Answer: Hydrangeas are usually pest free but can sometimes develop rust, which is a fungal disease. There will be orange/yellow spores or spots on the leaves. A fungicide application should control this problem. Or, your plant may need to be fed. Yellowing leaves can be a symptom of nitrogen deficiency. Bigleaf hydrangea is also susceptible to iron chlorosis. Because iron becomes less available as pH increases, iron chlorosis is most likely to be found on plants growing on high pH soil. In contrast to nitrogen deficiency, which is expressed as yellowing of old leaves, iron chlorosis is found on new leaves.
Iron chlorosis can be corrected by the addition to iron to the soil. The best way to do this is to use a chelated iron product. You should be able to find one of these products at a garden center or the garden section of a hardware store or mass merchandiser. Follow package directions carefully. You should begin seeing results in a couple of weeks.
As long as the soil pH remains high, you will probably need to re-apply the chelated iron product yearly or whenever symptoms reappear. Lowering soil pH through application of aluminum sulfate or mulching with acid organic materials like pine bark is a long-term solution for iron chlorosis.
Hope this information helps you determine just what might be causing the problems with your hydrangea.
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