Equisetum

Taxonomy

Species

Currently, 18 species of Equisetum are accepted by Plants of the World Online. Some researchers speak of 25 species. The living members are divided into three distinct lineages, which are usually treated as subgenera. The name of the type subgenus, Equisetum, means "horse hair" in Latin, while the name of the other large subgenus, Hippochaete, means "horse hair" in Greek. Hybrids are common, but hybridization has only been recorded between members of the same subgenus.

Two Equisetum plants are sold under the names Equisetum japonicum (barred horsetail) and Equisetum camtschatcense (Kamchatka horsetail). These are both types of E. hyemale var. hyemale, although they may also be listed as separate varieties of E. hyemale.

The Smith et al. classification of 2006, based on molecular phylogeny, placed Equisetum and Equisetaceae within leptosporangiate ferns. Subsequent classifications have maintained this placement.

Evolutionary history

Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests. Some equisetids were large trees reaching to 30 m (98 ft) tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period.

The oldest remains of modern Equisetum horsetails first appear in the Early Jurassic, represented by E. dimorphum from the Early Jurassic of Patagonia and Equisetum laterale from the Early-Middle Jurassic of Australia. Silicified remains of Equisetum thermale from the Late Jurassic of Argentina exhibit all the morphological characters of modern members of the genus. The estimated split between Equisetum bogotense and all other living Equisetum is estimated to have occurred no later than the Early Jurassic.

Subgenus Paramochaete

• Equisetum bogotense Kunth – Andean horsetail; upland South America up to Costa Rica; includes E. rinihuense, sometimes treated as a separate species. Previously included in subg. Equisetum, but Christenhusz et al. (2019) transfer this here, as E. bogotense appears to be sister to all the remaining species in the genus.

Subgenus Equisetum

• Equisetum arvense L. – field horsetail or common horsetail; circumboreal down through temperate zones
• Equisetum braunii Milde – northern giant horsetail, syn. E. telmateia subsp. braunii (Milde) Hauke.; west coast of North America
• Equisetum diffusum D.Don – Himalayan horsetail; Himalayan India and China and adjacent nations above about 450 metres (1,480 ft)
• Equisetum fluviatile L. – water horsetail; circumboreal down through temperate zones
• Equisetum palustre L. – marsh horsetail; circumboreal down through temperate zones
• Equisetum pratense Ehrh. – shady horsetail, meadow horsetail, shade horsetail; circumboreal except for tundra down through cool temperate zones
• Equisetum sylvaticum L. – wood horsetail; circumboreal down through cool temperate zones, more restricted in east Asia
• Equisetum telmateia Ehrh. – great horsetail; Europe to Asia Minor and north Africa. The former North American subspecies Equisetum telmateia subsp. braunii (Milde) Hauke is now treated as a separate species Equisetum braunii Milde

Subgenus Hippochaete

• Equisetum giganteum L. – southern giant horsetail or giant horsetail; temperate to tropical South America and Central America north to southern Mexico
• Equisetum hyemale L. – rough horsetail; most of non-tropical Old World. The former North American subspecies Equisetum hyemale subsp. affine (Engelm.) A.A.Eat. is now treated as a separate species Equisetum praealtum Raf.
• Equisetum laevigatum A.Braun – smooth horsetail, smooth scouringrush; western 3/4 of North America down into northwestern Mexico; also sometimes known as Equisetum kansanum
• Equisetum myriochaetum Schltdl. & Cham. – Mexican giant horsetail; from central Mexico south to Peru
• Equisetum praealtum Raf. – scouringrush horsetail, syn. E. hyemale subsp. affine (Engelm.) A.A.Eat.; temperate North America
• Equisetum ramosissimum Desf. (including E. debile) – branched horsetail; Asia, Europe, Africa, southwest Pacific islands
• Equisetum scirpoides Michx. – dwarf horsetail, dwarf scouringrush; northern (cool temperate) zones worldwide
• Equisetum variegatum Schleich. ex Weber & Mohr – variegated horsetail, variegated scouringrush; northern (cool temperate) zones worldwide, except for northeasternmost Asia
• Equisetum xylochaetum Mett. – Atacama Desert giant horsetail; southern Peru, northern Chile

Unplaced to subgenus

• †Equisetum dimorphum – Early Jurassic, Argentina
• †Equisetum laterale – Early to Middle Jurassic, Australia
• †Equisetum thermale – Middle to Late Jurassic, Argentina
• †Equisetum similkamense – Ypresian, British Columbia

Named hybrids

Hybrids between species in subgenus Equisetum

• Equisetum × bowmanii C.N.Page (Equisetum sylvaticum × Equisetum telmateia)
• Equisetum × dycei C.N.Page (Equisetum fluviatile × Equisetum palustre)
• Equisetum × font-queri Rothm. (Equisetum palustre × Equisetum telmateia)
• Equisetum × litorale Kühlew ex Rupr. (Equisetum arvense × Equisetum fluviatile)
• Equisetum × mchaffieae C.N.Page (Equisetum fluviatile × Equisetum pratense)
• Equisetum × mildeanum Rothm. (Equisetum pratense × Equisetum sylvaticum)
• Equisetum × robertsii Dines (Equisetum arvense × Equisetum telmateia)
• Equisetum × rothmaleri C.N.Page (Equisetum arvense × Equisetum palustre)
• Equisetum × willmotii C.N.Page (Equisetum fluviatile × Equisetum telmateia)

