Equisetum arvense L., Field Horsetail
Account Summary
Native, very common, widespread and locally abundant. Circumpolar wide-
boreal and introduced widely in the southern hemisphere.
1881; Stewart, S.A.; Co Fermanagh.
March to December.
Growth form, identification and preferred habitats
As any gardener or farmer will testify, E. arvense is a too frequently found rhizomatous and tuber producing perennial weed, which grows so deeply and spreads so relentlessly and rapidly that it is widely regarded as nearly impossible to eradicate (Holm et al. 1977; Grime et al. 1988). The acute, green, spreading teeth on the side branches of E. arvense help distinguish its sterile stems from the rather similar ones of E. palustre (Marsh Horsetail), in which the corresponding branch teeth are black-tipped and tightly clasp the stem for their whole length. In terms of identification it is fortunate that at least some of the characteristics of the side branches are unvaryingly reliable: irrespective of the size of the sterile stem of the plant, the lowest internode on each side branch of the stem of E. arvense is always equal to or longer than the adjacent stem sheath on the node from which it arises (Jermy & Camus 1991).
Typical habitats include rough grassland and disturbed areas in damp open woods, hedgerows, field-, river- and roadside banks, lakeshores, cliffs, screes, quarries, gardens, gravel paths and waste ground. Although Field Horsetail is able to grow in a very wide variety of soil types, it has a definite preference for neutral or slightly base-rich conditions. It is really only common and vigorous where there is a high water table and the soil is poorly drained (Williams 1979; Cody & Wagner 1981). Having said this, E. arvense is recognised to be the most plastic and adaptable horsetail in Britain & Ireland and it can survive and function over a very wide spectrum of soil moisture and nutrient regimes (Page 1997).
Field Horsetail occurs in a huge range of more or less open habitats and like Ulex europaeus (Gorse), Pteridium aquilinum (Bracken), Tussilago farfara (Coltsfoot) and other vigorous perennial weeds, it particularly avails itself of soil disturbance. Frequently such disturbance is due to mans' activities, but it can also be naturally occurring, for example in waterside habitats. Indeed, it has been suggested that river banks may have been the original natural habitat of E. arvense (Hauke 1966), and possibly of all Equisetum species.
Whatever the cause of soil disturbance, it creates an opening for the horsetail, providing bare ground or a gap in the vegetation into which the species then readily invades (Salisbury 1964; Grime et al. 1988). The last mentioned authors found that in their main study area around Sheffield, E. arvense was absent in 'skeletal habitats' (ie bare rocks, cliffs, scree and walls). However, in Fermanagh, we do have records of it colonising cliffs and screes, which might be another reflection of our very wet, more or less hyper-oceanic climate.
Fertile sporing and green vegetative shoots
Stems of E. arvense are of two types; the short, stout, pale- or buff-brown, cone-like, fertile sporing branches appearing for a fortnight or so in early April. As the non-photosynthetic sporing shoots appear before the green vegetative ones, they are sometimes referred to as 'precocious'. The fertile shoots are soon followed by the much more slender, green sterile or barren stems, branched to varying degrees. The green shoots are deciduous, growing and persisting until the frosts in late autumn, and then dying and disappearing in November, or by early December at the latest.
Rhizome extent and ecology
The extremely persistent, perennial, underground rhizome is long and branched and can reach a depth of 1.5 to 2 m., or more (Page 1997). In warmer climates it can exceptionally extend down to around 6 m deep (Cody & Williams 1981). Having said this, Williams (1979) found that in a sandy loam in England, 50% of the rhizome dry weight occurred in the uppermost 25 cm of soil, a further 25% in the next 25 cm and only 10% of the total rhizome material was found between 75 and 100 cm deep. He also found that if arable ground containing E. arvense was left fallow for one or two years, rhizomes and starch-filled tubers occurred much more shallowly, more than 80% of them being resident in the uppermost 25 cm of soil. He concluded that this was most likely due to the horsetail being released from competition by the absence of any crop and other work he quotes from different countries appears to support this suggestion (Williams 1979). Climatic factors such as the depth of soil warming are known to affect the depth to which rhizomes and tubers penetrate, as will soil aeration and moisture status (Williams 1979).
