Tree with exfoliating bark and ovate leaves

Question

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:

The flowers:

... one flower in closeup:

And the trunk:

and a closeup of the bark (with newspaper to persuade my cell camera to focus):

(Feel free to correct my terminology, and thanks.)

Answer 1

I think its Clethra, and as it's in flower now, Clethra alnifolia, although you haven't mentioned fragrance in regard to the flowers, and they are fragrant. Common name sweet pepperbush or summersweet, it's a deciduous shrub hardy down to USDA zone 3. It likes moist soil, which explains why it's chosen to grow on the edge of the drainage ditch, and is native to your area. More info https://plants.ces.ncsu.edu/plants/all/clethra-alnifolia/