Gardenias need proper nutrition to maintain healthy green leaves. The University of Rhode Island recommends feeding gardenias with an acidic fertilizer once each month, beginning in April and continuing until November. A potassium deficiency can cause yellowing, then browning, of the leaf tips, especially on leaves that have recently reached their adult size.
Keep the soil in which your gardenia is growing moist at all times. If you allow it to dry out, the leaves of your gardenia can begin to turn brown at the tips, then turn completely brown, die and fall off. Water your gardenia regularly and monitor the soil moisture to prevent this condition.
Several plant diseases can cause gardenia leaves to develop brown spots. They include bacterial leaf spot, other types of leaf spot and the Rhizoctonia fungus. The brown spots these diseases cause can occur at the leaf tips, or elsewhere on the leaves.