Yellow hydrangea leaves start to wilt and turn yellow they are often suffering from leaf scorch. This is due to the hydrangea being exposed to full sun for too long. It’s a good idea to move your hydrangea into an area that will get morning sunlight and be shaded during the afternoon.
Another common cause for yellow hydrangea leaves is iron deficiency or chlorosis. You’ll know this is the issue if the leaves are turning yellow between the veins but still have green veins. This problem can be caused by hard water or a lack of iron in the soil. A simple solution for this is to give your plant a feeding of iron chelate.
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Other causes for yellow hydrangea leaves can include pest infestation, fertilizer deficiency, improper drainage and poor air circulation. Make sure your hydrangea is being fed properly with a general plant food two to three times a season. Also, test your soil for pH and adjust it with soil acidifiers if necessary. Finally, be sure your hydrangea has adequate drainage by working sand into the soil or planting in a well-draining garden bed.
Hydrangeas planted in the ground have the benefit of having healthy, well-developed roots that can take up nutrients with ease. However, hydrangeas in pots need a little extra help as their roots are more confined. When using a high-powered phosphorus or nitrogen fertilizer, be sure to dilute it with at least one-level teaspoon of citric acid found in most grocery stores (it’s the kind used to pickle and can). Adding a bit of organic matter to the top layer of the soil is also an excellent way to improve hydrangea nutrition.