Aerial stems lasting less than a year, occasionally overwintering in the southwestern United States, usually unbranched, 20--150 cm; lines of stomates single; ridges 10--32. Sheaths green, elongate, 7--15 × 3--9 mm; teeth 10--32, articulate and usually shed early, leaving dark rim on sheath. Cone apex rounded to apiculate with blunt tip; spores green, spheric. 2 n =216. Cones maturing in spring--early summer. Moist prairies, riverbanks, roadsides; 1530--3500 m; Alta., B.C., Man., Ont., Que., Sask.; Ariz., Ark., Calif., Colo., Idaho, Ill., Ind., Iowa, Kans., Mich., Minn., Mo., Mont., Nebr., Nev., N.Mex., N.Dak., Ohio, Okla., Oreg., S.Dak., Tex., Utah, Wash., Wis., Wyo.; n Mexico including Baja California. Schaffner named this species Equisetum kansanum because he applied the name E . laevigatum to what we now know is the hybrid E . × ferrissii . The coarser-stemmed, occasionally persistent forms in the southwestern United States have been called Equisetum funstonii .

AERIAL STEMS: monomorphic, persisting through 1 growing season, 20–110 cm long, 10–32-ridged, erect (to prostrate after flooding), the surface smooth, usually bright green at maturity, unbranched or with irregular, scattered branches in woundforms, these with 6–15 ridges. SHEATHS: longer than wide, green with a narrow, dark tip; teeth 10–32 per sheath (6–15 on branches), 2–3 mm long, gray to black, shed early. STROBILI 8–25 mm long, the tips rounded. SPORES 35–70 μm in diameter. NOTES: Banks of streams and rivers, marshy meadows, wet, sandy areas: Apache, Cochise, Coconino, Gila, Greenlee, Maricopa, Mohave, Navajo, Pima, Santa Cruz, and Yavapai cos. (Fig. 1C); 1000–2600 m (3500–8400 ft); May–Aug; w U.S. e to OH, Can. n Mex. For a discussion of the hybrid with E. hyemale, see the treatment of that species.