Datasheet

Enhanced

6 August 2014

Urtica dioica (stinging nettle)

Author: Ian PopayAuthors Info & Affiliations

Publication: CABI Compendium

55911

https://doi.org/10.1079/cabicompendium.55911

Datasheet Types: Pest, Crop, Invasive species, Host plant

Abstract

This datasheet on Urtica dioica covers Identity, Overview, Distribution, Dispersal, Hosts/Species Affected, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Uses, Prevention/Control, Further Information.

Identity

Preferred Scientific Name: Urtica dioica L.
Preferred Common Name: stinging nettle
Other Scientific Names: Urtica galeopsifolia Wierzb. ex Opiz; Urtica major Kanitz.
International Common Names: English
California nettle
common nettle
European nettle
giant nettle
nettle
tall nettle; Spanish
chichicaste
ortiga
ortiga mayor; French
grande ortie
ortie
ortie dioique; Russian
krapiva; Arabic
qurrays; Chinese
gan su yi zhu qian ma
wei jian yi zhu qian ma
yi zhu qian ma; Portuguese
ortiga
urtiga
urtiga-maior
Local Common Names: Brazil
urtiga-maior
urtiga-mansa
urtigão
urtiga-vermelha; Czech Republic
kopriva dvoudomá; Denmark
brændenælde; Germany
grosse Brennessel; Italy
ortica
ortica grande
orticone; Netherlands
grote brandnetel; Poland
pokrzywa zwyczajna; Slovakia
prhl'ava dvojdomá
žihl'ava dvojdomá; Sweden
brännässla; Turkey
büyük isirgan otu
EPPO code: URTDI (Urtica dioica)

Pictures

Habit
Urtica dioica (stinging nettle); common nettle shoots along a wall base.
©Trevor James/Hamilton, New Zealand-2014

Young vegetative shoots
Urtica dioica (stinging nettle); young vegetative shoots.
©Trevor James/Hamilton, New Zealand-2014

Flowering shoot
Urtica dioica (stinging nettle); flowering shoot.
©Trevor James/Hamilton, New Zealand-2014

Inflorescence
Urtica dioica (stinging nettle); close-up of inflorescence.
©Trevor James/Hamilton, New Zealand-2014

Habit
Urtica dioica (stinging nettle); large patch of a dense monoculture of common nettles.
©Trevor James/Hamilton, New Zealand-2014

Late season monoculture
Urtica dioica (stinging nettle); late season monoculture of common nettles.
©Trevor James/Hamilton, New Zealand-2014

New shoots
Urtica dioica (stinging nettle); new shoots growing from a rhizome section.
©Trevor James/Hamilton, New Zealand-2014

Habit
Urtica dioica (stinging nettle); colonies in an English pasture.
©A.I. Popay/Hamilton, New Zealand-2014

Summary of Invasiveness

Although Urtica dioica is distributed widely in many parts of the world, it is considered invasive because of its nuisance value even within its native range, particularly in waste places, especially since its stinging hairs can cause painful welts on human and possibly animal skin. In some circumstances it can be very hard to eradicate because of its large root mass which allows it to spread vegetatively once it has established. In some countries it invades and takes up space in grassland, where it can form very large, often monospecific patches, and it can also be a nuisance in urban areas, especially in nitrogen-rich habitats.

Taxonomic Tree

Notes on Taxonomy and Nomenclature

In their delimitation of Urtica dioica L., Henning et al. (2014) pointed out that “The taxonomy of subcosmoplitan Urticadioica L. is problematic.” The taxon is morphologically quite plastic and encompasses a large number of named forms on all continents (Henning et al., 2014). The same authors continue, saying that ‘Morphologically, the differences between the subspecies and varieties are small and refer largely to details of leaf morphology (narrowly or widely ovate, base cordate or truncate) and indumentum (density of trichome cover, number of stinging hairs).’ The authors then add, quoting Woodland (1982) and Pollard and Briggs (1982; 1984a), that these characters are quite plastic in individual plants.

ITIS (2015) lists three subspecies in U. dioica, namely subsp. dioica L. (stinging nettle), subsp. gracilis (Aiton) Selander (stinging nettle, American stinging nettle, California nettle) and subsp. holosericea (Nutt.) Thorne (stinging nettle, slim nettle, hoary nettle). The Plant List (2013), however, rejects U. dioica subsp. dioica L., seeing it as synonymous with U. dioica L., but accepts as valid, albeit with only a low level of confidence, the subspp. gracilis and holosericea, and additionally subsp. afghanica Chrtek and var. sicula (Gasp. ex Guss.) Wedd.

