Aerial stems monomorphic, green, branched or unbranched, 20--80 cm; hollow center small, to 1/3 stem diam.; vallecular canals nearly as large. Sheaths elongate, 4--9 × 2--5 mm; teeth dark, 5--10, narrow, 2--5 mm, margins white, scarious. Branches when present only from midstem nodes, spreading, hollow; ridges 4--6; valleys rounded; 1st internode of each branch shorter than subtending stem sheath; sheath teeth narrow. 2 n =216.

Cones maturing in summer. Marshes and swamps; 0--1500m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Calif., Idaho, Maine, Mich., Minn., Mont., N.H., N.Y., N.Dak., Oreg., Pa., Vt., Wash., Wis.; Eurasia s to Himalayas, n China, Korea, Japan.

The name Equisetum palustre var. americanum has been used for specimens from the flora that have longer teeth than those from Eurasia.

Perennial herbaceous fern ally 20 - 80 cm tall Spores: green, spheric, 30 - 45 microns in diameter, and released from lengthwise slit in spore sacs (sporangia). Stems: one to several, unbranched or branched at middle nodes, green (dying back in winter), 1 - 4 mm diameter, round with small diameter hollow center (up to one-third total stem diameter) and five to ten lengthwise angled ridges and alternating valleys on outside. The stem joints have distinct nodes, and the stomates are scattered on each side of the stem valleys, but never sunken. Branches: often absent or sparse at middle nodes, whorled, spreading, distinctly jointed at sheathed nodes, rounded with hollow center and four to six lengthwise ridges and alternating rounded valleys. The first internode of each branch is only 2 - 5 mm long and shorter than the subtending stem sheath from which it originates. Sheaths: green with black teeth at top, elongate in face view, 0.4 - 0.9 cm long, 0.2 - 0.5 cm wide. The sheaths are actually small, fused, whorls of leaves. Sheath teeth: five to ten, black with obvious papery white border, narrow, 2 - 5 mm long.

Similar species: Equisetum palustre is most similar to E. fluviatile, but that species has square stem sheaths, more than eleven teeth per sheath, and almost entirely black teeth that may have very narrow white edges. If unbranched, other members of this subgenus can be distinguished because the stems are not green. If branched, they can be distinguished because the first internode of the branches will be at least the same length as the subtending stem sheath that the branch arises from. Members of the subgenus Hippochaete will always have pointed tips on the fertile cones. However, E. laevigatum still may be confused since its cone is sometimes rounded, but that species has non-persistent teeth on the stem sheaths.

Habitat and ecology: Incredibly rare since the Chicago Region is somewhat south of its natural range with the only confirmed collection being from Walworth County, Wisconsin. In other areas of North America this species is found in marshes and swamps.

Occurence in the Chicago region: native

Notes: This plant has the aboveground stems surviving for only one year or even less. As in other members of this genus, E. palustre is able to absorb and deposit silica in its stems. The genus Equisetum (the only living members of the Equisetaceae family) has been studied often as a means to understand the evolution of vascular plants. They are often also used to interpret fossil plants in a group called the sphenopsids, which first appeared in the Devonian Period (about 360 to 415 million years ago). At that time, the plants were treelike and reached sizes of well over 10 m tall and even 1 m in diameter. The group reached a point of high diversity in the Carboniferous (300 to 360 million years ago) when the majority of our coal deposits started forming from these and other plants. Today only these small plants (up to about 2 m tall and under 4 cm diameter in our largest species) survive from this once land-dominating group.

Author: The Field Museum

Stems annual, all alike, 2-8 dm, deeply 5-10-sulcate, smooth to the touch but with minute transverse ridge-wrinkles on the angles, the stomates in a single broad band in each furrow, the central cavity small, less than 1/3 the diameter of the stem, about the size of the vallecular cavities; sheaths green, 5-10 mm, rather loose, with persistent teeth, these 3-7 mm, black or dark brown at least in part, with evident, often rather broad, pale and hyaline margins; branches few and irregular to numerous and whorled at the middle and upper nodes, 5-6-angled, simple, the first internode shorter than the associated stem-sheath; cone pedunculate, 1-3.5 cm, blunt, deciduous. Streambanks, wet meadows, and marshes; circumboreal, in Amer. s. to Pa., Ill., N.D., and Wash.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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