The rapidity of rhizome spread reported is both amazing and alarming; for example, a 10 cm length of rhizome planted in a growth room (presumably, although not specified, in the absence of competition) produced in one year a total of 64 m of growth in a vertical direction through branching (Cody & Wagner 1981). As far as the current author is aware, horizontal rhizome growth rates have not been accurately measured under field conditions, but an individual rhizome has been reported achieving a spread of 100 m (Weber 1903, quoted in Cody & Wagner 1981).
Nutrient requirements
The nutrient demands of E. arvense on soil fertility were concisely expressed in classic ‘crop versus weed’ terms by Salisbury (1964) when he wrote, "its filching of soil nutrients is very appreciable". Hill et al. (1999) placed Field Horsetail among other rather nutrient-demanding weeds such as Cirsium arvense (Creeping Thistle) and Rumex crispus (Curled Dock), which typically require moderately fertile soil. Sinker et al. (1985) characterised E. arvense as requiring a medium to rich supply of phosphate, nitrogen and other mineral nutrients. Field Horsetail, however, appears as variable in this respect as in other environmentally controlled characters, since in his Rothamstead pot experiments, Williams (1979) found that the horsetail seemed well adapted to growth in soil with low nitrogen and it produced little growth response to additions of it.
Silica content
The horsetails, particularly E. hyemale (Rough Horsetail), but to a lesser extent E. arvense, are well-known for their ability to take up and deposit silica in the walls of the epidermal cells of their shoots. The silica content of dry E. arvense can vary between 1.2 to 6.9% and the ash can contain anywhere between 6.2 and 76% silica. This is why these horsetails have in the past been widely used for scouring pots and as polishing and buffing tools (Cody & Wagner 1981). The low levels of silica in both limestone and deep peat soils probably limits the growth of Equisetum species where these soils occur (Page 1997).
In mining areas E. arvense and E. palustre (Marsh Horsetail) are relatively frequent on metalliferous spoil heaps, the soils of which most other plants find too toxic to colonise (Grime et al. 1988). E. arvense has an unusual ability to take up and accumulate heavy metals such as copper, zinc, lead and cadmium, so that it has been proposed as a biological tool to monitor levels of pollution (Ray & White 1979). Reports quoted in Holm et al. (1977), that horsetail accumulates gold in quantities of up to 4.5 ounces per ton of fresh plant material, are exaggerations based on mistaken chemical analytic methods (Grime et al. 1988).
Ecological flexibility
It is the degree of ecological flexibility E. arvense displays in terms of both requirements and tolerances, which enables it to become the only horsetail that is a common weed of cultivated ground. Following a detailed systematic study, Hauke (1966) concluded that despite appearances, E. arvense is a hydrophyte! This decision calls into question how we define an aquatic plant, but we can hardly believe that many biologists or ecologists would entirely agree with Hauke's use of the term for this species. He supports his contention by pointing out that although Field Horsetail is sometimes found growing in very dry habitats (for instance along roadsides and railways), on the basis of his observations and measurements, he believes that it is quite sensitive to moisture stress and that it is able to survive in places where other plants cannot, simply because its extensive, deeply-penetrating rhizome system always manages to tap groundwater supplies.
Variation
Being versatile in terms of its ecology, E. arvense also demonstrates a very wide range of environmentally induced modification (phenotypic variation) of both plant size and form, some of which are easily demonstrated even within a single clone (Hauke 1966). Examples of this variation are very clearly illustrated in Page (1997, pp. 439 & 442), but Hauke (1966) concluded that all this variation is only superficial and none of it merits taxonomic recognition.
Asexual reproduction and dispersal
E. arvense dispersal can readily be achieved by its lightweight spores, but without question it also spreads very effectively vegetatively, both by means of rhizome fragments and through its small, oval, root tubers being carried unwittingly in mud attached to machines, animals and boots. Establishment from spores involves the successful negotiation of several high-risk stages of development, involving the ready release and transport of asexual spores and the production of gamete-bearing prothalli, structures which are known to have an extremely narrow habitat tolerance. The prothalli require bare muddy ground of high nutrient status, neither too wet nor too dry, and the entire absence of shading and competition, even that from mosses and liverworts. In view of this, the rarity of prothallus observations in nature is not all that surprising. Nevertheless, the existence of Equisetum hybrids and the few studies of wild gametophytes that have been made, do prove that sexual reproduction does happen. Even if it is a rare event, sexual reproduction will be significant since it maintains genetic variability and heterogeneity within horsetail populations that could not otherwise occur (Duckett & Duckett 1980).
