Urtica dioica L., Common Nettle

Account Summary

Synopsis

The Stinging or Common Nettle is probably one of the first plants we learn to recognise as children, unfortunately all too often a lesson we learn the hard and tearful way. As Pigott (1964) rather dryly and somewhat heartlessly points out, "If our early contacts with nettles lead us to dislike them, we also acquire a respect for one of the few plants which makes its presence felt." Mechanical deterrents like prickles and spines we can see and avoid, but Urtica dioica (and its close relative U. urens (Small Nettle)) are the only hairy plants in Britain & Ireland that punish us for touching them. When touched they administer a painful, hot stinging sensation that can last for an hour or so, sometimes accompanied by a blistering weal. The attack is reminiscent of that of an angry bee or wasp.

Growth form and preferred habitats

This dioecious, rhizomatous and stoloniferous perennial is represented in a wide range of damp, disturbed habitats from woodland to fens, ditches, along the banks of rivers and streams, in hedgerows, rough grassland, including on roadside verges, plus numerous moderately disturbed sites associated with man, his habitation and grazing stock. This includes any ill-managed or unmanaged waste ground, especially where materials of any sort are stacked or heaped, or where rubbish is discarded (Bates 1933; Greig-Smith 1948; Godwin 1975).

On deep, damp, nutrient-rich soils in unshaded sites, Common Nettle forms large, conspicuous, dominant, clonal patches, often around 150 cm tall, but occasionally forming a canopy up to 255 cm in height under optimal conditions (Oliver 1993a). Under drier, more shaded, or less favourable nutrient conditions, nettle plants may only achieve a height of around 30-50 cm and they display very much lower reproductive and competitive vigour than their taller counterparts. Established tall-growing nettle patches often dominate sites that are naturally well placed to receive nutrient inputs, eg at the base of slopes or along waterways. These clones are sometimes quite old (possibly 50 or more years in age). Indeed some stands, in old, long-established woodland, could even prove to be of ancient origin.

Common Nettle occurs on almost any soil except waterlogged ones, but it is only very rarely found on acid peat, or on heavily disturbed, regularly cultivated or frequently mown ground (Bates 1933; Greig-Smith 1948). Excessive repeated disturbance is the best method of extirpation of this terrible weed.

Fermanagh occurrence

Common Nettle is the ninth most widespread vascular plant in Fermanagh being present in 491 tetrads, 93% of those in the VC. In tetrad frequency it ranks ninth, just behind Angelica sylvestris (Wild Angelica) and immediately ahead of Ranunculus acris (Meadow Buttercup). In terms of record numbers, it is ranked much lower however, being the 29th most frequently recorded species in the VC.

British and Irish occurrence and status

The BSBI Atlas and the New Atlas both demonstrate that U. dioica is abundant throughout Britain and Ireland, being absent from only a few hectads on high and boggy ground in the N & W of Scotland. A Common Plants Survey organised by the conservation charity 'Plantlife' in GB and carried out for the first time in 2000 by volunteers (albeit on a small scale and totally unscientific in terms of coverage), nevertheless found that U. dioica was the most frequently recorded vascular plant in the country, being present in 93% of the sampled locations (Harper 2001).

U. dioica is probably definitely native only in fen carr alder-willow scrub and in associated 'tall herb' wetland communities. It might possibly also be indigenous in other forms of woodland, particularly under ash-alder, blackthorn scrub, or oak in some regions, although even here there may still sometimes remain an association with past human habitation (Tansley 1939; Rodwell et al. 1991a). Very occasionally, in decidedly wet or constantly humid environments, Urtica seedlings manage to colonise soils, and in similar conditions they may also grow in crevices on sheltered old walls or even as an epiphyte on trees. Otherwise we may safely consider Common Nettle to be a pronounced follower of man, occupying damp, fertile, moderately disturbed sites.

Common Nettle nutrition

U. dioica often grows luxuriantly around farms and houses, particularly beside gateways where animals wait to be brought in, along roadside verges where they are driven, and anywhere manure and excrement has been heaped. The association of Common Nettle with litter, manure, disturbed ground, habitation and rubble led to debate as to which factor most encouraged the growth of the species. Studies carried out by Pigott and Taylor showed that nettles only really thrive on luxury: when compared with many other wild plants they require, like most crop plants, large supplies of all plant nutrients. Although they are especially associated with high nitrogen and phosphate levels, Common Nettles are more 'greedy' than 'gourmet' when it comes to mineral nutrition (Pigott 1964; Pigott & Taylor 1964).

