Equisetum is an unmistakable element of our flora, and our 10 species are normally quite distinct. However, the scouring rushes (E. hyemale, E. laevigatum, and E. variegatum) commonly hybridize, and these hybrids are often frequent and distinctive elements of certain communities, especially dunes, beaches, and other moist to fairly dry plant communities with bare soil. They are intermediate and abortive-spored, but virtually behave as species and may occur without one or both parents. These hybrids are included in the key as in many areas they are the commonest Equisetum, but they do make constructing a key complicated, and some specimens of the hybrids may not key well. Specimens ideally should include the plant base, with enough material to assess the fate of last years shoots, whether evergreen, deciduous, or with the lowest several internodes more or less evergreen, in addition to cones.
Equisetum gametophytes are green and are mostly found on bare soil, which also explains the low competition habitats and early successional character of many species.
1. Stems with multiple regular whorls of branches (in fertile stems of E. pratense and E. sylvaticum, which sporulate early in spring, branch whorls become well developed only after the cones mature – check carefully for these young branches); apex of cones rounded.
2. Main stem sheaths with the apical half at least coppery and membranous, sheath teeth uniformly coppery, usually fused together in groups of 3–4 teeth; branches again branched; sheath teeth on small branchlets outcurved.
2. Main stem sheaths greenish, sheath teeth black or dark brown with white margins, at least apically, mostly separate (sometimes fused in pairs); branches mostly or entirely simple; sheath teeth on small branchlets straight, often appressed.
3. Central cavity of stem at least 4/5 of the stem diameter, the stem easily flattened; plant an emergent aquatic.
E. fluviatile (in part)
3. Central cavity of stem less than 4/5 of the stem diameter, the stem firm; plants upland or wetland.
4. First internode (including sheath and teeth) of branches of the second lowest branch-bearing main stem node clearly longer than the length of the sheath (plus teeth) on that main stem node; branch bearing stems almost never fertile.
E. arvense (in part)
4. First internode (including sheath and teeth) of branches of the second lowest branch-bearing main stem node slightly to clearly shorter than the length of the sheath (plus teeth) on that main stem node; branch bearing stems sometimes fertile (except in E. telmateia).
5. Teeth of the lower main stem sheaths usually 14–28 per sheath, 5–8 mm long; green stems never fertile; plant collected long ago in the Keweenaw Peninsula.
E. telmateia (in part)
5. Teeth of the lower main stem sheaths 5–14 per sheath, 1.2–5 (–6) mm long; green stems sometimes fertile; plants widespread.
6. Teeth on sheaths of first branch internode usually 5, brown with conspicuous white margins; teeth of the lower main stem sheaths of vegetative shoots 2.9–5 (–6) mm long; upper internodes of main stem of vegetative shoots minutely granular on the angles.
E. palustre (in part)
6. Teeth on sheaths of first branch internode usually 3–4, whitish green or very pale brown, lacking a conspicuous white margin; teeth of the lower main stem sheaths on vegetative shoots 1.3–2.7 mm long; upper internodes of main stem of vegetative shoots with elongated papillae on the angles.
E. pratense (in part)
1. Stems unbranched or occasionally sparsely and irregularly branched; apex of cones pointed or rounded.
7. Stems deciduous (stems from the previous year not persistent and green), the texture herbaceous with a soft surface or even fleshy, the ridges lacking papillae; apex of cones rounded; lower portions of stem sheaths lacking a dark band.
8. Stems pale brown, fleshy, quickly withering after the spores are shed in spring; sheaths ± inflated, membranous.
9. Teeth ca. 6–14 per sheath; plant widespread.
10. Translucent tissue between very dark centers of sheath teeth brownish; firm sheath bases usually sharply differentiated from soft fleshy internodes.
E. arvense (in part, fertile stems)
10.Translucent tissue between pale to dark centers of sheath teeth whitish; sheath bases often grading into internodes without sharp differention in texture and ribbing (except in very young material).
E. pratense (in part, fertile stems)
9. Teeth ca. 20–30 per sheath; plant collected long ago in the Keweenaw Peninsula.
E. telmateia (in part, fertile stems)
8. Stems green, herbaceous, not withering immediately after the spores are shed; sheaths appressed to stem.
11. Sheaths clearly widened above; teeth deciduous at maturity and mostly absent (leaving a blackish stub to form a dark ring at the sheath apex), if present, thin and fragile, often curled and shriveled, brown with clear margins, but almost totally translucent; plants of moist (but usually not wet) to dry habitats.
11. Sheaths nearly cylindrical, only slightly widened at the top; teeth firm, persistent, dark brown to blackish, often with a green center below, and narrow to broad whitish margins; wetland plants.
12. Sheath teeth 1.2–3 mm long, with at most inconspicuous, narrow, hyaline margins; central cavity of stem about 4/5 of the stem diameter, the stem easily flattened.
E. fluviatile (in part)
12. Sheath teeth 2.9–5 (–6) mm long, with broad and conspicuous hyaline margins, especially apically; central cavity of the stem less than 1/2 the stem diameter, the stem thus not readily flattened.
E. palustre (in part)
7. Stems evergreen or nearly so (stems from last year persisting and remaining green for a substantial portion of their length – check the lower internodes), the texture leathery and the surface hard, the ridges usually papillose; apex of cones pointed; lower portions of stem sheaths sometimes dark banded.
13. Stems curled and twisted, mostly less than ca. 15 cm tall; central cavity absent.
13. Stems straight (sometimes from a decumbent base), normally more than ca. 15 cm tall; central cavity present.
14. Widest stems (measured between the sheaths) 0.8–3.2 (–3.6) mm in diameter; sheaths usually with only a single apical dark band (this occasionally broad and nearly the full length of the sheath).
15. Stems with the ridges ± smooth or with low, rounded irregularly ± spaced papillae in one row (the ridge therefore rounded, not flat-topped); only the lower stem portion evergreen.
E. ×nelsonii (see E. laevigatum )
15. Stems with the ridges clearly tuberculate with conspicuous, often ± flat-topped (viewed from the side), tubercles in two rows (thus forming a ± flat-topped ridge); stems fully evergreen.
16. Sheaths (including teeth) of the widest stems 2–5.3 mm long; widest stems (measured between the sheaths) 0.8–1.8 (–2.2) mm in diameter; spores green, well-formed.
16. Sheaths (including teeth) of the widest stems 5.3–8 mm long; widest stems (measured between the sheaths) up to 3.6 mm in diameter; spores whitish, shriveled and irregular.
E. ×mackayi (see E. variegatum )
14. Widest stems (measured between the sheaths) 3.3–12.5 mm in diameter; mature sheaths (at least the lower ones) often with a dark blackish band at the base in addition to being dark at the apex.
17. Upper sheaths with a broad dark band at the base as well as a narrow one at the apex, often gray in between at maturity; sheaths (teeth excluded) about as long as wide, or slightly wider; stem fully evergreen.
17. Upper (and sometimes also the lower) sheaths green except for the upper dark band, lacking a separate dark band near the base, without a gray zone (except sometimes in old stems); sheaths (teeth excluded) about 1–2 mm or more longer than the width at the widest point; only the lower portion of stem evergreen.
E. ×ferrissii (see E. hyemale )