Stems usually glabrous, rarely sparsely pubescent proximally. Leaf blades: abaxial surface usually glabrous, rarely sparsely pubescent proximally. 2n = 32. Flowering Mar-Sep. Marshlands, pastures, prairies, meadows, swales, flats, sand bars, wet grounds, stream banks, moist depressions, ditches, estuaries, waste grounds, roadsides, sloughs, shores of lakes and ponds, bogs, thickets, grasslands; 0-3200[-4000] m; Alta., B.C., Man., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Europe; Asia; introduced in n Mexico, South America, Australia.