Equisetum hyemale

Equisetum hyemale L. subsp. affine (Engelm.) Calder & R.L. Taylor
(sp.: Latin: hiemalis, of or pertaining to winter, in reference to the evergreen habit; subsp.: Latin: affinis: related to, neighboring, akin to, the type description noting that this plant, which was originally named as a variety of a different species, “approaches very closely, sometimes too much so, to [E. hyemale], whence the name”)

Local names: scouring-rush horsetail, tall scouring-rush, American scouring-rush, common scouring-rush, great scouring-rush, rough horsetail, winter scouring-rush, cañuela

Plants often found in large colonies, spreading vigorously by vegetative reproduction via rhizomes (e.g., huge colonies are sometimes seen spreading along creeks forming virtual “thickets”); aerial stems usually perennial, monomorphic, unbranched (unless damaged or abnormal), 18–220(–300) cm tall, typically 5–10 mm in diam. (the plants tending to be more robust than those of E. laevigatum), with 14–50 ridges, the stem surface somewhat roughened (like sandpaper), dark green or bluish-green at maturity; leaf sheaths about as long as wide (length/width ratio 1:1), usually with 2 dark girdling bands/lines (sometimes irregular or absent) one at base and a narrow one at apex where teeth are attached and often with a whitish to ashy-gray or brownish band between (however, sheaths at upper few nodes often ± green and lacking basal dark girdle thus somewhat resembling those of E. laevigatum), the teeth of sheaths often persistent, blackish, or promptly shed; leaves 14–50 per node (number evident as teeth of sheaths); cones ovoid to cylindric-ellipsoid, (0.8–)1.5–2.5 cm long, 5–10 mm in diam., the apex usually pointed with a noticeable black apiculum; 2n = 216 (Hauke 1993). Stream banks, wet places, seepage areas, often forming large colonies; widespread from Pineywoods w throughout much of TX (e.g., Cameron Co., R. Runyon 114; Grayson Co., M. Nee & Diggs 43893; Val Verde Co., Correll & Correll 12891, all BRIT) (solid dots on the county distribution map represent specimens identified by J. Peck and G. Diggs, while open circles are herbarium and literature records identified by other authorities—see discussion below); throughout Canada and the entire U.S.; also Mexico, Guatemala, and e Asia. Sporulating Mar–late fall. [E. hyemale L. var. affine (Engelm.) A.A. Eaton, E. hyemale var. robustum (A. Braun) A.A. Eaton, E. prealtum Raf., E. robustum A. Braun var. affine Engelm.] Subspecies affine is native to both North America and e Asia (Kato & Iwatsuki 1983; Mickel & Smith 2004), while subsp. hyemale is European and w Asian in distribution. When the ranges of both subspecies are considered, the species demonstrates a classic Arcto-Tertiary distribution (Kato & Iwatsuki 1983) (see pages 32 and 33 for a discussion of the North America-e Asia disjunct distribution and the concept of an Arcto-Tertiary flora). Young or damaged stems or fragmentary specimens of this species and E. laevigatum can very easily be confused (such as a “top snatch”—specimens consisting of only a stem tip). For example, Mickel and Smith (2004) noted that “Young E. hyemale has longer than usual sheaths that often lack the lower dark bands, resembling those of E. laevigatum.” Indeed, when a plant that has multiple stems (from a rhizome) is available, it is evident that young stems or stems with anomalous regrowth can look remarkably like E. laevigatum, even though older stems are obviously typical of E. hyemale. Because of this and since the banding of the uppermost or lowermost nodes can often be deceptive, complete, mature stems should be collected whenever possible. Identification is further complicated because the hybrid E. ×ferrissii is vegetatively intermediate between its two parents and is morphologically quite variable. Because of the ease with which typical herbarium material can be confused, as noted above, only specimens identified by J. Peck and G. Diggs are represented as solid black dots on the county distribution maps of E. hyemale, E. laevigatum, and E. ×ferrissii. Jack Stanford (pers. comm.) pointed out that he has observed plants of E. hyemale with green rhizomes (even when they are underground); this is the only species he has seen exhibiting this phenomenon. This species is sometimes cultivated, including in water gardens in TX, but can spread aggressively through vegetative reproduction and become problematic (G. Diggs, pers. obs.). Poisonous (Burlage 1968; Burrows & Tyrl 2001).

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