Podostemum ceratophyllum Michaux. Threadfoot, Riverweed, Riffleweed. Phen: May-Jul. Hab: Attached to rocks and dams in rapidly or slowly flowing water. Dist: NS, ME, and QC south to sw. GA, s. AL, s. MS, se. LA (Florida parishes), AR, and w. TN; disjunct in the Ozark-Ouachita Highlands of w. AR and e. OK; Dominican Republic; Honduras.

Origin/Endemic status: Native

Taxonomy Comments: Fehrmann, Philbrick, & Halliburton (2012) demonstrated very low genetic diversity in the populations of Podostemum ceratophyllum north of the glacial maximum, in the Ozark-Ouachita Highlands, and in Central America, and high genetic diversity in the portion of the unglaciated southeast east of the Mississippi River.

Synonymy: = Ar, C, F, FNA6, G, K1, K3, K4, NE, NY, Pa, RAB, Tn, Va, W, WV, Graham & Wood (1975); = Podostemon ceratophyllum – GW2, Tx, orthographic variant; > Podostemon abrotanoides Nutt. – S, S13; > Podostemon ceratophyllum – S, S13, orthographic variant

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Wetland Indicator Status:

• Atlantic and Gulf Coastal Plain: OBL
• Eastern Mountains and Piedmont: OBL
• Midwest: OBL
• Northcentral & Northeast: OBL

Heliophily: 8