The association of Common Nettle with litter, manure, disturbed ground, habitation and rubble led to debate as to which factor most encouraged the growth of the species. Was the principal factor soil rich in nitrogen (Olsen 1921; Tansley 1939, p. 283), or was soil of loose texture – easily penetrated by the creeping, rhizome or stolon, most encouraging (Bates 1933; Greig-Smith 1948; Ivins 1952)? Pot and field experiments both showed that U. dioica always demonstrates a high demand for nitrogen, but it is at least equally greedy with respect to its phosphate requirement (Pigott & Taylor 1964). The rarity or absence of nettles from very acidic soils is explained by the fact that at pH levels below 6.0, soluble phosphate becomes chemically limited, and therefore is less available to plant roots. Furthermore, below pH 5.5 phosphate becomes almost insoluble, forming chemical complexes with iron, or with aluminium, which lock it away and render it completely unavailable to plants. In alkaline soils with a pH above 7.3, phosphate also becomes very tightly locked into calcium compounds, again making it unavailable to plants (Abeyakoon & Pigott 1975).

Soil phosphate availability, plant nutrition and nettle growth rate

Very wet, dry or cool soils also limit the availability of nutrients to plant roots in general, since the release of minerals from organic sources by the activities of micro-organisms slows down under such circumstances. Phosphate availability to plant roots is optimal between pH 6.0 and 7.0, and it is generally at a maximum at pH 6.5, but in reality other factors, eg the proportion and type of clay particles and the organic content of the soil, complicate the availability of all nutrients, and indeed these factors create a state of ever-changing flux in the soil-plant interface. This makes phosphate positioning in the soil important, since it does not migrate readily, or travel more than a few centimetres in the soil solution towards plant roots. On the plus side, this means that phosphate is not easily leached out of the soil, as happens with nitrogen, potassium and other more soluble, mobile plant nutrients. Finally it should be realised that the pH at the root surface can be significantly lower than that of the surrounding soil: organic acids are excreted by the roots themselves or by the bacteria and fungi in their rhizosphere (ie the zone around the root surface), and this also affects plant nutrient availability and obscures our understanding of it (Abeyakoon & Pigott 1975).

Growth of U. dioica colonies, eg in typical woodland soils, often becomes limited by the species reaching a point where it is unable to absorb any additional phosphate for its needs from the very dilute soil solutions that often occur in such situations. Pigott (1964) found this behaviour contrasted very strongly with that of other woodland species such as Mercurialis perennis (Dog's Mercury) and Deschampsia cespitosa (Tufted Hair-grass), which were much more able to take up phosphate from low soil concentrations. Healthy and vigorous plants of M. perennis may have as little as 40 mg of phosphorus per 100 g of dry-leaf tissue, while experimental pot-grown plants of Common Nettle show deficiency when the concentration on the same basis is around 200 mg. He found that wild grown nettle plants often contained as much as 700 mg phosphorus. This may to some extent also reflect a difference in phosphorus metabolism between the species (Pigott 1964). Most soils supporting more or less 'natural vegetation' in Britain and Ireland, when judged by the performance of nettles, are phosphate deficient. However, the plants that naturally grow on these soils are obviously able to tolerate low phosphate concentrations, and often they do not respond to experimental additional supplies of the compound.

The soils from habitats where U. dioica appears to be most likely native are usually very rich in soluble phosphate, eg moist soils in lowland alder woods, often beside streams or rivers. On the other hand, disturbed sites, especially those associated with past or present human habitation, are also phosphate enriched, due to the presence of the chemical compound in bones and other forms of discarded food waste. Rorison (1967) showed that additional calcium reduced the permeability of Common Nettle root cells to phosphorus, and that relative growth rates of the nettle and other species are correlated with soil phosphate levels. At low levels of phosphate availability, U. dioica shows deficiency symptoms and grows poorly and slowly (Nassery 1970).

Phosphate deficiency is most obvious in unfertilised upland rough pastures, particularly in the N and W of Britain and Ireland where the majority of soil parent material is highly siliceous, and where strong leaching occurs due to high levels of precipitation. Together these factors produce nutrient-poor, strongly acidic substrates unsuitable for U. dioica growth. Young stock farm animals, which require a large amount of phosphorus for their bone growth are often grazed on such ground. When they are eventually marketed, their removal from the land represents an export of phosphorus from the soil.

Severe phosphate deficiency has an important indirect effect on the supply of nitrogen in the soil, and this can also significantly affect the distribution of U. dioica. Around sites of human habitation soils are always phosphate enriched due to the incorporation of bones, crop-derived manure and household waste, including faeces. For this reason, Rackham (1990, p. 136) described man as a "phosphate gathering animal", and it explains why nettles are so commonly associated with farm buildings. Free-living, soil borne nitrogen-fixing Azotobacter bacteria appear to have a high phosphate requirement, and clover or other legumes, which have captive nitrogen-fixing bacteria in their root nodules, are often sparse or even absent from phosphate-deficient pastures (Pigott 1964).

European ecological studies of U. dioica, reviewed by Srutek & Teckelmann (1998), have shown that increasing levels of nitrogen fertiliser up to 240 kg/ha always increased both leaf and shoot biomass of the species. At high levels of nutrient supply there is a shift in assimilate partitioning in Common Nettle towards additional leaf production (Weiss 1993). On the other hand, self-shading gives rise to permanent leaf abscission, a phenomenon observed in U. dioica stands in the wild. The young nettle shoot builds a dense canopy as early as the seven expanded leaf stage, but it continues to produce new leaves and discard older ones as it grows, so that the total leaf canopy of the shoot is replaced three times during each growing season. Despite translocation of around 60% of the nitrogen content of the plant, the leaf pool of this nutrient has to be replaced from the soil twice during each growing season (Teckelmann 1987). Measurements of this kind underline the exceptionally high nitrogen demand U. dioica makes on the soil for this very soluble and mobile chemical element.

