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 .
Stems all alike, annual (reputedly sometimes perennial toward the southwest), mostly unbranched, 3-10 dm, 3-8 mm thick, with mostly (10-)16-32 ridges, these smooth or commonly with inconspicuous, low, transverse wrinkle-ridges; central cavity mostly 2/3-3/4 the diameter of the stem, much larger than the small vallecular ones; stomates in 2 lines in each furrow; sheaths mostly 7-15 mm, mainly green, black-banded at the apex only, or the lowest ones sometimes black also at the base or throughout, the dark, scarious-margined teeth promptly and regularly deciduous; cones short-pedunculate, 1-2.5 cm, blunt or often inconspicuously short-apiculate. Open wet places; s. B.C. to Baja Calif., e. to s. Ont., w. N.Y., O., w. Va., Ill., Okla., and Tex. (E. kansanum)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
VPAP (Yatskievych and Windham 2008), Allred and Ivey 2012, Heil et al. 2013
Common Name: smooth horsetail Wetland Status: FACW General: Rhizomatous perennial, 20-150 cm tall; stems smooth, hollow, slender, often flexulous, with longitudinal ridges and distinct nodes, usually unbranched; branches, if present, in whorls of up to 6 at one or few upper or middle nodes, sterile. Leaves: Minute and fused into a sheath that wraps around the stem at each node; sheath green with a black band around the top, tipped with a row of tiny black pointed teeth with narrow white margins; the teeth promptly fall off the lower stem nodes but remain attached to the nodes just below the stroboli; in autumn the sheaths can be partially and irregularly whitish and lack the black band as the stems start to die back for the winter. Sporangia: Plants dioecious (male and female structures are on separate plants); stroboli (spore-producing structures) located at tips of stems, the strobilus apex rounded or apiculate. Spores green and spheric. Ecology: Found in damp seeps, along streams, and at the bottom of canyons, from 3,000-8,000 ft (914-2438 m); stroboli maturing in spring to early summer. Notes: This is a very common horsetail along streams in the southwest. It is distinguished from other Equisetum spp by having mostly unbranched stems; having green spores; the sheath subtending each node is green with a black band around the top and tipped with a row of tiny black pointed teeth with thin white margins. The teeth on the sheaths at the lower nodes fall off early but the teeth are usually still present at the upper nodes throughout the growing season. E. hyemale is also common and widespread and appears similar, but has sheaths with a distinct dark band well below the top of the sheath; the teeth at the top of all sheaths reliably fall off early; and the stems do not die back to the ground each winter. Ethnobotany: Used medicinally as a contraceptive; to stimulate the kidneys; to treat bladder ailments, hemmoroids, high blood pressure, backaches, and colds; also used as a hair wash and to treat parts of the body affected by poison ivy. Etymology: Equisetum is from equus, horse and seta, bristle, while laevigatum means smooth or slippery. Synonyms: Equisetum funstonii, E. kansanum, E. laevigatum subsp. funstonii, Hippochaete laevigata Editor: SBuckley 2010, AHazelton 2017