Equisetum palustre L., Marsh Horsetail

Account Summary

Native, very common and widespread. Circumpolar boreo-temperate.

1881; Stewart, S.A.; Co Fermanagh.

March to December.

Growth form and preferred habitats

Marsh Horsetail is a characteristic rhizomatous perennial of wet to moist, moderately to slightly base-rich, often calcareous or dolomitic (ie magnesium rich limestone) habitats. It tends to occur as scattered individuals or clonal patches on the margins of small streams and ditches in marshy ground, including lakeshore fen-scrub woods, damp to wet meadows that seasonally flood and hedgerow ditches. In upland areas, Marsh Horsetail frequents calcareous flushes in acid, peaty, heather or grass dominated moorland. It grows beside lakeshores and pools at every altitude, including those on cut-over bogs, in disused quarries and gravel pits – always provided there is some lateral water movement and a degree of base-enrichment.

As its English common name suggests, Marsh Horsetail is more or less confined to these wetland habitats and in this respect it differs somewhat from its close relative, the much more weedy E. arvense (Field Horsetail), which is frequently found growing in relatively dry ground, including arable fields – if there were any of those around Fermanagh!

Plants of E. palustre vary greatly in the amount of branching they produce (see below), but they can readily be distinguished from the more frequently found E. arvense by the first internode of the side branches always being shorter than the adjacent stem sheath. Also, when the plant bears its black spore-producing cones, they are found on the tips of the slender green branches.

Like E. arvense, E. palustre readily colonises disturbed ground, and on marshy ground and in peatland situations it often becomes abundant after the original surface vegetation has been broken or trampled, for instance along tracks and beside streams, or along freshly dug boggy ditches. Borg (1971) has reported similar behaviour of E. palustre on Finnish bogs and fens.

In a detailed Dutch study of 1,000 agricultural fields, Marsh Horsetail was found to be more common on 29% of the fields surveyed that were regularly mown, than on those which had been under continuous grazing (8% of the fields surveyed). E. palustre was also much more frequent on soils low in potassium or phosphorus than when these nutrients were in good supply. In general, the study found Marsh Horsetail was a rare weed of well-drained, grazed pastures that were supplied with a manure rich in phosphorus and potassium (Sonneveld 1953 (in Dutch), quoted by Holm et al. 1977). Typically, Marsh Horsetail is a minor component of moderately fertile, grazed, herb-rich wet grassland conditions.

Variation

E. palustre is extremely sensitive to environmental change, varying markedly in size and form with degree of exposure. In the past, this variability led to numerous growth forms being named which are nowadays regarded as taxonomically insignificant. Plants in sheltered, somewhat shaded positions are much taller and are well furnished with whorls of upswept branches compared with the stunted and almost or entirely unbranched forms that occur in more open or windswept upland sites (Page 1988, 1997). These often very small, unbranched horsetails require careful identification, as sparsely scattered shoots of several horsetail species occasionally co-exist, eg E. arvense, E. palustre and E. variegatum (Variegated Horsetail).

Shade, trampling, spring frosts, or even air turbulence associated with road traffic can cause damage to the delicate growing point of E. palustre, causing the normally erect plant to become prostrate and bushy (Holm et al. 1977).

Spore production, dispersal and sexual reproduction

Dispersal in all horsetail species, including E. palustre, is surrounded by a degree of mystery, but we have to presume that in the case of transport to occupy a freshly created vegetation gap, it either involves wind dispersal of spores, their germination in a suitable soil and then subsequent delicate prothallial sexual process or, alternatively, vegetative spread. The sexual production of a new generation of the sporophyte plant is probably the least likely, or least frequent mechanism of increase and dispersal, since spore production and release is seasonal, the spores are short-lived and the many stages of the sex process require very specific environmental conditions. All stages of the sexual reproductive process involve high levels of risk and a failure of any of them negates the whole venture (Duckett & Duckett 1980).

