I live near Nashville, TN. It's planted in clay soil. Gets fertilized regularly. Also, it gets very few blossoms.
Great question............... let me know if you find out what to do. I live in Ohio and my gardenia is in a large pot. I bring it in for the winter months. It's old and I do not want to lose it.
This site has helpful info:
Here are some good tips on that. https://www.gardenia.net/guide/Gardenia-Leaves-Turning-Yellow
Two options to try: (1) Spray with Neem Oil mixed with a tablespoon of Hal Soap and spray thoroughly, including underside of leaves. Spray on a cloudy day, or when the bush is in the shade. (2) Worst case, cut the bush way back - leaving at least a foot of branches and remove all the leaves. I live in Alabama and did a cut back once and the Bush was three feet tall again the next summer. Since then I've used the Neem Oil / Sal Soap twice a year since.
It might be the clay; it may need more organic matter and like azaleas needs more acid. Leftover coffee ground provide both. Working peat moss or other organic matter into the soil before planting will loosen the clay soil so it can absorb the fertilizer better.