If it's grafted onto an herbaceous peony you are now growing a piece of that instead of the tree peony part.

^^ Yep, that's what I was going to say -- they're typically grafted. Hey - free peony!

Huh, I had no idea it could be grafted onto an herbaceous peony. I might email Song Sparrow nursery to see if it's grafted or not. Thanks!

When I have seen the rootstocks allowed to grow up through the tree peony and flower in my area the flowers happened to be mediocre in those instances.

The convention with herbaceous peonies is that you have to get a section of the crown with at least one shoot bud on it for a new plant to result. So it may be that your piece is going to make a little root growth for a time but ultimately fail due to being incomplete - I don't know.

Embothrium, that's my thought too. I'm hard pressed to see how this can sprout. I'll hold out a bit longer, just for kicks.

It may be the convention but if it was and if you have done any grafting you would know that the roots commonly used contain no crown tissue. There are too many tree peonies that produce herbaceous shoots for that idea to be true.

Gardenguides.com says ability to form new eyes on root sections varies with "some hybrids" being able to do it.

Just fyi, it never sprouted and when I emptied the pot there were no active rootlets growing. Oh well! Thanks to all who offered advice.