IUCN threat status:

Least Concern (LC)

Comprehensive Description

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Giant horsetail, or great horsetail (Equisetum telmateia), is a deciduous herb in the Equisetaceae (Horsetail) family. One subspecies, telmateia, is native to Europe, western Asia and northwest Africa (Flora of North America 2018). A second subspecies, braunii, is native to western North America, specifically the Pacific Coast (Alaska, British Columbia, Washington, Oregon, and California); it is also found in Idaho and Michigan (USDA 2018). The river horsetail (E. Maximum) is the most abundant horsetail species in Europe, while a different species of wood horsetail (E. Sylcaticum) is found in Sweden. Giant horsetail is usually found in lowland habitats and alluvial forests that scale from moist to wet. Giant horsetail is commonly found growing in stream sides, cliffs, seepage areas, gullies, swamps, and roadsides (Klinkenberg 2017; Alden 1998; Knoke 2018).

Giant horsetail has two stem types, fertile and sterile, with distinct differences. Sterile stems, hollow in the center, grow to between 50 and 300 cm tall and .5 cm and 2 cm thick (Knoke 2018). The sterile stems are segmented into layers several centimeters long, with the longest segments at the bottom. The bottom of each segment is a light green/cream, which abruptly transitions to dark brown, and then morphs into a bright green; the pattern then repeats. More than ⅔ of the stem is regularly and abundantly branched (whorled) with the branches protruding stick-like in a circle (Kinkenberg 2017). These brush-like branches make giant horsetails particularly noticeable. The sheaths on the stems have 15-30 teeth. Fertile shoots are 25 to 60 cm tall and 1.5-2 cm thick, are pale yellow and brown, unbranched, and have a spore-bearing strobilus at the top (Knoke 2018). The cone is primarily pale yellow with brown spores dotting it in a circular pattern; it is 4-8 cm long, hollow, and rounded at the tip (Klinkenberg 2017). The stems of both types of shoot are smooth. There is no notable difference between the juvenile plants and mature horsetails. The plant has a persistent, rhizomatous rooting system that makes eradication difficult. These plants characteristics make them rather unique and easily identifiable.

Giant horsetail is a perennial that alternates generations, switching between a sexual phase and an asexual phase, with each generation being an independent plant. One generation reproduces by spores born on stalks, forming into a fruiting, terminal cone on the fertile stem. Once these spores germinate, plants producing sperm and eggs grow. This generation is a sexual generation; the fertilization of the egg and the subsequent development of the plant results in the next asexual generation (Cody 2006).

Giant horsetail grows in dense clusters, often creating clonal colonies (Alden 1998). It is not known to be associated ecologically with any particular species of plant, but because they grow in coastal marshes, stream banks, and other wet places they are often near other plants that prefer those conditions. The IUCN Red List places giant horsetail in the category of “Least Concern,” despite habitat degradation (Akhani et. al. 2014).


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