This Checklist combines and updates the original checklist (1935)and supplementary list (1953) of the vascular flora published by Ellsworth P.Killip and Sydney F. Blake. Plummers Island, a small (4.8 ha) island in the PotomacRiver near the Maryland shore in Montgomery County about 14.5 kmupriver (northwest) from the center of the District of Columbia, has been intensivelystudied by biologists for over 100 years, mostly by members of the WashingtonBiologists' Field Club. Floristic studies have always included 14.2 ha ofthe adjacent Maryland shore (so-called “mainland”), part of the original holdingsthat belonged to the Club for many years. A total of 885 vascular plant specieshas been recorded over the years, including 76 known only from the mainland.Of these, 703 (79%) are native, 182 (21%) introduced, and 92 are being reportedhere for the first time. Based on the 300 species seen again in 2003–2005, it isestimated that the present flora totals about 350 species. The history of floristicwork here, changes in the vegetation cover and flora, including a section by Wellson the present plant communities, and known deliberate introductions arediscussed.