I purchased seed from a reputable supplier that was labelled as buttonbush or Cephalanthus occidentalis. One seed germinated and managed to grow two inches tall in the first two years. I then planted it in a tiny pocket of soil with access to water. Five years later this plant is now almost 15' tall (5 m) and flowering. The species is described as having " hard spherical ball-like fruits". The plant I have has elongated flowering bodies.
I wonder what exactly I have growing...it's taller, the flower is different but it sure likes water. Here is a picture of a buttonbush from Ontario trees