This is a wonderful plant. It has an interesting shape in every season, the “nutlets” make a terrific cut flower and the deer don’t like it one bit. Below is the description from Fedco Trees, which is one of the only places I could find information on this lovely plant.
6-10′ x 8′ Loose rounded branchy shrub with bright green foliage and masses of highly attractive unusual spherical fragrant white flowers. Suitable for the garden, the stream or pond, or even an old floating log. Don’t be surprised if you come across buttonbush growing in the river next time you head out in the canoe. But this is no invasive plant. Flowers appear for 4-6 weeks in summer, a magnet for honey bees. They also make interesting dried flowers. The seeds (nutlets) make good fodder for the ducks in the fall. Prefers moist soils! Recommended for naturalizing. Native to eastern U.S., west to New Mexico. Z4.
I haven’t tried growing it on a floating log, but it does very well in the culvert next to the driveway.