I could have sworn I knew this plant as sweetgale or sweet pepperbush or something. But turns out it's meadowsweet. Not the meadowsweet of England that's also called meadwort and was the orginal source of aspirin, but a Spirea that is host for the lovely azure blues, those little blue butterflies that seem to be everywhere sometimes, and nowhere oftentimes.
Those blues have an intricate relationship with ants, but that's a story for another day.