Are nettle colonies an ecological invasive problem or not?

The striking question as far as the widespread distribution and abundance of U. dioica in Fermanagh is concerned, is how to relate this and the species' high levels of nutrient demand to the very widespread infertile, gleyed or shallow calcareous ranker soils that are so characteristic of large areas the county (Cruickshank 1997, 2012). Clearly there must be very many local soil pockets of sufficient fertility to support Common Nettle, or it simply would not be present to the extent that it is. Have changes in pastoral agriculture practices over the last 50 years or so, involving impressive additional local drainage measures plus the massively increased use of fertilizers, both artificial chemical and even more commonly, the widespread use of organic manure and slurry, resulted in recent greatly augmented soil nutrient status throughout the county? Upland afforestation and the use of fertilizers and lime on the more acidic peaty soils where this activity typically occurs, would also point in the same direction. Would the increase in soil fertility be localised, or could run-off nutrients become sufficiently widespread and available to benefit weeds like U. dioica to the extent that it spreads into more than 90% of tetrads in the county? In other parts of Britain and Ireland, downwind of large-scale industrial regions, airborne nitrogen-rich pollutants might also assist nettle growth and perhaps enable the species to spread (eg in the Ochil Hills, Clackmannanshire (VC 87), Welch 2000). Although this factor (while present worldwide) is well recognised, it is most unlikely to be significant in Fermanagh or in other areas of W Ireland where the prevailing wind and weather is dominated by the proximity to the Atlantic ocean.

I believe that, while undoubtedly there have been increases in soil fertility, and particularly in nitrogen and phosphate levels in Fermanagh and elsewhere in Ireland, in general the soil fertility effects derived from the atmosphere are confined to soils of less extreme pH, to lower slope and valley ground levels, and they would be much too temporary in their duration to drastically alter plant growing conditions throughout our regional area. We might concede that U. dioica should have benefitted from such fertility factors to some unmeasured extent, and probably it has become taller, and possibly more abundant and dominant in its existing lowland sites. However, the county is not overrun with a plague of Stinging Nettles, and in numerous tetrads where infertile soils are very much the norm, the species remains scarce, sterile and depauperate, and it has to be actively searched for when plant recording!

Winter vegetative growth and horizontal spread

Irrespective of the height to which nettle stems grow locally, they die down every December after the first really hard frost of the winter, sometimes leaving slender, bare, grey ghost shoots standing throughout the remainder of the season. While frosty midwinter conditions halts the vertical growth of the nettle, it does not prevent continuing horizontal growth of its rhizomatous underground stems. These spread out under the loose but bulky sheltering soil litter layer, formed by the fallen leaves and withered shoots of the previous year. The buried, creeping rhizomes send up cream, pink or red surface-running stolons, and in milder spells of weather during the four coldest winter months, individual stolons can achieve a total horizontal spread stretching to somewhere between 10 and 120 cm. The stolons branch frequently and root freely at their numerous nodes (Oliver 1993b, 1994). Field measurements show that an overwintering individual nettle plant can spread up to 2.5 m in diameter from its original starting point entirely by this vegetative means (Oliver 2001).

In contrast to the appearance of the slender stolons, older portions of rhizome and deeper, established roots are covered with an extremely distinctive bright yellow, furrowed corky layer. With increasing age, these tissues become rather woody and mechanically very tough (Olsen 1921). Given time, the rhizome material in soil can become very extensive, eg a riverbank study in England which sampled one square metre of soil unearthed a total rhizome length of 63.41 m (K.G.R. Wheeler, pers. com., 1995, quoted by Oliver (1997)). Similar rates of midwinter horizontal stem growth and creeping dispersal are possible under any form of sheltering and insulating debris, organic or mineral. All winter extension growth of this type consumes stored energy present in the rhizome and root tissues (Bates 1933; Oliver 1993b, 2001).

Spring growth

Fresh green vertical shoots, generally unbranched, arise from the rhizome and stolon system, usually appearing in early March. In 2004, which was an exceptionally mild winter, these young shoots first appeared in mid-February. The leaves on the early shoots are extremely variable in size, shape and degree of hairiness, yet because of their unforgettable stinging ability, we quickly learn to recognise nettle leaves in all their guises

The roots usually lack mycorrhizas, and Abeyakoon & Pigott (1975) found none in over 20 root systems they sampled from natural habitats. The aerial stems are generally unbranched, although often later in the season some lateral branches may be produced towards the top of the stems. This typically occurs as a response to frost injury of the bud at the stem apex, or as a result of other physical damage, including trampling or browsing when the stems are young and still palatable to animals (Greig-Smith 1948).

