Gardenias are popular in the southern U.S. and need proper fertilization to grow. They are very temperamental, requiring humidity, and thriving in temperatures of 68 to 74 degrees Fahrenheit during the day, with 60-degree evenings. Gardenias are acid-loving plants, so it is best to test the pH level of your soil to be sure it is no higher than 7. If so, you will need to add an acidifier to bring the pH level down. Fertilizer comes in various forms such as pellets, powder or liquid forms.
Mix the acid-forming fertilizer or granular fertilizer into the soil or mix it with water. If you are using a pellet fertilizer, mix it into the soil around the plant.
Avoid getting any fertilizer on the leaves. Fertilize each month from April through November. If you are in southern Florida, fertilize monthly between March and October.
Apply water to the soil to avoid burning the roots.