Answer: There are several reasons why you might see yellowing. In June, you will see some of the older leaves yellow and drop as a natural occurrence.
Over watering can cause yellowing; the soil should be evenly moist like a wrung out sponge, not sopping wet or saturated. Since you did not have roots showing signs of rots, this is probably not the cause.
Pests can cause yellowing/ Look for signs of spider mites (in addition to stippled discoloration and in advanced stages yellowing and also brown dry leaves, you should also find fine webbing especially along the stem and undersides of leaves.) If you find these, they can be treated with commercially formulated insecticidal soap per the label directions.
Another cause of yellowing is soil that is not acidic enough. A quick fix for this problem is to use Ironite or a chelated iron product per the label directions. With a foliar application you should seen greening in a few days if this is the problem. If the Ironite gives good results, then you know pH is the culprit. You would test your soil pH and adjust it as indicated by the test results.
While Hollytone would help increase the acidity of the soil slightly, it works very slowly and would not provide immediate relief to the plants. It is however an excellent fertilizer to use routinely on your gardenias -- per the label directions -- so it is a good investment.
I would also suggest you consult with your local county extension for help in diagnosing the problem. They will be familiar with any localized conditions that might be contributing to the problem and can ask to see samples of the foliage as well as photos of the plants. And they can help with soil tests and interpreting the results.
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