Recognition of three subspecies by ITIS (2015) is presumably based on the work of Woodland (1982), who suggests that U. dioica subsp. dioica was introduced relatively recently from Europe into the range of the native North American U.dioica subsp. gracilis, based on its distribution in the Maritime Provinces of Canada near abandoned fishing villages and seaports along the Atlantic coast. From these sources, presumably subsp. dioica has been carried inland so that in the USA the only perennial nettle in Virginia, District of Columbia, North Carolina, Tennessee, Georgia and Alabama is this taxon.

U. dioica subsp. holosericea is native to the western USA (Arizona, California, Colorado, Idaho, Montana, New Mexico, Nevada, Oregon, Utah, Washington, Wyoming) and northern Mexico (Carey, 1995; USDA-NRCS, 2015). According to the Flora of North America Editorial Committee (2015), U. dioica subsp. holosericea is highly variable in leaf shape and degree of pubescence. The least pubescent plants appear to grade into U . dioica subsp. gracilis, and it is sometimes difficult to separate the two.

In the UK, Stace (2010) lists two subspecies, dioica and galeopsifolia, the latter without stinging hairs and with narrower leaves. Taylor (2009) notes that the former subspecies occurs throughout Europe but only as an introduced weed in some places; the latter subspecies is only known from scattered sites in Britain but occurs in European Russia, Ukraine, Bulgaria, Czech Republic, Slovakia, Hungary, Romania and the Netherlands. The galeopsifolia taxon, however, is recognized by the Plant List (2013) as a distinct species, U. galeopsifolia J.Jacq. ex Blume, although under the name U. galeopsifolia Wierzb. ex Opiz it is a synonym of U. dioica L.

Henning et al. (2014) dismiss Chinese specimens of U. dioica as belonging to another species or group of species, but say “their taxonomic identity will have to be attempted elsewhere.” They suggest transferring the American clade from U. dioica and transferring it to U. gracilis, with five subsidiary subspecies. This expanded U. gracilis can be differentiated by its subcircular (as opposed to ovate) achenes, unbranched flowering shoots, the formation of mostly short stolons, and strict monogamy. These suggestions have not yet been universally accepted and for the purposes of this datasheet the commonly accepted U. dioica sensu lato will be used.

The generic name Urtica is derived from the Latin urere meaning to irritate by burning, in reference to the burning sensation obtained from its stinging hairs. The specific epithet dioica refers to its dioecious nature. The common name nettle has its root in the ancient Proto-Indo-European word ned meaning to twist or knot, reflecting the early use of the plant as a source of fibre (Harper, 2015).

Plant Type

Herbaceous

Perennial

Broadleaved

Seed propagated

Vegetatively propagated

Description

Descriptions from some countries probably depend on the locally available subspecies, with U. dioica subsp. dioica being more common in Europe and U. dioica subsp. gracilis more prevalent in North America. The former subspecies is predominantly dioecious, has ‘weak’ stems, a sprawling, branching habit, and leaf blades and stems usually strongly hispid with stinging hairs on both leaf surfaces. The latter subspecies, by contrast, is mainly monoecious, with rigid, upright stems, and leaf blades and stems hairless but with stinging hairs only on the underside of the leaves (Bassett et al., 1977).

All subspecies are erect with extensive, brightly yellow-coloured rhizome or stolon systems, stems to about 2 m tall, leaves in opposite pairs, broadly ovate to lanceolate with rounded or more or less cordate bases, leaf margins toothed, leaf tip acute or acuminate. Inflorescences are axillary, spike-like, many-flowered, flowers small, green and unisexual. In subsp. dioica, pistillate (female) and staminate (male) flowers are almost always on different plants, but in subsp. gracilis the two flower types are in separate clusters on the same plant. The fruits are achenes, and these are tiny and light and readily carried by the wind. For a detailed description of U. dioica, see Reaume (2010).

Distribution

In its broad sense U. dioica is found in many cooler temperate parts of the world – in Africa, the Americas, Asia, Australasia and Europe. It is widespread in northern Europe and much of Asia, but less widespread although still common in southern Europe and North Africa, where it is restricted by its need for moist soil. In North America, it is widely distributed in Canada and the USA, where it is found in every province and state except for Hawaii, and also can be found in northernmost Mexico. It grows in abundance in the Pacific Northwest, especially in places where annual rainfall is high (GBIF, 2015). Taylor (2009) shows the distribution of U. dioica in Europe and its very wide distribution in Britain. DAISIE (2015) reports it as an alien in the Faroe Islands, Greenland, Iceland and Sweden, and also Bulgaria.

