Harding yellow leaves
You will have to bring the gardenia into the house. The foliage will most likely yellow and drop due to less light and acclimation to the indoors. Gardenias have somewhat specific cultural requirements for best growth. Focus on proper care.
Gardenias grow best in bright light but keep them out of direct sunlight. When flower buds are forming try to maintain a steady temperature of 62-63 degrees F. Sudden changes can cause flower buds to drop. When flower buds are not forming the normal room temperatures can be 60-75 degrees F. A high degree of humidity is essential when flower buds are forming. Stand the pot on a tray of moist pebbles and spray plant at least once a day with a fine mist spray using water at room temperature. Do not wet the flowers if plants are in bloom.
Gardenias grow less actively during the winter. From October-February allow the top 1-2 inches of the potting mixture to dry out completely before watering again. During the summer, water to make the potting mixture moist and allow the top half-inch of the mixture to dry before watering again.
Fertilizer - apply acid fertilizer every two weeks from March to September.
Here is more information http://www.clemson.edu/extension/hgic/plants/landscape/shrubs/hgic1065.html