I have both vines and shrubs with the leaves turning varigated. I’ve fertilized them with miracle grow (the nursery said this would address any potential iron issues) and still this has been happening every year for the last 4 yrs! they face south but are fairly protected, in clay soil, watered and mulched. I thought it might be the black walnut tree that is close but they seem ok with it according to research. Thoughts? just add some iron sulphate? plus I have a tree at the front that gets way more sun and is drooping in this heat. I’ve been soaking it daily for about 20 min and it revives briefly but then droops again and some leaves are turning yellow and falling off. aaggghhh… too much H20 or is it needing something else? plus it starts to flower but they never open! please help and thank you sooo much!
First, the yellow (chlorotic) leaves on your hydrangea shrubs. You are right in thinking that this may be an iron deficiency … either not enough in the soil or, more likely, a difficulty in it being carried to the plant due to the alkalinity of the soil. So, the application of a foliar spray of iron sulfate or iron chelate may result in a temporary improvement. But for long term results, you should test your soil’s alkalinity to see if that needs improving, in which case aluminum sulfate may be the answer. See Purdue University’s discussion of the issue here.
Regarding the tree, do you mean the standard form of Pee Gee hydrangea?
I agree with your suspicious that you have been over watering, since 20 minutes daily is a LOT of water, especially in clay soil, where it does not drain away quickly.
See the details of our answer to a previous inquirer here.
Once again, the clay soil may be quite alkaline and hydrangeas do prefer acidic soil.