With regard to infraspecific taxa, the different subspecies or varieties have quite different distributions (for examples, see Woodland et al., 1982). The monoecious subsp. gracilis for example seems to be restricted to North and South America (Henning et al., 2014), and subsp. dioica recorded in North America can probably be regarded as an introduced invasive from Europe (Woodland, 1982). According to Bassett et al. (1974), U. dioica subsp. gracilis occurs as far north as 53^oN latitude in the east and to 62^oN in the west of North America

The occurrence of subsp. dioica in South Africa is also probably the result of introduction, accidentally or deliberately, from Europe. Although subsp. gracilis is reported from New Zealand (Webb et al., 1988), Henning et al. (2014) suggest that this identification is erroneous.

Distribution Map

Distribution Table

History of Introduction and Spread

U. dioica subsp. dioica appears to have been widespread in most of its current range for a very long time, although some reports of its appearance or that of different subspecies in North America, Australia and New Zealand may be related to European settlement of those countries. The first record from Australia was in 1912 (Council of Heads of Australasian Herbaria, 2015), and from New Zealand in 1877 (Thomson, 1922). Taylor (2009) suggested that the species had increased in southern England in recent decades. Carey et al. (2008) in their UK Countryside Survey reported that U. dioica was one of the ten species that had increased most between 1998 and 2007 and had become the most abundant dicotyledonous plant recorded in the Survey, at least partly the result of reduced farm maintenance.

Introductions

Introduced to	Introduced from	Year	Reasons	Introduced by	Established in wild through		References	Notes
Introduced to	Introduced from	Year	Reasons	Introduced by	Natural reproduction	Continuous restocking	References	Notes
Australia		1912			Yes	No	Council of Heads of Australasian Herbaria (2015)
New Zealand		1840-1870			Yes	No	Esler (1988)

Risk of Introduction

U. dioica may easily be transported to countries where it is not yet present because it has long been used as a source of fibre, food and herbal medicines. It is also seen as a useful food source for many kinds of caterpillar and is sometimes deliberately planted for that purpose (for example, see the Monarch Butterfly New Zealand Trust (2015)).

Means of Movement and Dispersal

Natural Dispersal

Taylor (2009) suggests that the perianth assists in wind dispersal of U. dioica seed. Seeds can also be dispersed by water as they can survive floating in water for a week (Bond et al, 2007).

Vector Transmission (Biotic)

According to Taylor (2009), seed dispersal in U. dioica “is effected when the persistent, hispid perianth segments of the fruits adhere to animal fur, feathers and clothing”. Farm animals are known to eat nettles after they have been cut and dried so seeds can also be ingested by browsing cattle, as well as by deer and birds, pass through the digestive system and be excreted (Greig-Smith, 1948). Seeds are also ingested by earthworms, to be later excreted in wormcasts (McRill, 1974).

Accidental introduction

Seed of U. dioica has been found contaminating commercial timothy (Phleum pratense) seed lots (Gooch, 1963).

Intentional introduction

Further deliberate introduction to countries where U. dioica does not yet occur or is of scarce distribution is quite possible because of its much-touted medicinal qualities. Seed is available to buy on the Internet.

Pathway Causes

Pathway cause	Notes	Long distance	Local	References
Crop production (pathway cause)	grown as a fibre crop	Yes	Yes
Digestion and excretion (pathway cause)			Yes
Disturbance (pathway cause)			Yes
Hitchhiker (pathway cause)		Yes	Yes	Taylor (2009)
Internet sales (pathway cause)		Yes
Medicinal use (pathway cause)		Yes	Yes	PFAF (2015)
People foraging (pathway cause)		Yes	Yes	PFAF (2015)
Seed trade (pathway cause)	occurs as a contaminant of grass seed lots	Yes	Yes	Gooch (1963)

Pathway Vectors

Pathway vector	Notes	Long distance	Local	References
Clothing, footwear and possessions (pathway vector)			Yes	Taylor (2009)
Livestock (pathway vector)	seeds adhere to hair or fur, or can pass through digestive systems		Yes
Plants or parts of plants (pathway vector)	occurs as a seed contaminant	Yes
Water (pathway vector)		Yes	Yes
Wind (pathway vector)		Yes	Yes

Plant Trade

Plant parts liable to carry the pest in trade/transport	Pest stages	Borne internally	Borne externally	Visibility of pest or symptoms
True seeds (inc. grain)			Yes

Hosts/Species Affected

Bayer CropScience (2015) indicates that U. dioica is a weed of wheat, oats, barley, rye and other field crops, its weediness being attributed to its spread by rhizomes, allowing it to form dense colonies that exclude other species.

Host Plants and Other Plants Affected

Host	Family	Host status	References
Avena sativa (oats)	Poaceae	Main
Brassica napus	Brassicaceae	Unknown	Aghajani and Safaei (2008)
Hordeum vulgare (barley)	Poaceae	Main
Secale cereale (rye)	Poaceae	Main
Triticum aestivum (wheat)	Poaceae	Main

Growth Stages

Pre-emergence

Seedling stage

Vegetative growing stage

Similarities to Other Species/Conditions

Uncertainty reigns over the nomenclature of Urtica species, and many taxa within the genus are easily confused with others. Some of the species and subspecies also form hybrids, further adding to the confusion.