Nettles spread both vegetatively, by means of its underground rhizome and overground creeping stem, and also by seed production. The annual spread of nettle rhizomes is between 35 and 45 cm but, in addition, the rhizome also branches frequently, enabling quite rapid and effective colonisation and almost simultaneous dominance by the tall aerial stems to take place (Salisbury 1942, p. 216-7).

Flowering

The age of the plant at first flowering is not known, but it does not flower in the first year (Greig-Smith 1948). Typically, flowering occurs from late May or early June through to September. Since the species is normally dioecious (ie having separate male and female plants), some colonies may be unisexual. Some of these unisexual colonies might perhaps have formed from a single individual, so that seed production is sometimes localised. However, U. dioica has been widely introduced to many countries, and as a result, gene exchange has taken place with other closely related Urtica species. The increased genetic variation that this gives rise to means the traditional taxonomic distinguishing characters between species have become unreliable. Sometimes the variation is so great that only quantitative differences can be made between forms, and the species complex becomes unclear, with transitional forms and numerous named varieties being proposed (Hultén 1974, p. 294). This applies even to normally very conservative characters, including those governing the reproductive strategy of the species. Consequently, in some areas of the world U. dioica has become so variable that monoecious forms occur.

In any event, huge numbers of male flowers are produced, up to about 1,200 per stem node, and they each release their pollen explosively (Hickey & King 1981). Efficient wind dispersal of the vast number of pollen grains normally ensures adequate fertilisation of the female flowers, each of which produces a solitary oval seed (ie an achene), after obligatory cross-pollination (Greig-Smith 1948).

Seed production, dispersal and longevity

Seed is shed from late June onwards, although often some viable seed still remains on dead stems through until the following January (Greig-Smith 1948). Each achene is enclosed by four roughly hairy (ie hispid) persistent perianth segments of the female flower. These structures allow the achene to adhere like a burr to fur, feathers and cloth, and this same structure probably also assists with wind dispersal (Ridley 1930). The occurrence of nettles sometimes growing high above ground level on walls strongly supports the premise of wind dispersal, but it is possible that seed ingested by birds could also be transported to such elevated sites. The fruits are probably quite often dispersed in multiples, since the inflorescence may remain intact and be dispersed as a unit. Dispersal may also occur after ingestion by animals, achenes having been recorded in the faeces of cattle, fallow deer and magpies (Ridley 1930).

U. dioica produces large quantities of seeds (some large clones probably generating billions of seeds (Bassett et al. 1977)). The seed or achene develops a persistent soil seed bank (Odum 1978, quoted in Grime et al. 1988). This being the case, dispersal of the species by seed and rhizome fragments in transported soil is also highly probable.

Germination

Germination is stimulated by direct sunlight, or in a shaded site by fluctuating temperatures. Germination rates are rather variable, even under favourable conditions, typically falling between 26% and 96% (Greig-Smith 1948). Seedlings are found mainly in the spring on open, disturbed ground, particularly on soils that have previously been very wet (eg areas where puddles have formed) (Greig-Smith 1948; Grime et al. 1988).

Variation and taxonomy

The opposite and stalked, coarsely toothed, rough textured leaves are extremely variable in size, shape and degree of hairiness. Almost hairless (ie subglabrous) forms also exist (Stace 2010). The slender, tapering stinging hairs are made of silica and are mingled with non-stinging ones on the leaves and stems of the plant, their density being very variable. A rare almost completely non-stinging form of nettle occurs in Britain. It was at first almost exclusively associated with Wicken Fen and Chippenham Fen both in Cambridgeshire (VC29), but later reported in Berkshire (VC22), Norfolk (VCs27 &28), SE Yorkshire (VC61) and Angus (VC90) in E Scotland. In the past, the non-stinging nettle has been given the names, var. subinermis Uechtr. and var. angustifolia (Butcher 1961; Pollard & Briggs 1984a; Cook 1997; Beckett & Bull 1999). Most recently the non-stinging form has been elevated to species rank as U. galeopsifolia Wierzb. ex Opiz., by the Russian botanist D.V. Geltman. He regards the non-stinging English nettle as part of a mainly E & C European segregate which he considers "presumably diploid" (2n=26). Geltman regards the much more widespread stinging Common Nettle, U. dioica, as being a tetraploid with 2n=52 or 48 chromosomes (Geltman 1992). The existence of transitional intermediates, however, obliges Geltman to admit the possibility that U. galeopsifolia may not be a "completely good" species in terms of the species concept of Flora Europaea (Tutin et al. 1, 2nd ed., 1993). Therefore Geltman recognises that many botanists may prefer to accord the non-stinging nettle subspecific rank.

Geltman (1992) also suggests that U. dioica is probably of hybrid origin, the likely parents being U. galeopsifolia (or a species closely related to it) and U. sondenii (Simm.) Avrorin ex Geltman, a form which occurs in W & C Siberia and in N Scandinavia (see Jalas & Suominen 1976, Map 323 – where it appears as U. dioica subsp. sondenii).

