Since a gardenia is an acid-loving plant, this is the type of fertilizer you need. You should also buy a fertilizer that is enriched with iron compounds. A water soluble type fertilizer is the easiest and the most preferred type to use. Many people finds that fertilizers that are also good for azaleas will work perfectly for gardenia plants as well.
Wet the soil before adding any fertilizer mixture to it. This will keep the fertilizer from burning the roots of the plant.
Mix your fertilizer at about 1/4 of the recommended strength. If you use it at full strength, you may actually over fertilize the gardenia, which may harm it. Causing the leaves to turn colors and to eventually fall off of the plant.
Repeat every two to four weeks up until the month of November. You should never fertilize a gardenia between the months of November and February for this is their dormant months.