What causes my gardenia leaves to turn yellow?

Answer 1

Yellow leaves normally signal that the plant is under some type of stress. In order to remedy the problem, you must first try to pinpoint the issue. This article should help you with that: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/helping-a-gardenia-bush-with-yellow-leaves.htm

This article may also help: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm