Is it possible that one of the gardenias has poor drainage? That can cause the leaves to yellow and drop off. Also, take a look at the color of the leaves. If they are green with yellow tips, that can be a sign the plant needs iron. If it’s the opposite, yellow leaves with green tips, your plant needs magnesium. One more cause of yellow leaves is the pH of the soil is too high. The pH should be 5.0-6.5. I hope this helps and your gardenia gets better soon.
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