Aerial stems dimorphic; vegetative stems green, branched, 2--60(--100) cm; hollow center 1/3--2/3 stem diam. Sheaths squarish in face view, 2--5(--10) × 2--5(--9) mm; teeth dark, 4--14, narrow, 1--3.5 mm, often cohering in pairs. Branches in regular whorls, ascending, solid; ridges 3--4; valleys channeled; 1st internode of each branch longer than subtending stem sheath; sheath teeth attenuate. Fertile stems brown, lacking stomates, unbranched, shorter than vegetative stems, with larger sheaths, fleshy, ephemeral. 2 n =ca. 216. Cones maturing in early spring. Roadsides, riverbanks, fields, marshes, pastures, tundra; 0--3200 m; Greenland; St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; all states except Fla., La., Miss., S.C.; Eurasia s to Himalayas, c China, Korea, Japan. Among the many infraspecific taxa that have been named in this species, Equisetum arvense var. boreale Bongard has been most generally accepted and has been applied to plants with tall, erect stems with 3-ridged branches. Because both 3-ridged and 4-ridged branches may occur on a single stem, the variety boreale is not recognized here as distinct (R.L. Hauke 1966).

AERIAL STEMS: dimorphic, erect, the surface smooth; fertile stems 7–35 cm long, ephemeral, tan to pink, unbranched; vegetative stems 10–60 cm long, 5–14-ridged, persisting through 1 growing season, green, with regular whorls of branches, these 3- or 4-ridged. SHEATHS: as long as or longer than wide, lacking bands or markings; teeth 5–14 per sheath (3–4 on branches), 1–3 mm long (-8 mm on fertile stems), mostly black, persistent. STROBILI: 5–35 mm long, the tips rounded. SPORES: 35–70 μm in diameter. NOTES: Wet soil of montane meadows and stream banks, less commonly on moist rock ledges: Apache, Coconino, Gila, Graham, Mohave, Navajo, Pima, Yavapai cos. (Fig. 1A); 1400–2700 m (4500–8800 ft); nearly worldwide. The fertile stems of E. arvense are produced in April and May before the vegetative ones and wither soon after the spores are shed. They tend to be stouter than the vegetative stems, with fewer ridges and larger sheaths. The vegetative stems have regular whorls of branches, these with only 3–4 ridges. Care must be taken not to confuse these branches with those of wound-forms of other Arizona species, which develop following damage by flood-waters, and which occur irregularly and generally have more ridges. REFERENCES: G.Yatskievych and M.D. Windham , 2008, Vascular Plants of Arizona: Equisetaceae. CANOTIA 4 (2): 41–45.