The stinging hair and its chemistry

The stinging hair or trichome (or "stinging emergence", as E.L. Thurston (1974) prefers to call it after his careful study of its fine structure), consists of a fine capillary tube calcified at its lower end and silicified at its upper end. It is closed at the tip by a tiny bulbous swelling. The silica-rich upper portion of the hair is brittle like very thin glass (Salisbury 1964), and the bulb at the hair tip readily breaks off along a pre-determined line when it comes in contact with skin. The break produces a fine, needle-like point formed by an oblique fracture at a line of weakness in the upper tapering region of the hair. It only requires very slight pressure for this extremely sharp, slender needle to penetrate the skin, and the attendant compression of the unsilicified bladder-like hair base injects the contained fluid into the minute wound (Emmelin & Feldberg 1947).

The burning pain of a nettle sting is so strong that Germans call the plant 'Brennessel', 'brenn' meaning 'burning, branding or stinging', and 'essel', the equivalent of our, 'nettle' (Betteridge 1957; Simons 1992). The proverbial advice to 'grasp the nettle' is good advice, since whenever the nettle plant is handled roughly the hairs tend to be broken lower down rather than at their tip, and thus they are not so sharply pointed and do not penetrate the skin (Salisbury 1964).

The nature of the sting has been a topic of investigation ever since Robert Hooke examined the hairs with his microscope in 1665, yet despite a great amount of biochemical and pharmacological research over the past 120 years, the precise nature of the nettle sting toxin still remains something of a mystery (Thurston & Lersten 1969; Pollard & Briggs 1984b). It is still commonly thought by many members of the general public that the active chemical producing the sting is formic acid. However, this is now recognised as being incorrect. Formic acid is almost certainly absent from the stinging fluid, or else it is in much too low a concentration to produce the painful stinging effect (Thurston 1974).

The first investigators to use pharmacological techniques involving in vitro bioassays to test the effects of stinging hair extracts on living systems were Emmelin & Feldberg (1947). Their study found a combination of histamine and acetylcholine present, which they concluded produced the stinging sensation, the former irritating the skin and the latter producing the burning sensation. They also showed that acetylcholine on its own had little irritant action, but in combination with histamine it produced an immediate stinging pain (Emmelin & Feldberg 1947). A few years later, a third substance in the sting fluid was identified as 5-hydroxy-tryptamine (serotonin), which like histamine and acetylcholine is also present in animal tissues and causes inflammation and a rash on the skin (Collier & Chester 1956). In animal tissues, these three chemicals are neuro-transmitters that can induce contractions in smooth muscles, accompanied by a fall of arterial blood pressure and inhibition of the heart muscles. In plants, they appear to exist purely to sting and deter herbivore browsers (Starkenstein & Wasserstrom 1933; Emmelin & Feldberg 1949). However, it should be noted that an Urtica cell extract completely free of these three chemical compounds still elicits a painful response on the skin, indicating that additional compounds are involved in producing the characteristic deterring reaction (McFarlane 1963).

Chemical constituents have been found in Rumex obtusifolius (Broad-leafed Dock), which inhibit 5-hydroxytryptamine, and this helps explains why rubbing dock leaves on a nettle sting is so soothing (Brittain & Collier 1956).

Further doubts and debate regarding the chemical nature of the sting consider the quantities of the three supposed pain-inducing compounds are much too low to produce such a significant irritant effect (see Pollard & Briggs 1984b, pp. 508-9). Similar investigation of the stinging irritants in the related genus Laportea carried out by MacFarlane (1963) found the same three chemicals present, but another unidentified substance that is not dialyzed through cellophane, appeared to be much more active in producing pain than acetylcholine, histamine and 5-hydroxytryptamine.

Leucotrienes

In 1979, biochemists discovered and named compounds called leucotrienes (also spelt 'leucotrines' in some papers) in animal tissues. These substances are capable of inducing persistent cutaneous wheals after injection into human skin, even when present in only very minute quantities below the nanogram per millilitre level. Leucotrienes have also been found in insect venom and in the stings of sea-animals (Czarnetzki et al. 1990a). Using RP-HPLC (reverse phase high pressure liquid chromatography) and RIA (radioimmunoassay), Czarnetzki et al. (1990b) were able to show high levels of leukotriene B4 and leukotriene C4 and histamine in the urticating (ie stinging) fluid of U. urens. Leucotrienes are a family of biologically active compounds described as eicosanoid inflammatory mediators. They were first discovered in mammalian leukocytes, being produced by the oxidation of arachidonic acid (AA) and the essential fatty acid eicosapentaenoic acid (EPA) by the enzyme arachidonate 5-lipoxygenase. They have since been found in other immune cells. In animals, they participate in host defence reactions and pathophysiological conditions, such as immediate hypersensitivity and inflammation. In mammals, these compounds have potent actions on many essential organs and systems, including the cardiovascular, pulmonary and central nervous system as well as the gastrointestinal tract and the immune system. In addition, leukotriene production is usually accompanied by the production of histamine and prostaglandins, all of which act as inflammatory mediators (https://en.wikipedia.org/wiki/Leukotriene).

