Most gardeners grow petunias as warm-season annuals, planting them anytime after danger of frost is past. In the low and intermediate deserts (Sunset Western Garden Book climate zones 12 and 13), plant petunias in the fall. Choose a series like Jamboree or Supertunia, which can set flower buds during the cool season, when there are fewer daylight hours.
Light. Full sun produces the most blooms. Petunias will take some shade, but the deeper the shade, the fewer the flowers and the more leggy plants will become as they stretch for light.
Soil. Petunias like soil that's rich in organic matter, which also facilitates drainage and retains moisture.
Watering. Never let soil dry out completely. In hot weather, potted plants may need water twice daily.
Feeding. At planting time, fortify the soil with slow-release fertilizer, then feed plants twice a month with a half-strength liquid fertilizer.
Deadheading. You'll get more flowers and longer bloom if you remove faded blossoms. Self-cleaning varieties drop spent flowers naturally.
Pest control. In California, geranium (tobacco) budworm can chew up petunia leaves and buds. The most effective biological control is to spray plants with Bacillus thuringiensis (Bt) at the first sign of damage.