My hydrangeas have been yellowing for a couple of years. Up until now I‘ve treated them with an iron supplement. Note – Yellow leaves are a very different issue than ASTER YELLOWS.
After doing some research, I’m ready to work on some probable causes:
– lower the pH
– correct a drainage problem
– As always, I should be doing a soil test before doing anything else!
Here are three excellent articles on identifying and dealing with chlorosis: