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Answer

The most common cause for yellow leaves on hydrangea is iron chlorosis, iron becomes less available in high pH soils. Iron chlorosis in hydrangea is expressed as the new leaves turning yellow as they develop. Iron chlorosis can be corrected by adding a chelated iron product which can be found at your local garden center Follow the directions on the package carefully, you should see results in a couple weeks. It may need to be added once a year in soils with a high pH.

The other possible cause for yellow leaves on hydrangea is a nitrogen deficiency which is expressed as the yellowing of older leaves. Nitrogen deficiency can be correct by simply adding a balanced fertilizer or a hydrangea specific formula.

Hope this helps!