The horsetails are vascular plants, comprising 15 species of plants in the genus Equisetum. This genus is the only one in the family Equisetaceae, which in turn is the only family in the order Equisetales and the class Equisetopsida. This class is often placed as the sole member of the Division Equisetophyta (also called Arthrophyta in older works), though some recent molecular analyses place the genus within Pteridophyta, related to Marattiales. The molecular data, however, are somewhat ambiguous as of yet. Other classes and orders of Equisetophyta are known from the fossil record, where they were important members of the world flora during the Carboniferous period.


The name horsetail arose because it was thought that the stalk resembled a horse's tail, the name Equisetum being from the Latin equus, "horse", and seta, "bristle". Other names, rarely used, include candock (applied to branching species only), and scouring-rush (applied to the unbranched or sparsely branched species). The latter refers to the plants' rush-like appearance; the stems were used for scouring cooking pots in the past (due to them being coated with abrasive silica).

The genus is near-cosmopolitan, being absent only from Australasia and Antarctica. They are perennial plants, either herbaceous, dying back in winter (most temperate species) or evergreen (some tropical species, and the temperate species Equisetum hyemale, E. scirpoides, E. variegatum and E. ramoissimum). They mostly grow 0.2-1.5 m tall, though E. telmateia can exceptionally reach 2.5 m, and the tropical American species E. giganteum 5 m, and E. myriochaetum 8 m.

In these plants the leaves are greatly reduced, in whorls of small, segments fused into nodal sheaths. The stems are green and photosynthetic, also distinctive in being hollow, jointed, and ridged (with (3-) 6-40 ridges). There may or may not be whorls of branches at the nodes; when present, these branches are identical to the main stem except smaller.

The spores are borne in cone-like structures (strobilus, pl. strobili) at the tips of some of the stems. In many species the cone-bearing stems are unbranched, and in some (e.g. E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic sterile stems. In some other species (e.g. E. palustre) they are very similar to sterile stems, photosynthetic and with whorls of branches.


Horsetails are mostly homosporous, though in E. arvense, smaller spores give rise to male prothalli. The spores have four elaters that act as moisture-sensitive springs, assisting spore dispersal after the sporangia have split open longitudinally.

Many plants in this genus prefer wet sandy soils, though some are aquatic and others adapted to wet clay soils. One horsetail, E. arvense, can be a nuisance weed because it readily regrows after being pulled out. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. It is also unaffected by many herbicides designed to kill seed plants. The foliage of some species is poisonous to grazing animals if eaten in large quantities.

The horsetails were a much larger and more diverse group in the distant past before seed plants became dominant across the Earth. Some species were large trees reaching to 30 m tall. The genus Calamites (family Calamitaceae) is abundant in coal deposits from the Carboniferous period.