Gardenias bloom heaviest in the spring/early summer. Some varieties rebloom in the later summer. They are triggered to bloom when the days are shorter than the nights (short day plants) and there's not much you can do about it.

There are probably cultivars bred to cross more into longer days, but you'll need to do some research. Even August Beauty exhibits a strong propensity for short day behavior.

I have a gardenia in my greenhouse that is blooming profusely and has been for the last 3-4 mo. It was a gift; I have never had one before. I can try to figure out what it is if anyone thinks that my experience is relevant to South Carolina.