Hybrids

E. palustre forms hybrids with E. arvense ( E. x rothmaleri C.N. Page) and E. fluviatile (E. x dycei C.N. Page). The former has not yet been recorded anywhere in Ireland although there are 30 records widely scattered in Britain (Stace et al. 2015). E. x dycei has been found in five sites across Ireland and in 33 well scattered sites, mainly in the N & W of Britain (Stace et al. 2015).

Vegetative reproduction

Vegetative reproduction or spread, on the other hand, may prove less risky and could be achieved in several ways, either by the fortuitous arrival of a tuber or rhizome fragment, perhaps transported by water, or in mud attached to animal or human traffic. Alternatively, if an already established clone exists nearby, E. palustre may spread laterally by means of normal rhizome growth, both horizontal and vertical, eventually to occupy the vegetation gap at the soil surface, a form of dispersal known as 'diffusion'. In addition, soil disturbance could fragment the hardy and resilient rhizome and attached tubers, allowing the possibility of transport to fresh suitable sites.

Underground stem or rhizome

The rhizome of Marsh Horsetail is remarkably robust and wide in diameter when compared with its slender aerial stems and while the individual rhizome lives only a few years, it produces buds at its nodes which allow it to branch and spread underground and then develop fresh aerial shoots. Occasionally the rhizome bears tuberous outgrowths. Individual plants in a favourable habitat are known to have spread over 100 m in a few years (Holm et al. 1997). The plant does not however tolerate firm, packed soils, and rhizomes have been known to travel considerable distances in soft subsoil to avoid compacted upper horizons before the photosynthetic shoots re-emerge in more porous substrates (Borg 1971).

A pioneer colonist, competitive ability poor

It appears that Marsh Horsetail only competes effectively if its optimal growing conditions are met. As a result, it frequently behaves as a pioneer colonist, being abundant in suitable open, disturbed, muddy habitats in the early more open stages, but it declines thereafter as other species arrive, compete and become dominant. However, E. palustre is usually able to survive as a scarce, often diminutive companion species, provided the potential dominants are restricted in their vigour by grazing, cutting or some other destabilising environmental factor(s). Marsh Horsetail has a rather sparse canopy of slender shoots and this, together with its comparatively poor competitive ability, means it is not normally a dominant species in any plant community in which it occurs (Borg 1971; Grime et al. 1988).

While E. palustre occurs in a very wide range of wetlands, yet in all the vegetation communities it frequents, it remains a minor component. It is difficult to typify its role and status beyond that of a pioneer colonist of gaps and bare ground associated with disturbance. It may be reasoned that since the main part of the horsetail species grows well below ground level, it does not really belong to the same vegetation stratum as most of the more shallow-rooted species with which it associates. This revelation helps explain the wide range of vegetation types in which Marsh Horsetail occurs, albeit in many instances having a very low presence in them, and its relationship with associated species and vegetation communities are not firmly set. This matter is further complicated, since the rhizome of E. palustre confers exceptional persistence on the species, even in a changing environment. Possessing a vigorous rhizome allows the horsetail to survive and effectively ignore dynamic vegetation modifications happening around and above it, so that the species may become a relict of past plant communities in a particular spot of ground (Borg 1971).

Plant associates and communities

Having said this, we can identify some of its more regular associates in particular habitats. For instance, in moderately fertile marsh grasslands, the common associates of E. palustre include Filipendula ulmaria (Meadowsweet), Caltha palustris (Marsh Marigold), Geum rivale (Water Avens), Lycopus europaeus (Gypsywort), Angelica sylvestris (Wild Angelica) and Cirsium palustre (Marsh Thistle), together with numerous species of rush and sedge. In some of the marshy grasslands, swampy fens, woods and heathland, E. palustre overlaps and associates with E. arvense and E. fluviatile (Water Horsetail), and especially in hedgerow ditches, it can associate with E. sylvaticum (Wood Horsetail) and E. telmateia (Great Horsetail).