Budavari (1996) describes the leukotrienes as potent broncho-constrictors with a role in immediate hypersensitive reactions and some as potent chemotactic agents. She suggests it is the chemotactic role of the leukotrienes that gives a longer, stronger stinging effect to the nettles. Budavari (1996) characterizes histamine as a potent vasodilator involved in allergic reactions.

In common with the other compounds involved in generating the stinging effect, exactly how these chemicals are produced from fatty acids in plant cells remains mysterious. It is certainly beyond the chemical understanding of the present writer. However, the fact that these several different biochemical compounds have been located in stinging hairs of both U. dioica and U. urens, and their role in animal cells is known to involve or include the induction of inflammation, makes it very likely that they are actively involved in producing the nettle sting.

Further studies indicate that the chemical cocktail in the Urtica trichome includes significant levels of tartaric and oxalic acids, both of which induce a pain reaction and help extend the duration of the pain experienced when histamine, acetylcholine and serotonin are present (Han Yi Fu et al. 2006).

Treatment for stings

When stung, to minimise the pain it is important to avoid touching the affected area for at least 10 minutes. The best approach is to wash the stinging fluid off the skin without touching it, using liquid soap and lukewarm water. Applying the juice from a leaf of an Aloe vera plant, or using a manufactured product with a high concentrations of aloe vera, can help to manage the red and inflamed skin area and reduce the painful burning sensation. Cold compresses or bathing in tepid water are also recommended as ways of relieving the burning skin reaction (Cooper & Johnson 1998).

The deterrent effect

While many invertebrates (particularly insect larvae, slugs and snails), can attack nettle leaves with impunity, mammalian herbivores (eg rabbits, sheep and horses) are positively deterred by the numerous irritant hairs. The greater the density of stinging hairs, the more the plant is avoided, a learned behavioural reaction which occurs to the extent that clonal nettle patches in pastures become free to expand, unless they are mechanically cut or otherwise managed (Pollard & Briggs 1984b). Protected in this way, dense nettle colonies may smother out grass and reduce the grazing area available in pastures, since unlike the case of isolated thistles or many other poisonous or distasteful weeds, the stock animals cannot browse vegetation between the plants, for fear of the burning sting (Bates 1933).

The protective function of the sting

The highly specialized structure and chemistry of the nettle stinging hair suggests that it is unlikely to have any function other than defence against herbivores (Pullen & Gilbert 1989). While it may appear obvious that possession of the sting affords such protection, it took considerable care and effort to design and execute the experiments which proved that many invertebrates (particularly insect larvae, slugs and snails) can attack nettle leaf tissues with impunity, yet mammalian herbivores (eg rabbits, sheep and horses) are positively deterred by the irritant hairs.

Having said this, the situation is not one of complete mammal avoidance. In common with some animals' reaction to toxins in poisonous plants, mammals will browse stinging or mechanically protected plants if they are sufficiently hungry. It has been reported that some breeds of domestic cattle avoid nettles, while others eat them readily (Uphof 1962). Stinging hairs of U. dioica have also been found in the faeces of a number of mammalian herbivores (see Seed Dispersal below). In general, however, it has been shown that the greater the density of stinging hairs the more the plant is avoided by browsing mammals, a learned behavioural reaction that occurs to the extent that clonal nettle patches in pastures expand unless they are cut or otherwise managed (Pollard & Briggs 1984b).

Significant variability in stinging hair density exists within many examined nettle populations, and it has been shown to have a genetic basis (Polland & Briggs 1982). Later experiments by these workers found that the interaction of large animal herbivores with variation in stinging hair defences could be an important selective force in nettle populations displaying a typical range of variation. While the experimental results do not suggest that stinging is unimportant to invertebrate herbivores, the stinging mechanism does seem particularly well suited to deter larger animals. Large herbivores cannot eat 'around' stinging hairs in the way that insect larvae or molluscs can, and the immediate deterrence produced by a sting's burning sensation will usually act before significant quantities of plant biomass have been consumed by the larger grazing animals (Pollard & Briggs 1984b).

After browsing damage by vertebrate herbivores, or after mechanical clipping to manage or control nettle patches, the density of stinging hairs on regrowth stems and leaves is significantly higher than on the initial nettle growth. Since stinging hairs are presumably energetically expensive for the plant to produce, it would be strategically advantageous for an individual to be able to produce only as many of them as existing herbivore pressure necessitates. The observed increase in stinging hairs after grazing is thus an example of an induced response to environmental pressure (Pullin & Gilbert 1989).

Subsequent experiments using degrees of leaf and stem apex clipping to mimic grazing showed that there are differences in response even between the sexes of plants, eg with respect to regrowth, branching, reproduction and stinging hair density. In the case of the latter, density was higher on the new leaves of female plants than on males, which might be explained by the greater demand for defence in females due to their higher and longer allocation of resources to reproduction (Mutikainen et al. 1994). Similar earlier experiments by Pullin (1987) found that there was an increase in nitrogen levels and water content in fresh leaves re-grown by nettle plants after clipping, when compared with mature leaves. This increase in leaf quality allowed higher growth rates of the specialist herbivore larvae of Aglais urticae (Small Tortoiseshell butterfly).

