buttonbush, (genus Cephalanthus), genus of at least six species of shrubs or small trees of the madder family (Rubiaceae) native to Africa, Asia, and North America. Buttonbrush plants are named for their fragrant creamy white spherical flowers. They are sometimes used in landscaping and are a source of food for ducks and other waterfowl. In North America the common buttonbush (Cephalanthus occidentalis) is the best-known member of the genus and can reach up to 6 metres (20 feet) high in marshes and swamps.