Fertilizers meant for acid-loving plants are best for gardenia plants. Adding a regular dose of coffee ground to the mulch surrounding the plant is another good way to provide an acidic mulch to help gardenias thrive.
Gardenias perform best when they are in full sun. They prefer an acidic, humus-rich soil with good drainage. Gardenias should be mulched, not cultivated. Feed gardenias in mid-March with an acid plant food, blood meal or a fish emulsion. They can be fed again in late June to encourage more flowers. Do not fertilize in the fall, as that stimulates the tender growth that is easily killed during the winter.