The tree looks like it was planted really close to the wall. I wonder what the soil is like in that area. I found some of my peonies that show signs of partial foliage yellowing were grown in an area with a bad subsoil layer. The growing roots cannot get the proper nutrients in such compacted soil and develop chlorosis. No amount of iron is going to fix the yellowing unless the soil problem is fixed. I guess if it does not impact the bloom productiion, it should not matter. If the tree peony starts to decline, I think that is a definite sign that it needs to be moved to better location.
