Equisetum arvense

Equisetum arvense L.
(Latin: arvum, cultivated field, applied to plants to mean of cultivated fields or land)

Local names: common horsetail, field horsetail, bottle-brush

Aerial stems annual, dimorphic, the fertile unbranched, succulent fleshy, light brown to tan or pinkish, non-photosynthetic, ephemeral (produced in spring, withering as the sterile appear), the sterile much-branched, to 60(–100) cm tall, with 4–14 ridges, green, persisting through growing season, the branches numerous in regular whorls, solid, to ca. 25 cm long and ca. 1 mm in diam., with 3–4 ridges; leaves 4–14 per node (number evident as dark teeth of sheaths); cones cylindric to ovoid, to ca. 4 cm long and 10 mm in diam., the apex rounded; 2n = ca. 216 (Hauke 1993). Stream banks, lake shores, marshes, other moist areas; in TX known only from the Panhandle in Lubbock (Correll 1956 cited E.L. Reed 3613, 1932 at Buffalo Spring), Wheeler (W.C. Holmes, J.R. Singhurst and J.N. Mink 15094A [BAYLU]), and possibly Potter (B. O’Kennon, pers. comm.; the Potter locality is not mapped on county distribution map) cos.; nearly throughout the U.S. and Canada except absent from the far se U.S. w through most of TX; also Eurasia. Sporulating early spring. We have been unable to locate or confirm the 1932 Reed collection cited by Correll. Correll (1956) noted that his 1952 attempt to relocate this species at the Buffalo Spring locality was unsuccessful. Jack Stanford (pers. comm.) and Jim Blassingame (pers. comm.) both question whether the identification of the Reed collection is correct—based on their extensive experience in the area, they believe the collection was a specimen of either E. hyemale or E. laevigatum that was branched due to meristem damage. Marshall Johnston and Bob O’Kennon (pers. comm.) saw a few plants of what they thought was E. arvense at the edge of a playa lake in Potter Co. (near Amarillo) in the early 1990s. Mink et al. (2011b) recently reported a 2010 collection from Wheeler Co.; this collection was of a single tiny specimen clinging to the roots of a plant of another species. After examining the specimen, James H. Peck (pers. comm.) and G. Diggs agree that the material is consistent with a very immature specimen of E. arvense; additional specimens consisting of more mature material will be important in confirming this species for TX. Although many infraspecific taxa have been named in this species, we are following Hauke (1993) in not recognizing varieties or subspecies. This species occurs broadly across the n hemisphere and has a classic Arcto-Tertiary distribution (see page 33) (Kato & Iwatsuki 1983). Jones (1987) noted that instances of “death are known where hay contaminated with stems of E. arvense was fed to horses.” In fact, among horsetail species, E. arvense is the one that causes most cases of thiaminase poisoning (Vitamin B1 deficiency) in North America. Horses, particularly young ones, eating large amounts of E. arvense are most susceptible (Burrows & Tyrl 2001). Given its rareness and limited distribution in the state, we consider this species to be of conservation concern in TX.

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