Answer: Proper fertilization is important for gardenias. Most gardenias grow and flower well with applications of slow release or water soluble fertilizers with moderate levels nitrogen, phosphorous and potassium. Suggested fertilizer ratios are 3:1:2 or 3:1:3 (e.g. 15-5-10 or 15-5-15). If you can't find these specific fertilizers, use something with a similar ratio of nutrients. I prefer the general purpose, slow release fertilizers. Most last for 3 or more months. The first application should be made in March, and the second in July. If you are using a liquid plant food, begin applying in March and continue fertilizing every two or three weeks through September.
Frequently plants will become yellow due to a deficiency of one or more micro nutrients, usually iron. This can be corrected with the addition of a chelated form of iron, most often sold in a liquid form in the garden centers. Remember to apply "chelated" iron; other forms of iron just won't work. Make the application of iron on an as-needed basis directly to the soil around the gardenia.
Yellowing of gardenia leaves can be caused by things other than nutritional deficiencies. Potential causes include; insufficient light, over watering, poor drainage, and too low soil temperatures. Several investigations have indicated that a soil temperature below 70 degrees causes leaf yellowing. Inconsistent watering can also cause leaf yellowing and a browning of the leaf tip and edges. Some leaf yellowing of older leaves is natural and will occur in the winter prior to new spring growth.
Hope this information is helpful!
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