Fertilize Every Few Weeks
To maintain big blooms across an entire field, petunias need ongoing feeding. These plants are hungry! I apply a balanced liquid fertilizer every two weeks once they start flowering. A bloom-boosting formula with a higher middle number (phosphorus) helps fuel all that petal production.
You can also work in slow-release granular fertilizer at planting time, then supplement as needed through the season. Regular feeding helps keep plants from getting tired or sparse, especially in lean soils. A well-fed petunia plant is full of energy—and it shows in the size and intensity of the flowers!