I have both tree peonies and herbaceous peonies in the garden. The tree peonies have woody stems, bloom earlier and have larger, silky flowers (and more $$$). My garden is pushing the coldness zones a little for the tree peonies. This year they bloomed when we had that spell of silly 95 degree weather and the flowers only lasted two or three days. Jeez.
The old fashioned herbaceous peonies are tougher than nails and are the plants that what most people think of peonies. The limits of herbaceous peonies are that the flowers can be too heavy to support and they are limited in color. I have modern crosses with smaller flowers, but all of the herbaceous peonies fall somewhere between white, pink and dark red. The tree peonies have some that lean into a purplish and others that are clear yellow.
There were thousands of attempts to cross tree peonies with herbaceous peonies. In the late 1940s a Japanese horticulturist, Toichi Itoh, managed to pull it off, but he didn't know what he had because he died before they bloomed. All of the yellow herbaceous peonies in existence are intersectional crosses that trace their heritage back to Dr. Itoh. And damn they were expensive - hundreds of dollars apiece. Someday they may be as cheap as old fashioned peonies, but I am 73 and if so, they will have to be on my grave. Eventually the prices have come down some, still expensive, but within my range.
