I received a Corsage Gardenia plant I ordered online. It was in beautiful condition, I potted in good rich soil, watered with a little liquid iron and plant food and also sprayed it with insecticidal soap. All of a sudden the lower area developed some yellow black spotted leaves which I removed. Now new leaves are popping out where the leaves I removed were. None of the upper leaves were affected. I know it is a ground bush but not in my area in Illinois, it potted and will be indoors come Fall. Thanks John Styler firstname.lastname@example.org
Certified GKH Gardening Expert
Gardenia are a very finicky plant and will let you know if it's not in the perfect growing conditions. (and that's not easy!)
Transplant stress is a common reason for yellowing leaves.
You may want to leave the plant in a container and you can more easily control the conditions. Digging up the plant in the fall to winter over will again cause a root disruption.
These articles will help you with care of your new plant.