Do not fertilize gardenias in the fall. Doing so will stimulate tender growth, which may be killed if the temperature in winter drops below 15 degrees. Gardenias are cold-sensitive and during severe winters can be killed to the ground in the Upstate. Often they regenerate in spring. Prune shrubs after they have finished flowering to remove straggly branches and faded flowers.
Water gardenias regularly.
Drip-irrigating the shrubs will keep water off the foliage and blossoms and prevents leaf spots. Regular watering is necessary after blooming also to keep the plants in good condition and able to withstand winter weather. When sooty mold coats gardenia leaves in summer, it is usually due to an infestation of whiteflies. These sucking insects look like bits of cotton. While eating, they excrete excess moisture in the form of honeydew and this, in turn, supports the growth of a black fungus.