Yellow Gardenina leaves can be a result from several causes. There are a few Gardenia varieties. The bigger the leaves the less sun it will tolerate. They do not like heat, prefer cooler places because heat will turn the creamy white flower into yellow color fast. Beside heat, yellow leaves may be caused by basic soil ( they like acid soil.) It also can be not enough fertilizer ( if the yellow one is the older one, and the whole plant leaves are smaller than normal or lighter green.) Gardenia leaves should be a bit darker gree to be normal. Lack of micro-nutrients can also be the cause. Too dry between watering will also cause the old leaves yellow and drop. If it is the young leaves turn yellow ( usual with green vein), then it is possible not enough Iron or mico nutrients. If the leaves edges turn dark brown, then it can be poison by too much of a chemical nearby the roots, or not enough micro-nutrients. Worst thing is too much water condition, you will not only see many soft yellow leaves, but also dark brown edges and tips. Always keep it a little bit dry between watering will prevent over watering.