McAllister (1999) suggested that U. dioica and U. galeopsifolia could be differentiated by the distribution of stinging hairs (few, especially on the upper leaf surface in U. galeopsifolia, but common on both leaf surfaces in U. dioica) and the diameter of the non-stinging hair bases (fine with hair bases 20-25 μm across in U. galeopsifolia, but coarser, with hair bases 25-35 μm across in U. dioica), a difference observable with a 20X hand lens.

Habitat

U. dioica occurs in a wide range of habitats, as a common understorey species of riparian communities, but also in or near marshes and meadows, including grazed pasture land (Carey, 1995; Popay et al., 1982). Bayer CropScience (2015) records the weed as present in pastures, nurseries, orchards, neglected yards, waste places, roadsides, flood plains, stream banks and ditches. It is also a common weed of disturbed areas, preferring to grow on deep rich, moist soils, being intolerant of poor fertility, dense shade and frequent disturbance. According to Olsen (1921), in Denmark it thrives in full light as well as in light shade, but does best in half-shade. The same author found that, in general, more vigorous plants grew in soils with higher nitrate content, supporting observations by others that the species commonly grows where soils are rich in nitrogen. In pastures it invades areas of bare, nutrient-rich ground caused by overgrazing, poaching, stock feeding, spoil deposition and bonfires (Natural England, 1999).

The North American subspecies gracilis is reported by Bassett et al. (1974) as occurring in moist shady woodlands, thickets and mountain slopes, along streams and roadsides, and near fencerows, generally in deep rich soils, a description which matches the habitats of the European subspecies dioica well.

Greig-Smith (1948) said that, at least in Britain, outside its native habitat of woodland, it is “an almost universal follower of man, occurring on heaps of wood, metal, stones, earth and sand, along the foot of walls, around sheep and cattle yards and at the foot of stacks”. He adds that it occasionally appears as an epiphyte on willow (Salix spp.), ash (Fraxinus spp.), hornbeam (Carpinus spp.), oak (Quercus spp.) and poplar (Populus spp.). In Canada too, Bassett et al. (1977) note that subsp. gracilis is often associated with human habitation.

In Britain, U. dioica has been noted as favouring areas which have been cleared of other invasive weeds; Lockton and Godfrey (2015) reported U. dioica invading areas which have been subjected to eradication programmes for Japanese knotweed (Fallopia japonica) and Himalayan balsam (Impatiensglandulifera), while Cadbury (1976) also reported it in areas from which bracken (Pteridiumaquilinum) had been cleared.

Habitat List

Category	Sub category	Habitat	Presence	Status
Terrestrial	Terrestrial – Managed	Cultivated / agricultural land	Secondary/tolerated habitat	Harmful (pest or invasive)
Terrestrial	Terrestrial – Managed	Cultivated / agricultural land	Secondary/tolerated habitat	Natural
Terrestrial	Terrestrial – Managed	Cultivated / agricultural land	Secondary/tolerated habitat	Productive/non-natural
Terrestrial	Terrestrial – Managed	Managed grasslands (grazing systems)	Principal habitat	Harmful (pest or invasive)
Terrestrial	Terrestrial – Managed	Managed grasslands (grazing systems)	Principal habitat	Natural
Terrestrial	Terrestrial – Managed	Disturbed areas	Principal habitat	Harmful (pest or invasive)
Terrestrial	Terrestrial – Managed	Disturbed areas	Principal habitat	Natural
Terrestrial	Terrestrial – Managed	Rail / roadsides	Secondary/tolerated habitat	Harmful (pest or invasive)
Terrestrial	Terrestrial – Managed	Rail / roadsides	Secondary/tolerated habitat	Natural
Terrestrial	Terrestrial – Managed	Urban / peri-urban areas	Secondary/tolerated habitat	Harmful (pest or invasive)
Terrestrial	Terrestrial – Managed	Urban / peri-urban areas	Secondary/tolerated habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Natural forests	Principal habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Natural grasslands	Principal habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Riverbanks	Principal habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Wetlands	Principal habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Rocky areas / lava flows	Secondary/tolerated habitat	Natural
Terrestrial	Terrestrial ‑ Natural / Semi-natural	Scrub / shrublands	Secondary/tolerated habitat	Natural

Biology and Ecology

Genetics

There are differences in chromosome number between U. dioica subspecies in North America, with subsp. dioica being tetraploid (2n = 52) and subsp. gracilis diploid (2n = 26) (Woodland, 1982). Oddly enough, in Canada, both diploid and tetraploid races of subsp. gracilis occur (Bassett et al., 1974), with the tetraploid race occurring from the Rocky Mountains to the coast of British Columbia, and the diploid race from the eastern side of the Rocky Mountains to Newfoundland; no distinctive morphological differences were found between the two races. Bassett et al. (1974) reported that crosses between diploid U. dioica subsp. gracilis and tetraploid U. dioica subsp. gracilis in Canada resulted in F₁ hybrids that produced over 80% aborted pollen, no seed set and irregular pairing of the chromosomes in pollen mother cells.

