Abstract
A new species of Helianthus (Asteraceae), Helianthus waccamawensis Ungberg, sp. nov., is described from the Cape Fear Arch Region of North and South Carolina. Most recently, it has been included in H. floridanus A. Gray ex Chapm. but differs in its narrower leaves, achene and floral morphology, and habitat preferences. It superficially resembles H. angustifolius L. but differs in its extensively rhizomatous habit, shorter leaves, and achene and floral morphologies. It is further differentiated from the diploids H. floridanus and H. angustifolius by being hexaploid, and molecular phylogenetic data do not indicate a close relation to either species. In situ photos, diagnostic morphological photos, molecular phylogenetic data, cytological data, a range map, a list of additional collections, and a revised key for relevant southeastern Helianthus species are provided.