The authors of the original Typescript Flora of Fermanagh noted that Common Nettle was particularly abundant (and viciously stinging!) on the screes below the limestone cliffs at Knockmore, a site long frequented by feral goats whose droppings over many generations must certainly have encouraged the plant's growth (Meikle et al. 1957).

Environmental factors encouraging nettle population spread

Changes in pastoral agriculture practices, common in Fermanagh and elsewhere, over the last 60 years, have involved impressive additional land drainage measures, plus the massively increased use of fertilisers, both artificial chemical and organic. The widespread, regular spraying of fields with liquid manure and slurry has resulted in greatly augmented soil nutrient levels throughout most of the county. In addition, upland afforestation and its use of fertilisers and lime on the more acidic, peaty soils, has also led to further soil nutrient enrichment downstream. The increase in soil fertility may sometimes be very localised, but due to high rainfall levels, nutrient run-off also occurs and is sufficiently widespread to benefit common weeds like U. dioica. The question remains, has soil nutrient enrichment occurred to the extent that it encouraged U. dioica to spread into more than 90% of Fermanagh tetrads?

I believe there have been widespread, significant increases in environmental nutrient enrichment of freshwater bodies and soil fertility, and especially in nitrogen and phosphate levels. However, the greatest effects of this are confined to waters and soils with less extreme pHs, and to those situated on lower slopes and in valleys. The effects would be much too temporary in their duration to drastically alter plant growing conditions throughout our whole area. U. dioica will certainly have benefited from the increased fertility to some unmeasured extent, and probably the plant has become taller and perhaps more abundant and dominant in its existing lowland sites. However, the county is not overrun with a plague of Common Nettle, and in numerous tetrads where infertile soils are the norm, the species remains scarce, sterile and depauperate, so that it has to be actively searched for during plant recording.

Dominance and Plant Associations

Nettle patches in unshaded, relatively undisturbed sites are often either pure (ie single species) stands, or else they have very low species diversity, reflecting the very strong competitive ability of this often dominant, tall, herbaceous plant. Frequently, the only closely associated species in dense nettle clumps is the clinging climber Galium aparine (Goosegrass), which is so vigorous it can scramble over and sometimes smother the tall supporting stems of Urtica dioica. The National Vegetation Classification (NVC) recognises this vegetation as OV24, the Urtica dioica-Galium aparine community (Rodwell 2000, 5, p. 406-9). The NVC also lists Common Nettle as a component in 18 other communities of open vegetation, and it is a constant species in another one of them, OV25, the Urtica dioica-Cirsium arvense community. Open vegetation nettle patches, such as these communities represent, might give way to a more closed canopy woody vegetation, but the dense habit of the dominant nettle plant – associated with its tangle of rope-like rhizome, a deep layer of litter in autumn, and rapid spring canopy regeneration – all tends to greatly hinder the invasion of U. dioica's territory by any other plant species, including woody ones (Srutek & Teckelmann 1998).

Nettle stems frequently provide mechanical support for Galium aparine, but in parts of Britain they are also quite often entangled and parasitized by Cuscuta europaea (Greater Dodder), for which the Stinging Nettle appears to be the primary host plant (Holland 1981). This parasitic annual or rarely perennial species does not occur in Ireland; it is regarded as native in S England, but as an introduction it may be spreading northwards in Britain, while at the same time apparently declining in some of its southern stations (F.J. Rumsey, in: Preston et al. 2002).

European occurrence

U. dioica is shown in the Atlas Flora Europaea as commonly and continuously present throughout Europe, the distribution thinning only towards the east – although, in reality, this might merely reflect recording effort in those parts of the continent (Jalas & Suominen 1976, Map 322). U. galeopsifolia and all its synonyms fail to feature in this particular European Atlas, being simply subsumed in U. dioica. Beyond Europe, the distribution of U. dioica s.l., which includes up to eleven forms variously ranked as species, subspecies or varieties, is shown by Hultén (1974, Map 285) as being almost completely circumpolar in the temperate regions of the N Hemisphere.

World occurrence

In the stricter sense (often recognised as U. dioica subsp. dioica), the Stinging or Perennial nettle is mapped by Hultén (1974) and Hultén & Fries (1986, Map 635) as occurring as a native form throughout Europe and along the coast of N Africa, extending east to Lake Baikal where it meets the usual E Asia form, U. dioica var. angustifolia. U. dioica subsp. dioica also extends southwards into Asia Minor and eastwards through Iran to Pakistan. This form of the species has also been recorded as an introduction in South Africa, St Helena, Ethiopia, as well as in N & S America, including Mexico.

In N America, north of Mexico, in addition to three native annual Urtica forms recognised at the species level, there is just one perennial species, U. dioica, and within it three subspecies (Woodland et al. 1982). Of these, the American Stinging nettle is U. dioica subsp. gracilis (Ait.) Selander; it exists in both diploid and tetraploid forms (2n=26 & 52), the latter having a strictly western distribution – the Rocky Mountains forming an effective barrier between the two ploidy levels. Subspecies gracilis extends south to Louisiana, New Mexico, Arizona and California (for a map of these N American forms of U. dioica occurring in Canada, see Basset et al. 1977, Fig. 3).

