When hydrangea tree leaves turning yellow turn yellow they often indicate that the plant isn’t getting enough sunlight. This can be a result of the upper leaves shading the lower ones, or it could be a sign that the plant is deficient in nutrients like iron. To correct this issue, water the hydrangea deeply twice or three times per week and fertilize it with nitrogen-rich fertilizer to keep the leaves green.
If the leaves have a yellowish cast to them, it’s more likely that they are suffering from root rot or other fungal diseases. This can occur when the soil is too wet and doesn’t drain well. A soil test can help determine which disease or fungus is causing the yellowing leaves so it can be treated.
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Hydrangeas can also suffer from nutrient deficiencies, especially when the soil pH is too high, which prevents them from absorbing the necessary micronutrients during flowering and growth. This is a condition called iron chlorosis. In this case, the new leaves on the hydrangea turn yellow and then brown as they begin to die off. Hydrangeas can be fed with a magnesium sulfate fertilizer to treat this issue, or they can be drenched with a solution of two-level teaspoons of citric acid per gallon of water to bring the soil pH down to a level that is healthy for the hydrangea.
Another symptom that hydrangeas can suffer from is leaf scorch. This occurs when the upper and lower leaves of the shrub start to look yellow and dry, with irregular dry blotches on the leaf tips and edges. It’s most common in older leaves on the top and side of the hydrangea that gets the most sun. When this happens, the hydrangea is simply trying to put its energy into the most important tasks, creating blooms, so it starts dropping less important leaves first.