Equisetum species

The Horsetails and Scouring Rushes

Wood Horsetail, Photo © by Earl J.S. Rook
Wood Horsetail
Photo © by Earl J.S. Rook

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The natural history of the northwoods


Name:

  • Equisetum, from the Latin, equus, "horse", and seta, "bristle, animal hair". A reference to the coarse black roots of Equisetum fluviatile, Water Horsetail
  • Other common names include prêle (Qué)

Taxonomy:

  • Kingdom Plantae, the Plants
    • Division Equisetophyta, the Horsetails
      • Class Equisetopsida, the Horsetails
        • Order Equisetales, the Horsetails
          • Family Equisetaceae, the Horsetails
            • Genus Equisetum, the Horsetails
  • Taxonomic Serial Number: 17148
  • North Country Species
  • Equisetum x ferrissii (StL)
    Equisetum x mackaii (StL)
    Most of those in Equisetum subg. Equisetum are still unknown in North America, but they should be sought, especially north of 45° N latitude.

    Species 15 (11 in the flora): nearly worldwide.

Description:

  • A genus of primitive rhizomatous, perennials related to the ferns.
  • Leaves in whorls, reduced and fused into a sheath at the stem joints. The number of leaves usually equals the number of stem ridges. Food is produced in the stems and branches (photosynthesis), the leaves being so reduced in size as to be nearly useless.
  • Stems upright, hollow, and cylindrical; jointed in appearance, with longitudinally ridged surface containing silica deposits. Stems with hollow center and series of small carinal (under the ridges) and larger vallecular (under the valleys) canals. Stem ridges traversing length of internode and continuing into sheaths, terminating in sheath teeth. Stems arise at close intervals from long, creeping, underground rhizomes, often forming colonies. Separate fertile and sterile stems may be similar, or distinctive, in appearance. Fertile stems emerge before the sterile, bearing a cone at the tip. Aerial stems of Equisetum vary considerably in habit and appearance, even on individual plants, because of environmentally induced modifications affecting height and branching.
  • Branches absent or produced at the nodes
  • Cones at tips of fertile stems; composed of whorls of hexagonal, sporangiophores bearing sporangia
  • Spores tiny, green, each with four hygroscopic, strap-like structures known as elators.

Identification:

  • Identifiable as horsetails by the upright, hollow, jointed, cylindrical stems with inconsequential and easily overlooked leaves.
  • Distinguished from one another by presence or absence of branching, number of leaves, and habitat.
  • Field Marks
    • presence or absence of branching
    • branch segment length at point of origin
    • branch angle at point of origin
    • size of central stem hollow and adjacent channels, if any
    • number and color of leaves (sheath teeth)

Identification of North Country Equisetum

  • Distinguishing between the two subgenera of Equisetum, the Horsetails (Subg. Equisetum) and the Scouring Rushes (Subg. Hippochaete) is a fairly simple matter. Horsetails have branches, in regular whorls, giving them a somewhat bushy look. Scouring Rushes lack branches or, when branches are present, they are few and very irregular. Because Equisetum are rarely encountered as lone stems in the field, a quick survey of the clump is sufficient to determine subgenus.
  • If you have a Horsetail (a branched Equisetum), look closely at the branching. If the branches themselves divide into multiple branches, then you have Equisetum sylvaticum, Wood Horsetail. The leaves (sheath teeth) will be reddish and papery. If the branches are themselves unbranched look at the segmentation of the branches.
  • If the first branch segment at the stem is noticeably shorter than the remaining segments, examine a section of stem:
    • If the stem has a very thin wall with a very large hollow in the center, then you hav eEquisetum fluviatile, Water Horsetail. It is typically found in an aquatic habitat (usually growing in water) and will have more than 11 leaves (sheath teeth) per joint, often black throughout or with narrow white margins.
    • If the stem has narrow central hollow with large side channels, then you have Equisetum palustre, Marsh Horsetail. It is typically found near, but rarely actually growing in, water. It will have stem joints with a stretched or elongated appearance and fewer than 11 leaves, with prominent white margins and dark centers, per sheath.
  • If the first branch segment at the stem is actually longer than the stem segment from which it emerges, look at the shape of the branch and the angle of the branching.
    • If the branches are three-sided and horizontal to drooping, then you have Equisetum pratense, Meadow Horsetail.
    • If the branches are four angled and vigorously ascending, then you have Equisetum arvense, Field Horsetail.
  • If you have a Scouring Rush (an unbranched Equisetem), look closely at the stem. If the stems are very slender (about the diameter of a #2 pencil lead), low growing and tangled rather than upright, then you have Equisetum scirpoides, Dwarf Scouring Rush.
  • If you have a typical upright Scouring Rush, look closely at the stem joints.
    • If the stem joints are marked by an ashy grey band, then you have Equisetum hyemale, Tall Scouring Rush. The stems will be rough textured.
    • If the stem joints are marked by a narrow, black band, then you have Equisetum laevigatum, Smooth Scouring Rush. The stems will be smooth and the leaves (sheath teeth) tiny or absent.
    • If the stem joints are marked by a distinct white margin, then you have Equisetum variegatum, Variegated Scouring Rush. The stem ridges will be minutely grooved and the sheath teeth persistent throughout the season.

