The epsom salts certainly help to keep the leaves a beautiful green, however, the leaves turn yellow for other reasons, which include stress caused by lack of water and needing a complete fertiliser. My own gardenia's also have the odd leaf turn completely yellow just before the buds open, and I have always assumed this is because of the huge resource they've just expended in producing all those flower buds. My own gardenias - and I have about 40 of them ranging up to 3 ft high are generally kept in an informal hedge and are fairly well cared for. Anyway I wouldn't have any issues feeding mine with Thrive soluble fertiliser, although I would prefer Searles complete plant food with trace elements.