This Eurasian alien has been widely distributed for "water gardens" and it has locally escaped across northeastern states, usually in artificial ponds (FNA 2). It may be slowly spreading into southeastern states, but it remains unknown on the Coastal Plain (W). Several other alien or adventive species in this genus may be expected in the south. In Ky. Marsilea is known from only two localities and it may have disappeared from both: (1) with colls. by F.T. McFarland and others during 1936-40 and perhaps earlier (KY, NY, PH etc.), from a large pond about 1-2 miles east of Lexington between Liberty Road and the C&O railroad (FAYE, probably at 38.0244, -84.4461); (2) with colls.by M.L. Branson and others during 1978-80 (EKY, APSC) from "pond in stream on NE corner of US 127 and KY 70" (CASE). See also notes in RC, who could not relocate plants after 1980-2010. The native range and ecology of M. quadrifolia remains somewhat mysterious in Eurasia, with two disjunct centers: central Europe, mostly from France to the Caucasus region; and Southeast Asia from India to Korea (Corli et al. 2021). Corli et al. stated: "In the past, M. quadrifolia was quite common in its range and gained the name "pig pasture weed"... However, at the end of the 20th century it became rare in its natural European range as a consequence of habitat loss and use of herbicides..."