In more upland acid grassy blanket peat moorland, E. palustre frequently occurs in sloping flushes and beside small streams where the flow of base-rich spring water enhances both the nutrient supply and the aeration of thinner layers of peaty or silty soils. Here, and around moorland lakes where there is similar spring water enrichment, for instance in Fermanagh, around Spectacle Lough and Drumcose Lough, Marsh Horsetail occurs with species such as Potentilla palustris (Marsh Cinquefoil), Galium uliginosum (Fen Bedstraw), Eriophorum latifolium (Broad-leaved Cottongrass), Parnassia palustris (Grass-of-Parnassus), Schoenus nigricans (Black Bog-rush or Black Beak-sedge), Pinguicula vulgaris (Common Butterwort), Carex dioica (Dioecious Sedge), Selaginella selaginoides (Lesser Clubmoss), Cirsium dissectum (Meadow Thistle) and, occasionally, E. variegatum (Variegated Horsetail). In a similar flush, but at lower altitude below the cliffs of Poulaphouca, close to the shore of Lower Lough Erne, E. palustre has again been found growing along with E. variegatum.

Fermanagh occurrence

In Fermanagh, it is the third most frequent horsetail being found in 248 post-1975 tetrads, 47.0% of those in the VC. It is especially frequent around Upper and Lower Lough Erne and on the upland limestone, bog and heath in the west of the county.

British & Irish occurrence

A common and widespread deciduous species throughout Britain & Ireland, E. palustre is perhaps somewhat less common in the S and W of Ireland due to the prevalence in this part of the country of heavily leached, base-poor, excessively acid soils (Jermy et al. 1978).

European and world occurrence

In Europe and beyond, its widespread distribution closely matches that of both E. arvense and E. fluviatile, extending from Iceland to almost all of Europe southwards to Spain, Italy, Greece and Turkey (Jalas & Suominen 1972, Map 35). The world distribution stretches eastwards through much of N Asia to almost encircle the arctic, although there is a distinct gap in N America in the central provinces of Canada (Hultén 1962, Map 89; Hultén & Fries 1986, Map 16).

Toxicity

In common with other species of Equisetum, E. palustre contains high levels of silica, plus the thiamine-destroying enzyme thiaminase, and the toxic alkaloid(s), palustrine and/or equisetine. When animals are allowed to graze selectively they avoid horsetails. However, in cut and stored hay, there is little way they can do so, and the toxins survive drying and storage. Tests in Finland showed that as little as 2 gm of dried horsetail in the daily fodder of cattle caused their milk yield to decline. Larger amounts of horsetail caused lack of appetite, diarrhoea and general illness of the animals (Borg 1971). Horses are not as sensitive as cattle, but if fed small amounts over long periods, they can also suffer serious poisoning (Holm et al. 1977).

Weed control

In spite of numerous and prolonged trials, a really effective method of controlling the growth of E. palustre has not yet been developed. The aerial shoots are easily destroyed by any of several herbicides, but the deep, wide-ranging rhizome survives and persists, and its starch reserves simply cannot be exhausted by attacking the aerial parts of the colony. The systemic herbicide MCPA gives the best penetration into the rhizome, and if low rates of application are used, the aerial shoots are not destroyed as quickly as at higher rates. This allows more time for the herbicide to be translocated downwards into the rhizome. Using this MCPA herbicide procedure in conjunction with both sub-surface drainage and intensive arable cultivation, rather than allowing prolonged grass leys to develop, appears to be the best available agricultural treatment of the Marsh Horsetail problem in pastures (Borg 1971).

Names

The Latin specific epithet 'palustre' is derived from 'palus', meaning a swamp or bog, and translates as 'growing in swampy places' (Gilbert-Carter 1964). E. palustre shares some of the English common names applied to horsetail species in general, and they are? similar to those applied to the more common species E. arvense. Example include Cat-whistles, Marshweed, Paddock Pipes and Snake Pipes (Britten & Holland 1886).

Threats:

Drainage and other agricultural improvements and acidification associated with forestry plantation.