Sexual and vegetative reproduction
As with other homosporous (one type of asexual spores only), sporophyte (spore-bearing plants) (ie the ferns and horsetails), vast numbers of spores are produced and released in the early spring (generally from early April into May). A small proportion of these asexual spores germinate to produce a prothallus, on which separate male and female sex organs develop. Huge numbers of male gametes (sex cells) swim in a film of water to fertilise the much fewer female ova, and young sporophyte plants successfully produced by this sexual cycle then grow and develop. Some of these juvenile sporophyte plants have been known to produce up to two dozen shoots and several tuber-bearing rhizomes by the end of their first season of growth (Page 1967; Duckett & Duckett 1980).
By comparison, vegetative dispersal and successful subsequent establishment appears very much more probable and this is generally regarded as the more usual method of reproduction of the species (Marshall 1986; Grime et al. 1988).
Sporophyte light requirements and colony development
Once the plant has colonised (by whatever means) and a rhizome has been established, growth of the horsetail can be incredibly rapid, allowing the plant to quickly form a clonal patch. E. arvense has such a terrible reputation among gardeners, fruit-growers and farmers (both arable and grass-managing), that along with its relative E. palustre, it appears in Holm et al. (1977) book entitled, The World's Worst Weeds. This might be pitching its status a little higher than it deserves. The reason for saying this is that, having no leaves, E. arvense entirely relies for its photosynthesis on its annual, green, wiry, branching stems, which grow up to 80 cm in height. Thus most Equisetum species cannot tolerate much deep shade, and the horsetail plant casts very little shade itself (Salisbury 1964).
E. arvense has been given an Ellenberg's indicator value by Hill et al. (1999) for its light preference in the British Isles of '7' on a scale from '1' to '9', meaning that it is regarded by these authors as a plant generally found in well lit places, although it may also sometimes occur in partial shade. This light requirement limits the species competitive ability, and being relatively low growing and possessing a comparatively insignificant canopy of its own, it never (or rarely ever), becomes a dominant plant after the manner of taller, aggressive and persistent invading weeds, such as gorse and bracken. Grime et al. (1988) draw a very interesting parallel between many of the features of E. arvense and Tussilago farfara (Coltsfoot), both in terms of their pioneering, colonising ability and weedy persistence.
Toxicity
Pretty well all species of Horsetails are dreaded worldwide, partly for their deep seated growth and their enduring survival ability, but also because the plants are so poisonous to cattle, sheep and horses. Here again, comparison may usefully be made with Pteridium aquilinum (Bracken), since both fern and horsetail contain a toxic enzyme called thiaminase, which destroys thiamine and creates a vitamin B1 deficiency in any monogastric animals which graze them (Cody & Wagner 1981).
Equisetum species also contain high levels of a number of toxic alkaloids, of which the best known is palustrine (Cooper & Johnston 1998). In this case, the poisonous principle is not destroyed by drying and storage and horses can show clinical signs of poisoning when their hay contains as little as 5% horsetail (Cooper & Johnson 1998). Fortunately the plants contain varying quantities of silicates, making them harsh to touch and unpalatable, at least in the fresh state, so that animals tend to avoid grazing horsetails and therefore poisoning is rare in Britain & Ireland.
The levels of toxins in Equisetum species suggest the possibility of allelopathy, ie toxic suppression, directed towards surrounding competing plants. Work on this topic in Russia is regularly quoted, which showed that when tested along with twelve other species, water extracts of E. arvense displayed the strongest inhibitory effect on seed germination and seedling vigour when it was applied to 30 species of meadow grasses (Zelenchuk & Gelemei 1967 (in Russian), quoted by Cody & Wagner 1981).
Weed control
Control of E. arvense is extremely difficult. Forking out, cutting and burning all prove a useless waste of time and effort. Eradication may be achieved in the long term by shading the weed out with taller plants, which is what normally happens to the species in undisturbed natural vegetation (Page 1997). In the garden, even as simple a matter as sowing a patch of Nasturtium has been recommended for this purpose (Allan 1978). Mulching with leaf compost is reputed to stop lateral movement of Field Horsetail, and it has been reported that black plastic sheeting placed over infested soil killed rhizomes in the upper 60 cm of soil within three to four years (Cody & Wagner 1981).
