Try putting some ironite on the ground around the gardenia.
You can get small bag of Ironite plus at Lowe's and most other garden centers.

Evergreen does not mean that the leaves live forever on broad-leaved evergreens, it only means that the plant will always have green leaves and not lose them all at once in the fall. I forget the exact number of years in the cycle for gardenias, but they live for X number of years and then die and are replaced or not, the latter especially if they were in a spot where the growth of the plant has shaded them out - leaves need light (in varying degrees) to live. Old leaves turn yellow. I think what you are seeing is normal. I would hold off on adding anything until you KNOW it is needed - sometimes adding stuff you THINK is needed does more harm than good.

sounds like good advice to me. i will keep an eye on them and see what shakes out. thanks

I get some light yellow leaves on my gardenias over the pass few years. So I add iron cleating and they turn green and I have never seen any harm. I have just planted some new gardenias and they have some light yellow leaves and have just added some iron cheating to the ground around them. My older plants have very dark green leaves now and good lots of flowers. dibbit, you always let your gardenias turn yellow?

I have planted 4 new gardenias (the small trailing kind) several weeks ago. When first planted they bloomed and looked great. Now they are yellow, brown and dying!! I have watered, fertilized (with Miracle grow) and nothing seems to help. I'm afraid they're goners. Any help-- fast!!!

Symptoms of iron chlorosis include paling and yellowing of leaves, frequently with the leaf-veins staying green in the early stages. If leaves uniformly turn yellow, and then fall off, I am much more inclined to think it normal aging which will be followed by normal leaf growth, and not a symptom of a soil problem. Since too much iron can cause problems, I am slow to add anything to the soil as a single-ingredient "solution", when I am not sure that the problem is truly as I THINK it is. If I were convinced, through the range of symptoms exhibited by the plant in question, that iron chlorosis was the problem, then I would treat for it.