Whether planted in the ground or growing in pots, yellow leaves on hydrangea can suffer from various conditions that can cause their leaves to turn yellow. The most common reason for this is simply a lack of water. Hydrangeas require consistent moisture to thrive and if their soil is too dry they will begin to conserve it by shutting down certain processes such as photosynthesis. This will cause their leaves to turn yellow and wilt. The best solution is to add a little bit of all-purpose plant food to the soil or garden and feed as directed by the package.
A lack of nitrogen can also cause a hydrangea’s leaves to turn yellow. This is called chlorosis and it typically affects older leaves first as they start to wilt. Nitrogen chlorosis is easily fixed by using a balanced fertilizer or ensuring that the soil is properly amended with nitrogen rich materials such as compost and manure.
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Other problems that can cause hydrangea leaves to turn yellow include disease, low light levels, and weather conditions. Fungal diseases such as powdery mildew can result in a white, powdery growth on the outside of the leaf and if left untreated will spread to the inside of the leaves turning them yellow. Fungicides can be used to control powdery mildew.
Hydrangeas planted in the ground or in pots can be affected by nutrient deficiency, particularly iron. When the pH of a soil is too high (alkaline) the hydrangea will not be able to absorb iron from the substrate and this can lead to a condition known as iron chlorosis where the leaves will turn yellow between the veins. This can be corrected by amending the soil with products that lower the pH such as elemental sulfur or aluminum sulfate.