Browning is the natural end result of gardenia blooms but it sounds like yours are turning brown way too soon. There are some bloom blights that can cause that problem. Such blights are increased by wet conditions such as frequent rainy spells or sprinkler irrigation hitting the blooms. Spraying fungicide sprays can help but you need to repeat sprays after rainy spells to maintain protection. It also can help to remove brown and dried blooms to reduce the incidence of disease.
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