Flowering petunias (Petunia spp.) come in a rainbow of colors and flower sizes suitable for indoor container growing. Grandifloras have blooms up to 5 inches across. Multifloras have smaller blooms but are more prolific than grandifloras. Millifloras have 1-inch blooms and a more compact plant size. There are also many spreading varieties.
Petunias need a well-draining, acidic potting medium, such as a soilless peat mix. The container needs to have drainage holes in the bottom. You can plant one petunia plant in a 4-inch container, or plant up to four petunias in a 10- to 12-inch container. If you use a larger container, give each plant about a 4-square-inches space. Keep the petunias at the same soil depth they were at in the nursery container.
Light and Temperature
Place the petunias in the sunniest window in your house. Petunias need full sun outdoors, and plenty of light indoors to produce multitudes of blooms and to keep them from getting spindly. Turning the plants every few days can help keep the growth even on both sides of the container. If the plants are under artificial lights, give them 14 to 16 hours of light per day. Keep the petunias in a warm room, between 70 and 75 degrees Fahrenheit during the day, and at a minimum of 60 F at night. Although they're usually grown as annuals, petunias are short-lived perennials in the frost-free climates of U.S. Department of Agriculture plant hardiness zones 10 through 12.
Water and Fertilizer
Water the plants with at least 1 inch of water per week. Allow the water to drain from the bottom of the pot to ensure you are watering deeply enough to establish a deep root system. Water the soil at the base of the plant to keep the leaves dry. Wet foliage can develop mildew or fungal diseases. Fertilize the plants beginning the first week after planting. Use a general-purpose fertilizer diluted to one-half strength every two weeks, usually a rate of 1/2 teaspoon per 1 gallon of water. Check the directions on the package because rates vary among brands.
Pruning and Pests
To keep the petunias looking neat and encourage new blooms, pinch off old blooms as they fade. In midsummer, the plants may get spindly. Cut stems back by about one-half to encourage new branching growth. Make the cut with pruning shears or snips just outside a leaf set. Millifloras and trailing petunias are exceptions to the pruning rules. Millifloras rarely get spindly, and trailing plants should not be cut back. Even indoors your plants are susceptible to pests. If you see moving dots on your leaves or webbing in the foliage, you may have aphids or mites. Spray the plant with a premixed insecticidal soap, making sure you target both the tops and undersides of leaves.