Stace (2010) reported that in Britain, subsp. dioica is tetraploid with 2n = 48 or 52, and that subsp. galeopsifolia is diploid (2n = 26), adding that these two subspecies do not differ constantly in morphology or even in chromosome number, and that intermediates occur but their chromosome numbers are unknown.

When Pollard and Briggs (1982; 1984a,b) explored the behaviour of what they assumed to be a stingless variety of U. dioica subsp. dioica, but which was later confirmed as belonging to the species U. galeopsifolia (McAllister, 1999), they found that the two species produced fertile hybrids. McAllister (1999) further determined that in most cases U. galeopsifolia was diploid (2n = 26) whilst U. dioica subsp. dioica was tetraploid (2n = 52), and speculated that the intermediate forms resulting from crosses between the two taxa were triploid.

Reproductive Biology

U. dioica subsp. dioica is, as its botanical name suggests, mostly dioecious, with male and female flowers on different plants. Glawe (2006) found that although male and female flowering plants occured at approximately equal numbers at her study site at Meijendel harbour in the Netherlands, seeds collected individually from 33 female plants and grown to flowering showed progeny sex ratios that ranged between 5% and 76% male offspring. The same author investigated the effects of physiological and environmental factors on sex ratios, and found that sex ratios remained constant under different soil nutrient conditions or when plant growth hormones were applied. However, in monoecious plants with both male and female flowers, the proportion of male flowers increased when plants were grown under more favourable conditions. In fact, nearly half of the plants that initially appeared monoecious exclusively produced male flowers when grown in more fertile soils. The author described these as ‘inconstant males’ (genetic males that occasionally produce seeds), which change the fraction of male flowers in response to different environmental conditions. She suggested, as a result of her crossing experiments, that males represent the heterogametic sex and females the homogametic sex. However, sex determination does not follow a clear-cut scheme and it seems that genes on different loci are involved in sex determination.

Shannon and Holsinger (2007) also reported a complicated picture of sex determination in U. dioica. Most significant was evidence for a maternal influence on sex determination and for the possibility of gynodioecy as an intermediate stage in the evolutionary pathway to dioecy. The North American subspecies gracilis is mostly monoecious, with staminate and pistillate flowers on each shoot (Bassett et al. 1977). However some panicles are entirely male, some entirely female and some are mixed, with hermaphrodite flowers occasionally being produced. Male flowers appear mostly in June and July and female flowers later in the growing season. Although the plants are self-compatible, this difference in flower time of males and females must to some extent lead to more crossing than selfing.

Pollen dispersal in

U. dioica

is by wind, but is aided by the rapid outflexing of the stamens at anthesis, which produces an explosive release of pollen (Pollard and Briggs, 1984a). Hyde (1959) found that the amount of nettle pollen present in the air in Cardiff, UK, approached that of grass pollen and was possibly a significant cause of hay fever.

Nettle plants produce abundant seed. Those growing in full sunlight produce 10,000-20,000 seeds per shoot (Bassett et al., 1977). The average 1000-seed weight is 0.2 g and the seeds have an oil content of 27.85% and a protein content of 15.5% (Kew Seed Information Database, 2015).

Besides producing seeds, plants spread vegetatively by means of rhizomes or stolons. Vegetative spread is initiated in a plant’s first season. A rhizome planted in late summer can spread into a 2.5 m wide monospecific patch by the following year (Bassett et al., 1977).

Physiology and Phenology

Pollard and Briggs (1984b) explored the structure and function of the stinging hairs of U. dioica subsp. dioica. When brushing contact is made with a hair the swollen tip is broken off obliquely along a more or less predetermined fracture line, leaving a sharp point ("resembling the beveled tip of a hypodermic needle"). This point penetrates the skin and the subsequent pressure squeezes the base of the stinging cell which thus actively injects the toxin contained within it. Pollard and Briggs (1984b) point out that despite a great deal of biochemical and pharmacological research over the past 100 years, the precise nature of the toxin is not fully understood, although it is known to contain serotonin and acetylcholine (Connor, 1977).

