Plant in wet area of Washington, D.C.

This plant is growing on the bank of a pond in an aquatic garden.

 

Buttonbush, Cephalanthus occidentalis.

 

Thanks. I couldn't find it in any of my identifier books, but I see now that I was looking in the wrong section. What I thought were lots of petals, were really lots of flowers each with 4 petals.

 

It's an old friend. Lots of it up in New Hampshire, where I used to spend summers as a kid. Always was intrigued with the Sputnik blossoms.
Looks as if you have some colorful insect action, too!