Equisetum arvense (field horsetail)

The horsetails are primitive perennial plants. Fossil material of Equisetum arvense has been found from the Carboniferous period, about 300 million years ago (Mitich, 1981). E. arvense is sometimes a serious weed in the temperate areas of the northern hemisphere. Its chromosome number is 108 (Hauke, 1978).

E. arvense is a perennial fern ally, with conspicuous nodes (joints) that are easily separated. It has fluted, prominently ridged stems (3-5 mm) and whorled branches which are usually regularly and abundantly branched, giving the plant the appearance of a Christmas tree. The branches are ascending or suberect, simple and nearly smooth. The leaves consist of small teeth set in a whorl on a closely adnate nodal sheath and at maturity are free of chlorophyll (Golub and Wetmore, 1948). However, the species is very variable and differences in the numbers of branch teeth, the nature of branching and the habit have been described (Hauke, 1966).E. arvense has extensive and deep-seated rhizomes that are dark brown to black, dull, covered with hairs and occasionally bear tubers. The tubers are formed as shortened swollen internodes consisting of starch-filled cells traversed by a few vascular bundles (Korsmo, 1954). The rhizomes send out shoots each year.The stems are annual, erect or decumbent, hollow and jointed, with sheaths at the joints. The fertile stems are unbranched, 10-25 cm high, terminating in a cone which may be 2.5-10 cm. The cone is formed of shield-shaped, stalked scales from which spores are produced. The spores germinate to produce the minute, sexual stage (gametophyte), which is seldom seen, from which the spore-bearing plants develop. The spores (33-48 µm in diameter) are globose and pale green to yellow. The fertile stem has large, easily separated joints, up to 8 mm thick. The sheaths are 14-20 mm long, with large, partly united teeth, 5-9 mm long, flesh-coloured yellowish or brownish. They are precocious and ephemeral. As small Arctic plants they occasionally persist and become branched and green (Tutin et al., 1964-1980). The sterile or vegetative stems are branched, 20-100 cm high, with smaller joints. The sheaths are cup-shaped, 5-10 cm long, gradually widening upward. The teeth are dark brown to light tan, free or partly united, 1.5-7 mm long, dry thin and membranaceous. The branches are 10-15 cm long with internodes 1.4-4.5 cm long and 0.8-4.5 mm in diameter. Internally, both carinal and vallecular collenchyma are present. Chlorenchyma is present under the ridges but is interrupted under the valleys. The central canal takes up 33-66% of the diameter of the stem.E. arvense is adapted to dry conditions because it has a silica-coated cuticle, sunken stomata, reduced leaves and an assimilating stem, while the rhizomes bear hydromorphic structures (Borg, 1971). The hollow rhizomes facilitate the passage of air far down into the soil (Uchino et al., 1984) and serve as routes for the uptake of water (Borg, 1971). The roots of E. arvense also associate with several strains of nitrogen-fixing bacteria in a nitrogen-free mineral nutrient solution (Uchino et al., 1984).

E. arvense is circumpolar in distribution, throughout Europe and Asia, south to Turkey, Iran, the Himalayas, and across China (except the southeastern part), Korea and Japan. It is also found throughout Canada and the USA as far south as Georgia, Alabama, Arkansas, Texas, Arizona, New Mexico and California (Hultén and Fries, 1986). Holm et al. (1991) classifies E. arvense as a principal weed in Belgium, Canada, England, Finland, Germany, Japan, New Zealand, the former Soviet Union, the USA, the former Yugoslavia and as a common weed in Alaska, Argentina, Brazil, the former Czechoslovakia, France, India, Iran, Madagascar, Mauritius, Netherlands, Poland, Romania, Spain and Sweden. In Chile, China, Iceland, Italy, Korea and Turkey it is only reported as present.

E. arvense is found in many arable crops (Håkansson, 1995c) as well as in open grassland, particularly in the temperate regions of the northern hemisphere. According to Holm et al. (1977) it is found in over 25 crops including beets, potatoes, and spring-sown and autumn-sown cereals. E. arvense is seldom ranked as a principal weed (Holm et al., 1977). In many parts of Europe, it is commonly found in orchards and other perennial crops (Håkansson, 1995b). It is also commonly found in first-year leys for cutting but, at least under Swedish conditions, is less commonly found in older leys for cutting (Håkansson, 1995a).

E. palustre is distinguished from E. arvense because it has no tubers on the rhizomes, its sterile and fertile spore-bearing stems are similar in appearance and emerge at the same time and its spore-bearing cones usually appear at the tips of the stems but sometimes appear at the tips of the branches (Holm et al., 1977). The sterile stems of E. palustre might also be mistaken for those of E. arvense, however, the first branch internodes of E. palustre are shorter than the subtending stem sheath rather than longer as in E. arvense (Cody and Wagner, 1981).

E. arvense grows under a variety of soil and climatic conditions, but mainly in unproductive habitats such as marshes, swamps, ditches, river banks, open fields, open woods and areas such as road sides and railway embankments (Holm et al., 1977; Cody and Wagner, 1981). E. arvense is more common on sandy soils than on clay (Buchli, 1936). It grows in almost any substrate but prefers neutral or slightly basic soils (Meusel et al., 1971).

