Don't panic as yellow leaves are a sign of many things including too much water, too much shade, poor drainage and even lack of fertilizer. I need to know if you are asking about an indoor gardenia in a container or a shrub gardenia in the landscape. On indoor gardenias spider mites often cause yellow leaves. They can be controlled by washing each leaf with mild soap or spraying with an insecticidal soap several times over a few weeks. Indoor gardenias also suffer from lack of humidity especially in dry climates during the summer. Send me more information about the plant and we will solve this mystery of the yellow leaves. Keep growing, Marianne Binetti