Herbicide experiments
The choice of herbicides used against Equisetum infestation is governed by the crop or vegetation type affected and the scale of the weed problem. For instance, it was found that MCPA applied after the horsetail had completed emergence gave 100% control of aerial growth for the rest of the season and also reduced the number of horsetail stems emerging in the second year (Hoyt & Carder 1962, quoted by Cody & Wagner 1981).
Glyphosate and other translocated herbicides, such as Asulam (which in particular is also effective in similar manner against Bracken), can be used in uncultivated areas. Glyphosate herbicides are most effective if the horsetail is allowed to fully emerge and its stems are then crushed before applying the chemical in late summer (late July or August is probably the best time) (Marshall 1986). Crushing is recommended because the slender stems and branches are covered with microscopic silica spicules and therefore they are not easily wetted.
Fermanagh occurrence
E. arvense is both the most common and widespread horsetail on a world basis and the most widespread horsetail in Fermanagh, present in 433 post-1975 tetrads, 82% of those in the VC. However, since Fermanagh is a county extraordinarily well supplied with lakes and wet marshy or boggy ground, in terms of record frequency, Field Horsetail remains second to E. fluviatile (Water Horsetail) by a margin of over a thousand records. E. arvense is found throughout the county, although it is uncommon on high ground and absent from aquatic situations, except the gametophyte, which is a pioneer coloniser of muddy water margins of lakes and reservoirs, very occasionally producing the sporophyte generation (Duckett & Duckett 1980).
British and Irish occurrence
Field Horsetail is the most common and widespread horsetail in Britain & Ireland, being present in every VC. The distribution thins slightly in areas of predominant deep peat soils in N Scotland and in the SW of both islands (Jermy et al. 1978; Page 1997; Preston et al. 2002).
European and world occurrence
E. arvense is very widespread throughout most of Europe. It extends into the far south of Italy, but is more thinly present in the Iberian Peninsula. It occurs in most of the Mediterranean islands, but not in Cyprus (Jalas & Suominen 1972, Map 38). It stretches eastwards through the Caucasus, the Himalaya, C China and Japan to the S USA. It was introduced into Mexico and New Zealand and may or may not have persisted (Hultén 1962).
E. arvense is the most common and widespread species of the genus, being circumpolar chiefly in boreal latitudes and present though less common in the Arctic, including Greenland, Iceland (Hultén & Fries 1986, Map 19). It also occurs in a prostrate form, subsp. boreale (Bong.) Á. Löve, in the Arctic circumpolar region including the mountains of Norway, Sweden, Iceland and the arctic islands (Jonsell et al. 2000).
Uses
The starch-filled rhizome tubers of E. arvense are eaten by ducks in Alaska (Hauke 1966) and in the past N American Indians both peeled and ate raw stems of the fertile cone-bearing stems and used the dried ashes of the sterile stems to treat sore mouths (Cody & Wagner 1981). Grieve (1931) list many medicinal uses for the fresh or dried sterile stems and their ashes, the main uses being as a diuretic and astringent, to staunch bleeding (including nose bleeds and ulcers) and to counter acidity of the stomach.
Names
The genus name 'Equisetum' was coined by the ancient Roman writer, Pliny and is thought to have been first applied by him to E. arvense. It is a combination of two Latin words, 'equus', a horse and 'saetum', a bristle or hair, and it is thought to refer to the bristly appearance of the jointed stems with their whorled branches (Gilbert-Carter 1964; Grieve 1931). The same notion also gave origin to the English common name 'Horsetail', which is a direct translation of the medieval Latin name, 'cauda equina', under which it was sold in apothecary shops (Prior 1879; Grigson 1974). The Latin specific epithet 'arvense' is a common one, being derived from 'arvum solum', meaning 'arable land', on which the plant is often found growing (Gilbert-Carter 1964).
There are a total of 21 additional English Common names for Equisetum species in general in Britten & Holland’s reference work (1886), many of which carry watery connotations involving pipes, frogs (and tadpoles), toads and rushes, eg Tadpipes, Tad-broom, Toadpipes, Snake Pipes, Water Grass, Cat-rushes. Several other names refer to the bushy appearance of the branched stem, for example, Cat's-tail, Colt's-tail and Bottle-brush (Britten & Holland 1886). The weedy, unwanted nature of the plant, or its malign, pernicious presence is featured in a Welsh name which is translated as 'Evil man's garters', the evil man being a standard euphemism for the devil (Awbery 1984).
Threats
None.