This is the buttonbush, commonly found in wet areas. It has a surprising distribution in nature, being found in eastern Canada down to southern Mexico; it's also found in Cuba. The flowers are fragrant and the butterflies soon find them. Botanically it belongs to the same family as coffee and madder.
It grows so well here that every few years I cut it back severely: its response is always the same - quick regrowth.
This is such a distinctive looking plant that it is not likely to be confused with any other common garden plant, but there is another shrub rarely seen in our gardens, Adina rubella , which is similar in bloom. The Adina however has much smaller leaves. Adina and Cephalanthus both belong to the botanical family Rubiaceae.