Equisetum is the only genus in an unusual group of pteridophytes (plants that have vascular tissue but do not have seeds). About 15 species exist; their closest living relatives are believed to be ferns, although more similar types of plant (some the size of trees) are found in the fossil record. The stems, which accumulate large amounts of silica, are jointed and hollow, and the leaves, borne in whorls at the nodes, are reduced to small teeth. Spores are produced in terminal cones; some species, including E. arvense, have two growth forms, one of which produces branches but no cones, the other of which is unbranched and produces cones. Cones have been considered edible when cooked; rarely, shoots have also been eaten. Horsetail, which is widely distributed, has been used at least since the time of Dioscorides as an aquaretic (to increase the flow of urine) for the treatment of urinary diseases, kidney stones, and edema. It is reputed to strengthen connective tissue and to treat wounds, and is used for hepatitis in Asia. Horsetail was also historically used, combined with other plants high in silica, to treat tuberculosis.

The German Commission E gives horsetail?s use for urinary problems a positive rating based largely on human use data; little clinical research has been done. Animal studies, and one human study, of horsetail?s aquaretic activity have used different species of Equisetum. Animal studies using E. arvense have reported that horsetail has anti-inflammatory, antioxidant, sedative, and anticonvulsant effects and enhances cognition in aged rats. The plant also contains hepatoprotective compounds. All species of Equisetum may affect thiamine metabolism when eaten in large quantities by livestock, although E. arvense is significantly less toxic than related species. Rodents fed both high doses of horsetail and a cholesterol-rich diet develop dermatitis.

Selected References

dos Santos, J. G., Jr., M. M. Blanco, F. H. M. Do Monte, M. Russi, V. M. N. B. Lanziotti, L. K. A. M. Leal, and G. M. Cunha. 2005. Sedative and anticonvulsant effects of hydroalcoholic extract of Equisetum arvense. Fitoterapia 76:508-513.

dos Santos, J. G., Jr., F. H. M. do Monte, M. M. Blanco, V. M. N. B. Lanziotti, F. M. Damasseno, and L. K. A. Leal. 2005. Cognitive enhancement in aged rats after chronic administration of Equisetum arvense L. with demonstrated antioxidant properties in vitro. Pharmacol. Biochem. Behav. 81:593-600.

Duckett, J. G. 1973. Comparative morphology of the gametophytes of the genus Equisetum, subgenus Equisetum. Bot. J. Linn. Soc. 66:1-22.

Guillon, J. M. 2004. Phylogeny of horsetails (Equisetum) based on the chloroplast rps4 gene and adjacent noncoding sequences. Syst. Bot. 29:251-259.

Hauke, R. L. 1966. A systematic study of Equisetum arvense. Nova Hedwigia 13:81-109.

Hauke, R. L. 1978. A taxonomic monograph of Equisetum subgenus Equisetum. Nova Hedwigia 30:385-456.

Hauke, R. L. 1993. Equisetaceae. Pp. 76-84 in: Flora of North America Editorial Committee, eds. Flora of North America, vol. 2. Oxford University Press, New York.

Maeda, H., K. Miyamoto, and T. Sano. 1997. Occurrence of dermatitis in rats fed a cholesterol diet containing field horsetail (Equisetum arvense L.). J. Nutr. Sci. Vitaminol. (Tokyo) 43:553-563.