pruned at all, which makes them really easy-care.
Plant Food: We recommend Jack's Classic Petunia Plant Food. Specially formulated for iron hungry crops such as petunia, calibrachoa, bacopa, diascia, nemesia, scaevola, and others. Feeds through both the roots and leaves.