A. I would need more information to be specific in helping diagnose your particular problem. However, let me provide you with some basic information on raising gardenias, which may help you discover and correct any problems in your garden.
Start by looking at the soil. If you do not know the pH of your soil, you may contact the County Extension Agency for help in testing the soil. Gardenias require a slightly acid soil, which may be achieved by the addition of organic matter, iron sulfate, or other materials. Mulch the plant with compost containing manure and pine needles. This mulch will help maintain the acidity in the soil as it breaks down.
Gardenias should be planted in sun or evenly filtered sun and require well-drained soil. Raised beds are usually the answer for gardenias in Galveston County.
Scale is one common problem with gardenias. Looking at the leaves of the plants can identify scale. Scale causes yellow spots on topsides of leaves, which later fall off. They secrete a honeydew substance in which a black sooty mold grows. To control scale, spray with a dormant oil such as Volck when temperatures are between 40 and 85 degrees. Spray once and repeat ten days later. For more information on controlling scale refer to Texas Cooperative Extension publication L-1827, Scale Insects on Ornamental Plants.
Iron deficiency is another common problem with gardenias. A symptom of this is yellow foliage with eventual defoliation. Correct the pH of the soil as described above, and periodically add iron sulfate to the soil.
Set up a regular feeding schedule about once a month using a food for acid loving plants, such as azalea food. Scratch into the soil, water well, and replace mulch as needed.
The Galveston County Extension agent will be happy to help you in diagnosing problems and offer suggestions for remedying the problem that you are having with gardenias.