In N America, the 'European Stinging nettle' (ie subsp. dioica) is considered a relatively recent introduction into or invading the range of subsp. gracilis. Subspecies dioica was first recognised from Stone Mills, Bay of Quinte, Ontario in 1877, and since then has been collected only rarely in scattered locations in E Canada & USA, usually near seaports, including abandoned fishing ports, ballast heaps and railway yards. Very likely it arrived with ships’ ballast or as a seed contaminant in cargo, and has become accidentally spread inland by man. Many herbarium specimens appear to bear aborted or underdeveloped female flowers, perhaps due to a lack of pollen, a fact which suggests that subsp. dioica may have increased its range largely by asexual means, ie through transport of fragments of rhizome (Woodland 1982; see his Fig 2 for distribution map of subsp. dioica and subsp. gracilis in N America).

The third N American form of U. dioica is subsp. holosericea (Hoary Nettle), a native diploid (2n=26), which is a polymorphic complex displaying considerable phenotypic variation. It is confined to western states where it scarcely ever meets subsp. dioca (see map, Fig. 1 in Woodward 1982). In experimental pairings, tetraploid U. dioica subsp. dioica was genetically compatible with other tetraploid taxa, but incompatible with diploids (Woodland et al. 1982).

U. dioica subsp. dioica and subsp. gracilis, plus U. urens, are all present in New Zealand as introduced plants colonising waste places and cultivated land. The European form, subsp. dioica, which was first recorded there in 1870, is more widely scattered than the N American subsp. gracilis, which wasn’t recorded until 1944. New Zealand also has five native Urtica species, of which the woody shrub U. ferox Foster (Tree Nettle), has an extremely vicious sting – indeed it is said to have actually killed people (Webb et al. 1988; Roy et al. 1998).

Hultén (1971, p. 294) remarked that where U. dioica s.s. (also known as U. dioica subsp. dioica) meets other Urtica taxa native of the respective country, gene exchange apparently often takes place, and very difficult taxonomic problems are created. Subsequent biosystematic study of Urtica in N American by Woodland et al. (1982) and Woodland (1982) greatly clarified and simplified the situation there, as detailed above.

Uses

Young nettle shoots are often boiled and eaten like spinach, and nettle broth is a well-known, delicious, healthy food. In medical situations, when nettle stinging hairs come into contact with a painful area of the human body, they can actually decrease the original pain of the patient. Scientists think the nettle sting ingredients do this by reducing levels of inflammatory chemicals in the body, and by interfering with the way the body transmits pain signals. Stinging nettle has been used for hundreds of years to treat painful muscles and joints, eczema, arthritis, gout, and anemia. Today, many people also use it to treat urinary problems during the early stages of an enlarged prostate (called benign prostatic hyperplasia or BPH). It is also used for urinary tract infections, hay fever (allergic rhinitis), or in compresses or creams for treating joint pain, sprains and strains, tendonitis, and insect bites (Konrad et al. 2000; Safarinejad 2005; Schneider & Rubben 2004 and many references available online).

In the past, the strong fibres present in stem tissue were used to make good quality cloth and paper, a practice remembered in fairy stories and revived during the First World War (see Grieve 1931, pp. 574-9, for a full account of many such uses, and see also Vickery (1995)).

Names

The genus name 'Urtica' is derived from the Latin 'uro', meaning 'to burn', the reference to the sting being all too obvious (Hyam & Pankhurst 1995). The Latin specific epithet 'dioica' is a Latinized form of two Greek words, 'di' and 'oikos', meaning 'two households'. This refers to the nettle plants being unisexual (Gilbert-Carter 1964).

The English common name 'Nettle' is derived from the Anglo-Saxon and Dutch word 'netel', which according to Prior (1879) is the "instrumental form" of 'net', itself the passive participle of 'ne', a verb common to most Indo-European languages, meaning 'to twist', 'to spin' or 'to sew'. 'Nettle' is thus connected with the plant's long, strong fibres providing good quality thread and cloth, which it did from prehistoric times up until the Industrial Age, when it was replaced by linen and cotton (Grigson 1987).

Nine alternative common names are supplied by Grigson (1987), several of which refer to the sting, eg 'Tenging-' or 'Tanging-nettle'. Names such as 'Heg-beg', 'Hidgy-pidgy' and possibly even 'Hoky-poky', originating in places as far apart as Scotland and Devon, may possibly be derived from the Anglo-Saxon 'hege' or 'haga', meaning 'hedge', often the place where nettles are found growing. Names such as 'Devil's Leaf', 'Devil's Plaything' and 'Naughty Man's Plaything' suggest connection with Danish nettle folklore which held that nettle patches marked where elves lived, and that stings were a protection from sorcery (Grigson 1987).

Threats

None.

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