Plant petunias in a sunny location in your garden. If you have potted petunia plants, place them on a sunny balcony or window sill. The flowers should receive at least 6 hours of direct sunlight each day to stay healthy. Petunia plants require moisture to avoid over-drying, which leads to wilted blooms. Water the plants moderately each day or second day so that the soil is damp but not soaked. Let the soil dry slightly between watering to avoid root rot. If you are growing petunias in a pot, aim to have up to 3 plants in a pot that measures 1012 inches (2530cm). If you are growing petunias in a garden, space them out about 3 inches (7.6cm) apart. Petunias have large blooms and need enough room to develop fully and stay healthy. Your petunia plants may get infected with a virus or disease that will diminish them. Check your plants for signs of disease, such as small, translucent spots or dark circles on the petals. If this is the case, removing deadheads or trimming the plants will not be enough to save them.

Summary:
Plant petunias somewhere that receives direct sunlight. Water petunias a small amount each 1 to 2 days. Space petunia plants about 3 inches (7.6cm) apart. Check petunias for signs of disease.