I have found that rain water ONLY has helped my gardenias tremendously.A visit to a local cows barn brings em to full bloom.
Mine are in pots on the patio,and are garage kept through the winter,and even then kept watered once a week with rain water.I always had the problem as well with dropping leaves, nowwhen I bring them out of the garage they shed old leaves,I keep any yellow ones plucked.
I live in Tennessee, and love my gardenias. It is well worth the extra work to smell the glorious blooms. Good luck