I purchased a Chinese tree peony with large yellow flowers about 5 years ago. Problem 1, Every year it sprouts perennial white peonies that grow quickly up through the tree peony. I have to pull them out. Problem 2, After 5 years the tree peony is no taller than it was at the end of the first year. It is about 18" tall, including leaves. New sprouts come from existing buds on the branches, buy the branches die back to about 6" right away in the early winter. I live in hardiness zone 4-5, southwest of the USA Great Lakes by about a hundred miles.
Plant it deeper, so the tree peony roots out above the graft. This doesn't work (is lethal) for most grafted plants, but does work well with tree peonies.
Tree peonies with P. lutea in the mix (the yellow ones) are not as hardy (Z5-6) as tree peonies that are primarily P. suffruticosa (Z4)