U. dioica is a long-day plant and may need up to 16 hours daylength for flowering (Bond et al., 2007). Flowering (in Britain) takes place from late May to early August and viable seed is shed or may remain on the dead stems until December or January. According to the Kew Seed Information Database (2015), tested seeds germinated readily after stratification at 5^oC or 6^oC for 8 weeks before being transferred to alternating temperatures of 25/10^oC, 8/16^oC, 33/19^oC or 12/12^oC. The seeds of North American plants of U. dioica subsp. gracilis apparently require no vernalization and fresh seed will germinate in 5 to 10 days.

With regard to plant development, new rhizomes are produced in late summer or autumn either from old rhizome material or from the base of aerial shoots (Greig-Smith, 1948). They continue to grow at or just beneath the soil surface until the death of the aerial shoots when they turn upwards to form new shoots. Young rhizomes are reddish in colour and have stinging hairs and scale leaves. Older rhizomes and roots have a yellow corky layer and so appear yellow in colour. The roots branch profusely and form many fine laterals.

Greig-Smith (1948) says that new aerial shoots of U. dioica continue growth until about 15 cm tall and then survive the winter (in Britain), resuming growth the following spring. Flowering begins in late May or June. In Canada, according to Bassett et al. (1977), the North American subspecies is killed back to ground level by frost each year but its rhizomes survive and sprout again in spring.

Taylor (2009) cites work by Grime and Hunt (1975) in saying that although U. dioica has a small seed mass its competitive strategy involves an exceptionally high relative growth rate, which coincides with tall stature, extensive lateral spread and the tendency to accumulate leaf litter, characteristics that facilitate the exclusive occupation of fertile sites.

Wheeler (1981), cited in Taylor (2009), compared the growth of woodland and pasture clones of U. dioica subsp. dioica at different light levels. Plants shaded by deciduous woodland grew better in their light regime of 37.3% of direct incident light from November to April and of 23.8% from May to October than did pasture clones in 84.3% of direct incident light in respect of height, internode length and shoot dry mass. However the pasture clones produced 82% more seeds than the woodland ones. When plants were grown in pots at 25%, 35%, 67% and 100% (full greenhouse light) irradiance, there was no significant difference between total dry mass of plants.

Taylor (2009) reported that plants wilted under very dry conditions, but they may be able to ‘harden’ to drought to some extent. The same author says that the plant cannot withstand flooding of its rhizomes and roots for long periods. Greig-Smith (1948) observed that the shoot tips are not affected by spring frosts but may die back after early autumn frosts. U. dioica does not persist in saline areas (Bassett et al. (1977).

Longevity

According to Bayer CropScience (2015), stands of U. dioica can persist for around 50 years, and Bassett et al. (1977) also suggest a conservative estimate of the age of some clones of at least 50 years.

Taylor (2009) quotes Thompson and Grime (1979) in saying the seed bank of U. dioica is of the persistent type: few seeds germinate immediately after dispersal and the seed bank changes little in size during the season and is large in relation to the annual production of seeds. Wheeler (1981, cited by Taylor, 2009) found a seed bank which varied between 1754 to 9090 viable seeds m^-2 in floodplain pasture. Roberts and Bodrell (1984) sowed seeds of U. dioica into sterilized soil in pots. The soil was periodically cultivated, and seedlings continued to emerge for 5 years, with peak seasonal emergence in April, and the greatest number emerging in the first year.

Population Size and Structure

U. dioica is often found in very large patches and forms pure stands under favourable conditions (Taylor, 2009). These patches are often the product of a single individual that has spread by rhizomes which can be 50 cm or more in length (Greig-Smith, 1948).

Nutrition

Greig-Smith (1948) reported the British occurrence of U. dioica on almost all soil types, but noted its absence from waterlogged soils and its rarity on acid peats. The same author, along with many others, commented on its apparent preference for soils with a high nitrate content. However, Bates (1933) suggested that the controlling factor was the soft, unconsolidated nature of favoured substrates rather than high nitrate. Ivins (1952) found that U. dioica only invaded sown plots of pasture species when one of the sown components was a legume, adding support to the idea that nettles prefer growing in nitrogen-enriched soils. Taylor (2009) describes a number of experimental studies on the interactions of nitrogen and phosphate and their effects on the growth of nettle plants. Unless phosphate is added, in some soils seedlings grow extremely slowly and exhibit the characteristic symptoms of severe phosphate deficiency. Hence, the distribution of nettles seems to be limited by phosphate availability (Taylor, 2009). Piggot (1971) found that in well-lit sites U. dioica seedlings responded strongly to the addition of phosphate, which allowed a short period of high relative growth related to a rapid but transient increase of leaf area ratio.

