Whats your PH? Where I live on this gyp hill, I have to constantly treat all my peonies with iron. We have alkaline soil and alkaline water both. i add sulpher to the soil every fall. Once I actually got one beds PH down to 7.2.Most of the beds run around 8. If I ever move again, the soil gets tested first. I buy iron from Rosecare.com. It's expensive but works quickly.

I have the same problem with one of my herbaceous peonies. My Alexander Woollcott herbaceous peony has the same exact yellow coloring as your tree peony with chlorosis and the 4 peonies growing 2 feet from this plant in the same bed show no such yellowing foliage condition. How can one plant be deficient in certain nutrients and the others do not when the soil condition is one and the same?

I really have no idea because many of my tree peonies have absolute no signs of chlorosis. I guess I might have to add some gypsum and work it in to improve the soil around the ones that are looking like this, and hope that it helps out with whatever is wrong with it.

Perhaps some ironite and rabbit manure may help too.

Most of them look nice and green like this.