If it's only a few, I would not worry. Leaves have to be occasionally shed.

How long has the plant been in the ground?

If it is competing with tree roots for moisture and nutrients, the leaves will yellow big time. The only answer to that scenario is to move the shrub.

If you want to get all kinds of advise and laugh your head off put the follow the link below.

So you want to grow a gardenia, huh? (also known as the infamous suicidal gardenia thread)

Here is a link that might be useful: So you want to grow a gardenia, huh? (also known as the infamous suicidal gardenia thread)

If the leaves are turning yellow and the veins stay green it needs nitrogen. They are very heavy feeders and the one of the first plants in my garden to show fertilizer distress when it is needed.

If it is the new young leaves then it is a Nitrogen problem. use a high Nitrogen fert.

timestocome said that if its has interveinal chlorosis (green veins on a yellow leaf) it is nitrogen but this is an iron problem.

get something like ironite. gardenias like acidic soil. if you have alkaline it will do very poor.

I would apply the iron and use a complete fert. that is made for acid loving plants.

the interveinal chlorosis could also mean a Manganese deficiency which is impossible to tell the difference in it and an iron deficiency without a soil or tissue sample. this is why a complete fert. should be used.

I have an August Beauty that is doing the same thing. It is also happening to my Grandmother's August Beauty as well as one of her old fashioned Gardenias. It is the old leaves that are turning yellow and the plants are still putting out new dark green leaves. I fertilized them with Hollytone and they all look much better.