Petunias are one of the most popular garden bedding flowers. Most varieties will bloom throughout the summer, except in extreme heat. Although some species of petunia are tropical perennials, today’s hybrids are usually grown as annuals, so hardiness zones aren’t a factor in growing them. Most varieties prefer full sun, but in the heat of mid-summer, partial shade will be appreciated. Extreme heat can cause petunia plants to stop setting flowers until the temperature drops. Petunias do best in full sun, but can handle partial shade, especially in hotter areas. They are very slow to grow from seed. If starting from seed, begin at least 10 to 12 weeks before your planting out date. Sprinkle it on top of the soil and pat lightly, for good contact. Keep reading how to grow petunias from seed and care for them here.
SOURCE:http://www.gardenoholic.com/how-to-grow-petunias-that-will-bloom-all-summer/