I'd like to identify this tree, growing on the margin of a drainage ditch next to a fairly dry gravel parking lot in coastal Maine, USA (hardiness zone 5b). It's currently about 8' tall, with opposite branching; exfoliating bark; 6" ovate leaves with fine-toothed serrated margins that occur in clusters. It is currently (August) flowering, with multiple small white buds along a 10" central stems emerging from the centers of most leaf clusters. Sniffing the (perhaps mostly gone by) flowers over a couple of days indicated no odor.
Some pictures. First, the whole tree:
Second, a leaf cluster viewed from the top and the side:
And the trunk:
(Feel free to correct my terminology, and thanks.)