Fixes for yellowing gardenia leaves include cutting back on watering, adjusting the pH of the soil and adding magnesium or iron, according to Gardening Know How. Chlorosis, or the condition that causes yellowing of the leaves, occurs when a plant does not have enough chlorophyll.
The first step to take for yellowing gardenia leaves is to check the soil moisture. While a gardenia requires moist soil, it does not do well when the soil is overly wet. Add some compost or organic matter to the soil to encourage a rich environment and ensure good drainage is possible.
If water is not an issue, check the pH of the soil. Gardenias require a soil pH between 5.0 and 6.5 to allow them to properly absorb minerals from the soil that are necessary for health. Because soil pH problems lead to mineral deficiency, which also causes leaf yellowing, minerals may need to be supplemented to stop the yellowing.
Magnesium deficiency produces leaves that are yellow at the base of the plant, but the tips remain green. A dose of Epsom salt added around the plant is a simple fix for this deficiency. Iron deficiency produces leaves with yellow tips, while the base of the leaves remains green. Supplement an iron deficiency in spring using chelate to stop yellowing.