Large subterranean buds develop during the summer from an extensive rhizome system. They emerge as achlorophyllous spore-producing stems (10-20 cm) the following spring. Coning takes place mainly in April and May at about 60°N in Sweden and in July further north. The vegetative shoots (20-30 cm) appear about 1 week after the fertile stems, growing from smaller terminal or lateral rhizomatous buds, which were also formed the previous season. The terminal buds are always larger than the lateral buds and produce the first and also the most vigorous vegetative shoots in height and diameter.E. arvense reproduces by spores on the fertile stem, but primarily by extensive rhizomes. Williams (1979) reported that about 50% of the rhizomes occupied the upper 25 cm of the soil profile, 25% occur in the next 25 cm of soil and the remaining 25% in the next 25 cm of soil. Under some circumstances, E. arvense reproduce by tubers that grow out from the nodes of the rhizomes and which, detached from the rhizome, may serve as a vegetative propagule.E. arvense is slow-growing (Andersson and Lundegårdh, 1998a). Rhizomes planted in March attain their maximum shoot growth in July, their maximum shoot height in August, their maximum shoot number in September and accumulate dry matter in the rhizomes until October. The tubers are initiated in late summer and grow in size and number until November (Kvist and Håkansson, 1985; Marshall, 1986).E. arvense is adapted to sunny habitats. It is dependent on its rhizomes and tubers for growth under heavy shade. For maximum growth, E. arvense has a high potassium demand, especially at high nitrogen levels (Andersson and Lundegårdh, 1998b). The shade sensitivity of E. arvense can partly be explained by its small, scale-like, non-functional leaves at the nodes (Holm et al., 1977). The assimilating capacity of the stem of E. arvense is not known (Borg, 1971). E. arvense seems to be sensitive to direct light competition. However, unfavorable conditions that do not decrease light may favour it. E. arvense is tolerant of low levels of nitrogen but will be overtopped by fast-growing species when competing for increased supplies of nitrogen. An increase in nitrogen supply and reduction in light, both increase the reduction of rhizome dry weight, whereas tuber production can be favoured by low nitrogen supply and high light intensities (Andersson and Lundegårdh, 1998a).E. arvense can tolerate extended periods without rain because its rhizomes can extend several metres down into the soil (Cloutier and Watson, 1985; Reuss and Bachthaler, 1988) to escape the effects of tillage and herbicides (Williams, 1979). The tubers on the rhizomes act as organs of storage and regeneration, but the tubers may also disseminate the weed (Holm et al., 1977). Tuber size increases with depth (Williams, 1979), contributing to the plant's strong regenerative capacity. It has been reported to emerge through silt layers up to 1 m thick following flooding (Holm et al., 1977).

Species pathogenic on E. arvense include Fusarium semitectum (in Alaska, USA), Leptosphaerie hiemalis (in Canada), Mycosphaerella tassiana (in Greenland), Phoma equiseti (in Nova Scotia, Canada) and Gloeosporium equiseti (in Ontario, Canada) (Cody and Wagner, 1981)Herbivores that feed on E. arvense include ducks (Hauke, 1966), the homopteran Macrosteles borealis and the hymenopteran Dolerus spp.; the coleopterans Grypidus equiseti, Grypus spp. and Hippuriphila spp. are found associated with E. arvense (Cody and Wagner, 1981).

Although E. arvense is found with many crops (Håkansson, 1995a,b,c) it is not competitive with a vigorous crop and therefore has little impact on crop yield (Williams, 1979). However, it can cause difficulty during grain harvesting by clogging harvesting and threshing equipment with its bulk (Hoyt and Carder, 1962).E. arvense can be toxic owing to the thiaminase that is primarily found in its leaves and stems. Thiaminase is an enzyme that splits the B vitamin thiamine rendering it inactive. Hay containing this weed may be more poisonous than fresh plants in the field. Thiamine is involved in decarboxylation reactions in animals. Deficiency of thiamine leads to accumulation of pyruvate in the blood, with a resulting impairment in energy metabolism and cellular shortage of ATP. Hay that contains E. arvense at a level of 20% or more may produce symptoms of thiamine deficiency in horses in 2-5 weeks. Ruminants are not generally affected by thiamine deficiency because thiamine is made in the rumen (Henderson et al., 1952; Cheeke and Schull, 1985).UsesE. arvense is an astringent herb and has a diuretic action. Fresh E. arvense can be bruised and applied to wounds to stop bleeding (Hauke, 1978). It has also been used to treat deep-seated lung damage such as that caused by tuberculosis or emphysema (Ode, 1993). What makes E. arvense valuable as a medicine is its mineral and silica content. It contains up to about 7.4 % silica (Carnat et al., 1991). Silica is important in strengthening many tissues in the body including bone, hair and nails. It promotes calcium absorption and helps fight against plaque deposits in the arteries. Historically, the dried stems of E. arvense were used to polish pewter and other metals. Other chemical constituents are flavonoids (0.3%), potassium (1.8%), isquercitroside (0.12%), phenolic acids (0.7%), calcium (1.3%) (Carnat et al., 1991) as well as traces of alkaloids (nicotine, palustrine and palustrinine), saponins, phytosterols, tannins, and the minerals manganese, sulfur, and magnesium (Ode, 1993).E. arvense accumulates heavy metals, which makes it a suitable indicator for heavy metals such as copper, zinc, cadmium and lead (Ray and White, 1979).