We can thank the Japanese, and a spectacularly successful breeding program, for the double petunia. Over the decades since its birth, the double petunia has been crossed and re-crossed and improved immensely. We now have several species of this gorgeous garden favorite. Plant your double petunias once the temperature has warmed to 60 degrees F.
Plant your double petunias 12 inches apart in an area that receives at least six hours of sunlight a day. Although they will grow in the shade, they will not flower as profusely as they will in the sunshine. Double petunias are especially in need of long days to bloom.
Amend the soil in the planting area by adding a 3-inch layer of compost or well-rotted manure. Using the gardening fork, mix the amendment in with the existing soil to a depth of 8 inches.
Add fertilizer to the soil. Extension agents at the University of Minnesota suggest 12-12-12 at a rate of 2 pounds per 100 square feet. Mix this in with the amended soil. Once the double petunias are established, starting in early July, use liquid fertilizer every two weeks, after watering.
Water the double petunias once a week, making sure the soil is wet to a depth of 6 inches. If you are growing your flowers in pots or containers you may need to water more frequently.