Hybrids between species in subgenus Hippochaete

• Equisetum × ferrissii Clute (Equisetum hyemale × Equisetum laevigatum)
• Equisetum × moorei Newman (Equisetum hyemale × Equisetum ramosissimum)
• Equisetum × nelsonii (A.A.Eaton) Schaffn. (Equisetum laevigatum × Equisetum variegatum)
• Equisetum × schaffneri Milde (Equisetum giganteum × Equisetum myriochaetum)
• Equisetum × trachyodon (A.Braun) W.D.J.Koch (Equisetum hyemale × Equisetum variegatum)

Phylogeny

Etymology

The scientific name Equisetum is derived from the Latin equus ('horse') + seta ('bristle').

Distribution and habitat

The genus Equisetum as a whole, while concentrated in the non-tropical Northern Hemisphere, is near-cosmopolitan, being absent naturally only from Antarctica, Australia, New Zealand, and the islands of the Pacific Ocean. They are most common in northern Europe, with ten species (E. arvense, E. fluviatile, E. hyemale, E. palustre, E. pratense, E. ramosissimum, E. scirpoides, E. sylvaticum, E. telmateia, and E. variegatum); Great Britain has nine of these species, missing only E. scirpoides of the European list. Northern North America (Canada and the northernmost United States), also has nine species (E. arvense, E. fluviatile, E. laevigatum, E. palustre, E. praealtum, E. pratense, E. scirpoides, E. sylvaticum, and E. variegatum). Only five (E. bogotense, E. giganteum, E. myriochaetum, E. ramosissimum, and E. xylochaetum) of the 18 species are known to be native south of the equator.

The plants grow in habitats including marshy areas.

Ecology

Equisetum species may have been a common food for herbivorous dinosaurs. With studies showing that horsetails are nutritionally of high quality, it is assumed that horsetails were an important component of herbivorous dinosaur diets. Analysis of the scratch marks on hadrosaur teeth is consistent with grazing on hard plants like horsetails.

As an invasive species

One species, E. fluviatile, is an emergent aquatic, rooted in water with shoots growing into the air. The stalks arise from rhizomes that are deep underground and difficult to dig out. Field horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is unaffected by many herbicides designed to kill seed plants. Since the stems have a waxy coat, the plant is resistant to contact weedkillers like glyphosate. However, as E. arvense prefers an acidic soil, lime may be used to assist in eradication efforts to bring the soil pH to 7 or 8. Members of the genus have been declared noxious weeds in Australia.

All the Equisetum are classed as "unwanted organisms" in New Zealand and are listed on the National Pest Plant Accord.

Toxicity

E. arvense contains thiaminase, which metabolizes the B vitamin, thiamine, potentially causing thiamine deficiency and associated liver damage, if taken chronically. Horsetail might produce a diuretic effect. Further, its safety for oral consumption has not been sufficiently evaluated and it may be toxic, especially to children and pregnant women.

If eaten over a long enough period of time, some species of horsetail can be poisonous to grazing animals, including horses. The toxicity appears to be due to thiaminase.

Uses

People have regularly consumed horsetails. The fertile stems bearing strobili of some species can be cooked and eaten like asparagus (a dish called tsukushi (土筆) in Japan). Indigenous nations across Cascadia consume and use horsetails in a variety of ways, with the Squamish calling them sx̱ém'x̱em and the Lushootseed using gʷəɫik, or horsetail roots, for cedar root baskets.

Extracts and other preparations of E. arvense have served as herbal remedies, with records dating over centuries. In 2009, the European Food Safety Authority concluded that there was no evidence for the supposed health effects of E. arvense, such as for invigoration, weight control, skincare, hair health or bone health. As of 2018, there is insufficient scientific evidence for its effectiveness as a medicine to treat any human condition.

In culture

The pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, is said to have inspired John Napier to invent logarithms.

Common names

The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble a horse's tail.

Other names include candock for branching species, puzzlegrass, and snake grass or scouring-rush for unbranched or sparsely branched species. The latter name refers to the rush-like appearance of the plants and to the fact that the stems are coated with abrasive silicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made of tin. Equisetum hyemale, rough horsetail, is still boiled and then dried in Japan to be used for the final polishing process on woodcraft to produce a smooth finish. In German, the corresponding name is Zinnkraut ('tin-herb') or Schachtelhalm ('nesting stalk'). In Spanish-speaking countries, these plants are known as cola de caballo ('horsetail').

A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally referred to as "horsetail", and adding to confusion, the name "mare's tail" is sometimes applied to Equisetum.