Petunias have smaller, but more abundant flowers and are ideal for summer bedding or in a mixed border 1. start them indoors 10 to 12 weeks before you want to set them outside. So 2. Petunia seeds are very small and needs lots of light in order to germinate. Remember to water them. 3. When the plants have three leaves, you can plant them outside. 4. Space the plants about 1 foot apart. 5. Petunias can grow in partial shade, but they will have fewer flowers. It's better if the plants have shelter from the wind. 6. If you're planting petunias in containers, use a soil-less mix.Petunias have smaller, but more abundant flowers and are ideal for summer bedding or in a mixed border 1. start them indoors 10 to 12 weeks before you want to set them outside. So 2. Petunia seeds are very small and needs lots of light in order to germinate. Remember to water them. 3. When the plants have three leaves, you can plant them outside. 4. Space the plants about 1 foot apart. 5. Petunias can grow in partial shade, but they will have fewer flowers. It's better if the plants have shelter from the wind. 6. If you're planting petunias in containers, use a soil-less mix.