Buttonbrush or Cephalanthus occidentalis is described as follows by the Wikipedia article, "Cephalanthus occidentalis."
Cephalanthus occidentalis is a species of flowering plant in the coffee family, Rubiaceae, that is native to eastern and southern North America. Common names include buttonbush, common buttonbush, button-willow and honey-bells.Here is a further quote from the Missouri Botanical Garden.
Buttonbush is a somewhat coarse, deciduous shrub with an open-rounded habit that typically grows 6-12’ (infrequently to 20’) tall. It is common throughout Missouri, most frequently occurring in wet open areas, low woods, thickets, swamps, upland sink-hole ponds, river bottomland and stream/pond margins (Steyermark). Tiny, tubular, 5-lobed, fragrant white flowers appear in dense, spherical, long-stalked flower heads (to 1.5” diameter) in early to mid-summer. Long, projecting styles give the flower heads a distinctively pincushion-like appearance. Flower heads are very attractive to bees and butterflies. Flower heads mature into hard spherical ball-like fruits consisting of multiple tiny two-seeded nutlets. Fruiting heads usually persist throughout the winter. Ovate to elliptic glossy bright green leaves (to 6” long) are in pairs or whorls. Leaves emerge late in spring (May). Genus name comes from the Greek words cephalo (head) and anthos (flower).We found these growing in Washington, D.C. at the Kenilworth Park and Aquatic Gardens.