Associations

In Denmark, Olsen (1921) observed that plant species growing along with U. dioica included Mercurialis perennis, Stachys sylvatica and Chrysosplenium alternifolium, species of damp woodland environments. Greig-Smith (1948) listed associated species in several habitats in Britain, including fen carr, different woodlands and scrub, boulder beaches and river banks. Taylor (2009), again in Britain, lists associated species in swamps and tall-herb fens, woodlands, scrub and hedges, mire vegetation, mesotrophic and calcicolous grasslands, in maritime vegetation and among crops in arable agriculture.

Plant species commonly associated with U. dioica in southern Finland include Alopecurus pratensis, Elymus repens and Filipendula ulmaria, along with other, less common species. U. dioica was sometimes associated with the stem holoparasite Cuscuta europaea (Koskela, 2002).

One of the few species capable of persisting in almost monospecific stands of U. dioica is the scrambling winter annual Galium aparine, which can grow through and destroy the canopy of U. dioica by its weight resting on the nettle stems (Taylor, 2009).

Environmental Requirements

Rorison (1967) found that for U. dioica germination was most rapid, seedling survival high and growth strong on calcareous soils (pH 6.1-7.4), but germination was slower and less complete on increasingly acid soils (pH 3.5-4.1). Taylor (2009) interpreted the results of Piggot (1971) and Jankowska-Blaszcuk and Daws (2007) on the effects of light quality and quantity on germination as meaning that seeds are only likely to germinate in micro-sites near the soil surface and in the absence of overtopping vegetation. Jankowska-Blaszcuk and Daws (2007) demonstrated that small seeds like those of U. dioica require light with a red to far-red ratio of 0.9 or greater for 50% germination. According to Bayer CropScience (2015), U. dioica plants are intolerant of poor fertility, dense shade and frequent disturbance. Their rhizomes have difficulty penetrating compacted soil, so open-textured soils of pH 5.0 to 8.0 are preferred.

U. dioica subsp. gracilis tolerates a wide range of fall and snow cover conditions in North America (Bassett et al., 1974).

Climate

Climate type	Description	Preferred or tolerated
BS - Steppe climate	> 430mm and < 860mm annual precipitation	Tolerated
Cf - Warm temperate climate, wet all year	Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year	Preferred
Cs - Warm temperate climate with dry summer	Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers	Preferred
Cw - Warm temperate climate with dry winter	Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)	Tolerated
Ds - Continental climate with dry summer	Continental climate with dry summer (Warm average temp. > 10°C, coldest month < 0°C, dry summers)	Tolerated

Latitude/Altitude Ranges

Latitude North (°N)	Latitude South (°S)	Altitude lower (m)	Altitude upper (m)
65	40

Rainfall Regime

Summer

Winter

Bimodal

Uniform

Soil Tolerances

Soil texture > light

Soil texture > medium

Soil reaction > acid

Soil reaction > neutral

Soil reaction > alkaline

Soil drainage > free

List of Pests

Notes on Natural Enemies

Slugs and snails find the leaves of nettles very palatable even though their resulting faeces may be packed with stinging hairs (Grime et al., 1970). In Britain and elsewhere the larvae of small tortoiseshell (Aglais urticae) and red admiral (Vanessa atalanta) butterflies are commonly or exclusively found on stinging nettles (Davis, 1972, 1973). Greig-Smith (1948) presents long lists of insects (Thysanoptera, Hemiptera, Homoptera, Heteroptera, Coleoptera, Diptera, Lepidoptera) and fungi associated with species of Urtica (mostly U. dioica) in Britain, as well as a nematode. Taylor (2009) presents an even longer list of phytophagous insects found on U. dioica, as well as the names of parasitic and saprophytic fungi associated with the species. In south Wales, larvae of Eupteryx cyclops and E. urticae were found to feed specifically on stinging nettles, while E. aurata only fed on nettles in spring and autumn (Stiling, 1980).

Bassett (1977) presents a long list of insects, microorganisms and viruses associated with U. dioica in Canada.

In New Zealand, the endemic New Zealand red admiral butterfly (Bassaris gonerilla) is restricted to species of Urtica, but has been in decline since 1928 (Monarch Butterfly New Zealand Trust, 2015), probably in part due to a reduction in populations of the endemic New Zealand tree nettle U. ferox. Both B. gonerilla and the yellow admiral butterfly (Vanessaitea, endemic to New Zealand and Australia) lay eggs that develop into adults on U. dioica.

According to Taylor (2009), fallow deer (Dama dama), roe deer (Capreolus capreolus) and red deer (Cervus elaphus) consume U. dioica.

Cuscuta europaea, the European dodder, is an annual rootless stem holoparasite that obtains resources for its own growth and reproduction from the host plant through haustorial connections that penetrate into the host vascular tissue. U. dioica is a common host of this parasite. Plants originating from parasitized populations flower later and allocate less biomass to asexual reproduction (stolon production) compared to plants from unparasitized populations, indicating possible selection by the parasite for late flowering and against asexual reproduction in U. dioica (Koskela, 2002).

