Petunias produce colorful blooms from summer throrough the first fall frost. There are small-flowering petunias that are just right for container planting and large-flowered varieties that are striking when grouped in flower beds. One of the quickest ways to add these brightly-blooming annuals to your garden is to transplant nearly mature seedlings into the beds once all frost danger passes in spring. Purchase petunia transplants from a nursery or garden center, or start them from seed indoors.
Put down a 3-inch layer of compost over a well-drained garden bed that receives at least six hours of sunlight a day. Till the compost into the top 8 inches of soil.
Apply 2 pounds of 8-8-8 (nitrogen-phosphorus-potassium) analysis fertilizer over every 100 square feet of garden bed. Use a slow-release, granular fertilizer.
Dig the planting holes to the same depth as the nursery pots and slightly wider. Space the holes 1 foot apart for most petunia varieties, except for spreading petunias, advises Iowa State University Extension. Space spreading petunias 2 feet apart.
Set the petunia transplants into the planting holes so they are at the same depth in the garden that they were at in the nursery pot. Refill around them with soil and firm it lightly around the plants with your hands.
Water thoroughly after transplanting, wetting the soil until it feels moist to at least a 6 inch depth. This collapses air pockets around the roots and allows the plants to begin absorbing water and nutrients from the soil.