The best fertilizer for petunias is a balanced, water-soluble fertilizer that is designed to be used on blooming plants. It should be applied consistently on a weekly basis.
Petunias should be watered once every week. For abundant blooming, petunias must be grown in full sunlight, and in soil with plenty of drainage. When blooms become spent, remove them before they seed. When removing spent blooms, always remove the base of the flower as well. Petunias grow well in cool weather, but they should never be left outside during a frost. Partial shade should be provided during the summer. Aphids and other pests can be removed with a strong spray from a garden hose.