Ants can carry thrips which will destroy a gardenias leaves and blooms.Ants like the honeydew that aphids and thrips produce when eating the leaves. Control the ants and the plant can survive. We used a Bayer product that was worked into the soil around the gardenias. I revived a full bed of gardenias in Florida that had been neglected for years. We had flowers almost year round. Keep soil moist and wash off leaves if any white or black powdery mildew appears. Gardenias like an acidic soil too.