8 (2) First internode of each branch shorter than subtending stem sheath; branch valleys rounded. (9)
+ First internode of each branch equal to or longer than subtending stem sheath; branch valleys channeled. (11)

  • 9 (8) + Branches hollow, branch ridges rounded. (10)

    10 (9) Sheaths square in face view; teeth more than 11 per sheath, dark, occasionally with narrow white margins, 2--3 mm. E. fluviatile
    + Sheaths elongate in face view; teeth fewer than 11 per sheath, with prominent white margins and dark centers, 2--5 mm. E. palustre

    11 (8) Aerial stem sheath teeth reddish, papery, coherent in 3--4 large groups; stem branches also branched. E. sylvaticum
    + Aerial stem sheath teeth dark, firm, separate or coherent in more than 4 small groups; stem branches unbranched. (12)

    12 (11) Branch sheath teeth deltate; branches spreading. E. pratense
    + Branch sheath teeth attenuate; branches ascending. (13)

    13 (12) Lowest whorls of branches with 1st internode longer than sheath; spores green, spheric. E. arvense


    14 (1) Aerial stems unbranched or with scattered branches; stomatal lines always single. (15)

    15 (14) Cone apex rounded; aerial stems annual. E. laevigatum
    + Cone apex pointed; aerial stems perennial (at least persisting over winter in California populations of E. laevigatum). (16)


    16 (15) Spores white, misshapen. (17)
    + Spores green, spheric. (19)

    17 (16) + Sheaths dark-girdled; teeth persistent or shed. (18)


    18 (17) Teeth 14 or fewer per sheath, persistent. E. ×mackaii
    + Teeth more than 14 per sheath, usually shed. E. ×ferrissii


    19 (16) Sheaths dark-girdled at most nodes of stem; teeth 14 or more per sheath, usually shed; articulation line visible. E. hyemale
    +
    Sheaths green or obscurely girdled at nodes near base of stem; teeth 32 or fewer per sheath, usually persistent, shed in some species; articulation line lacking. (20)


    20 (19) Teeth 3--32 per sheath; stem ridges same number as teeth; aerial stems erect and straight. (21)
    + Teeth 3 per sheath; stem ridges 6; aerial stems inclined and tortuous. E. scirpoides

    21 (20) Sheath teeth usually shed; cone apex rounded to apiculate with blunt tip; stem ridges flattened or convex. E. laevigatum
    + Sheath teeth usually persistent throughout; cone apex sharply apiculate; stem ridges minutely grooved. E. variegatum

Distribution:

  • Global, with many of our species also found in Europe and Asia.

Habitat:

  • Wet places; standing waters of shallow ponds and ditches, marshy areas, wet meadows, and moist woods.
  • Equisetum occurs in moist places such as riverbanks, lakeshores, roadsides, ditches, seepage areas, meadows, marshes, and wet woodlands.

Fire:

  • Fire kills above ground growth, but most species survive due to the deep and extensive rhizomes, from which the new, post-fire stems sprout.

Associates:

History:

  • Equisetum is the sole surviving genus of a complex group of primitive plants which covered the planet during the Carboniferous period more than 300 million years ago.

Uses:

Reproduction:

  • By spore and vegetatively by rhizome
  • Primary means of reproduction is asexual; conditions for the production of gametophytes from spores are limited and relatively rare.
  • Asexual reproduction: Spreads from extensive rhizomes. Even short segments of broken rhizomes will sprout. Overwintering buds develop at the nodes of the rhizomes.
  • Sexual reproduction:
    • Spores are equipped with elaters, long appendages that expand and contract with changes in humidity. Elaters function to dig the spore into the soil and to tangle spores together, thereby creating a larger propagule and increasing the probability that prothalli will be close enough to ensure fertilization.
    • Elaters may also aid in wind dissemination. Spores released by the cone bearing stems are dispersed by wind or water. The spores are thin-walled, short-lived, and quickly germinate under moist conditions.
    • Spores germinate to form prothalli: tiny plants only a few cell layers thick that are usually either male or female, producing only antheridia or archegonia, respectively. Swimming sperm are released by the antheridia and require water for transport to the egg-containing archegonia.
    • After fertilization takes place, the sporophytic generation (the identifiable large plant) develops in place, growing out of the prothallus.

Propagation:

  • By rhizome division

Cultivation:

  • Hardy to USDA Zones 1-3 (average minimum annual temperature -40ºF to more than -50ºF)
  • Good for bog gardens, pond margins, and naturalizing low, wet places.
  • Many species available by mail order from specialty suppliers.

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Last Updated on 26 February, 2004