Yes! Petunias are hard-working annuals that live for only one summer so they need plenty of nutrients to pump out the blooms. I am planting a bed of petunias this afternoon and I will work 1 capful (that would be 3 tablespoons) into each 4 foot-by-4 foot area of the soil before adding Wave petunias. I learned the hard way that if you don't fertilize petunias they bloom poorly and the leaves turn yellow. Osmocote that is worked into container gardens, hanging baskets, or in the ground will keep your petunias in bloom all summer. Keep growing, Marianne Binetti