Answer: Gardenias tend to lose their oldest leaves about this time of year, in preparation for producing healthy new foliage. If your plants have been in the ground for 3 years and have been doing well up until now, the yellowing, dropping leaves might be a reaction to an unseasonal warm or cold spell, drying winds, water stress or a need for fertilizer. It sounds as though you've already fed them and applied ironite so they should have nutrients available - it sometimes takes several weeks for them to show any improvement after being fed. Make sure they're getting adequate water and closely inspect the leaves for signs of insect pests. Aphids can cause yellowing leaves, as can spider mites. If you find webbing on the undersides of the leaves or where the leaves meet the stems, you've got a spider mite problem. A miticide will take care of them. Hosing the plants off once a week or so will keep spidermites from camping out in your gardenias and will also rinse dust off the leaves.
I'd just give them a little more time to perk up. Sometimes a light pruning will encourage a flush of healthy green growth.
Hope this helps!
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