This large shrub occurs mostly in the central and southern Appalachians, and within Ky. it is largely restricted to rocky banks along larger streams in the upper Cumberland Rv. watershed. In addition to its distinctive white flowers with long sepals that appear in late May to July (Cr, J, W), arborescens is glabrous in virtually all parts, and non-colonial with stems up to 5-6 m tall. Other "azaleas" (sect. Pentanthera) of Ky. reach only 1-4 (5) m. The related southeastern species, R. viscosum (L.) Torr., is known from some nearby counties in Tenn. and Va., and may be expected in boggy sites; it has shorter sepals (0.1-1 mm versus 1.5-5 mm) and corollas are pubescent within. No vwerified colls. have been located.