I was told to put Epsom Salts on Gardenias to help them bloom. Plant is a year old, green and healthy, but has not set any blossoms yet.
Help Gardenia Bloom
Certified GKH Gardening Expert
Epsom salt is magnesium sulphate. For gardenias, this will help prevent yellowing of the leaves but has nothing to do with blooming really. Give it some bone meal instead. That will add phosphorus. Gardenias that have too little phosphorus will look lush but will lack flowers. This article will give some other tips: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-care-to-get-a-gardenia-bush-to-bloom.htm