Answer: How often to apply this would depend on the condition of your plants and the pH of the soil they are growing in. You would only use liquid iron or chelated iron to counter yellowing foliage where the leaves are yellowing but the veins are staying green. If you spray it on you should see results in a few days, if you apply it to the soil around the plant you would see results in a week or two. Read and carefully follow all of the label directions.
In the meantime, you would also test your soil pH and correct it as needed to bring it into an acceptable range for gardenias. It should be below about 6.0 or so. Once the pH is corrected you would not need to use iron. Your county extension should be able to help you with the soil tests and interpreting the results. I hope this helps.
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