1. Equisetum arvense L. (common horsetail, field horsetail) Pl. 17e,f,g; Map 31

E. arvense f. diffusum (A.A. Eaton) Clute

E. arvense f. ramulosum (Rupr.) Klinge

Aerial stems dimorphic. Vegetative stems 15–60 cm long, dying back at the end of the growing season, green, smooth, 10–14-ridged, with regular whorls of numerous branches in the upper 1/2–3/4 Leaf sheaths green. Teeth of the main stem leaf sheaths persistent, sometimes fused together in pairs, narrowly triangular, black, sometimes with lighter margins. Teeth of the branch leaf sheaths green, narrowly triangular, attenuate. Fertile stems 7–32 cm long, unbranched, pink to tan or white, smooth, dying back after the spores have been shed (ephemeral). Strobili 0.9–3.5 cm long, the tips rounded. 2n=about 216. April–May.

Scattered nearly throughout the state, but most common in the Glaciated Plains north of the Missouri River (U.S., Canada, Europe, Asia. Banks of river and streams, margins of lakes and ponds, pastures, roadsides, and railroad ballast; less commonly in freshwater marshes, fens, wet prairies, and edges of bottomland forests.

The fertile stems of this species are ephemeral, lasting only a few weeks until the strobili have matured and shed their spores. The sterile stems usually become noticeable as the fertile stems begin to wither.

A report of E. pratense Ehrh., the meadow horsetail, in northeastern Missouri (Jacobs, 1989) was based upon misdetermined specimens of E. arvense. In E. pratense, the fertile stems become green and develop branches after the spores have been shed, and the branch leaf sheath teeth are triangular and not acuminate. Its range is to the north of Missouri, with the closest populations in northeastern Iowa.