ABSTRACT Harperella (Harperella nodosum) is the only federally listed endangered plant species in the Chesapeake and Ohio Canal National Historical Park (C&O Canal NHP) in the National Park Service in Maryland and one of four federally listed endangered plant species in West Virginia. This paper contrasts unsuccessful and successful efforts to reintroduce harperella seedlings in the main stem of the flood-prone Potomac River and on opposite sides of the river in damp sites in western Maryland and in streams in northeastern West Virginia. The reintroduction efforts in Maryland unsuccessfully transplanted seedlings on cobble bars on the Maryland side of the Potomac River and in artificial sites in the prism of the C&O Canal where harperella had never been found; the reintroduction efforts in West Virginia successfully transplanted seedlings on cobble bars in two tributaries of the Potomac River in West Virginia where harperella populations once flourished but in the last decade or so had been decimated by floods and ice damage. This investigation has contributed to defining harperella’s life-cycle characteristics and ecological requirements. The data and observations presented can be used as a guide for reintroducing harperella seedlings and, by extension, other semiaquatic plant species into appropriate sites.