Natural enemies

Natural enemy	Type	Life stages	Specificity	References
Aglais urticae (small tortoiseshell butterfly)	Herbivore	Leaves	not specific	Davis (1972)
Bassaris gonerilla	Herbivore	Leaves	to genus	Monarch Butterfly New Zealand Trust (2015)
Cuscuta europaea (European dodder)	Parasite	Stems	not specific	Koskela (2002)
Eupteryx aurata (leafhopper, potato)	Herbivore	Leaves	not specific	Stiling (1980)
Eupteryx cyclops	Herbivore	Leaves	to species	Stiling (1980)
Eupteryx urticae	Herbivore	Leaves	to species	Stiling (1980)
Vanessa atalanta (red admiral butterfly)	Herbivore	Leaves	not specific	Davis (1972)

Impact Summary

Category	Impact
Cultural/amenity	Negative
Economic/livelihood	Positive and negative
Environment (generally)	Positive and negative
Human health	Positive and negative

Impact: Economic

U. dioica is generally regarded as a weedy invasive species. Where it occurs in pastures and grasslands its monospecific clumps can take up considerable space and thus reduce hay yields and the amount of grass available to livestock. It is normally avoided by livestock, thus restricting their free movement. Although considered native to Canada, it is listed as a noxious weed in several provinces (Carey, 1995), including Nova Scotia and Quebec (Darbyshire, 2003) and Alberta and Manitoba (Bassett et al., 1977). In the USA, U. dioica is classed as weedy or invasive depending on state and authority (USDA-NRCS, 2015). According to Bayer CropScience (2015), it is a weed of cereal and other field crops, is difficult to eradicate and is an important alternative host of the economically damaging carrot fly (Psila rosae).

Impact: Environmental

Impact on Biodiversity

The weediness of U. dioica is attributed to its spread by rhizomes, allowing it to form dense colonies that exclude other plant species (Bayer CropScience, 2015). Where U. dioica occurs in dense patches in pasture or woodland it could be a problem if it interferes with the growth or occurrence of endangered species.

Impact: Social

Generations of children and adults have suffered the stinging hairs of U. dioica subsp. dioica. A number of toxins are present in the stinging hairs, including serotonin and acetylcholine (Connor, 1977). A report in 1982 (Anon., 1982), cited by CBIF (2015), noted that hunting dogs in the USA were poisoned after massive exposure to stinging nettles. Symptoms included trembling, pain, slobbering, dyspnoea and vomiting. Without treatment some dogs died 2-3 days after exposure. The action of the stinging hairs is neutralized by heat or by drying, so leaves can be used for edible purposes quite safely (PFAF, 2015).

The copious wind-blown pollen of U. dioica is a major contributor to summer hay fever (Hyde, 1959; Bassett et al., 1977).

Risk and Impact Factors

Invasiveness

Invasive in its native range

Proved invasive outside its native range

Has a broad native range

Abundant in its native range

Highly adaptable to different environments

Pioneering in disturbed areas

Tolerant of shade

Benefits from human association (i.e. it is a human commensal)

Long lived

Fast growing

Has high reproductive potential

Has propagules that can remain viable for more than one year

Reproduces asexually

Impact outcomes

Monoculture formation

Negatively impacts agriculture

Negatively impacts human health

Reduced amenity values

Reduced native biodiversity

Threat to/ loss of native species

Impact mechanisms

Competition - shading

Competition - smothering

Poisoning

Produces spines, thorns or burrs

Likelihood of entry/control

Highly likely to be transported internationally accidentally

Highly likely to be transported internationally deliberately

Difficult to identify/detect as a commodity contaminant

Difficult/costly to control

Uses

Economic Value

Bast fibres from U. dioica stems have been used as sources of fibre for textile making for centuries. In Europe, it was cultivated commercially during the 19th and 20th centuries, right up until the Second World War. Many traditional clones were developed, with fibre contents ranging from 1.2 to 16% dry matter and fibre yields from 140 to 1280 kg/ha. Harvesting starts in the second year of growth and the crop may produce well for several years. Interest in the crop is undergoing a revival, especially as an organically grown crop for the ‘green’ textile industry (Vogl and Hartl, 2003).

Nettle plants are commonly used by organic gardeners to produce a liquid plant feed (Royal Botanic Gardens Kew, 2015). Until recently nettles were cultivated in Scotland, Denmark and Norway for use in the food, textile and medicinal industries. According to Bown (1995), plants are harvested commercially for the extraction of chlorophyll for use as a colouring agent and in foods and medicines. Dried nettle leaves and extracts are available from many herbal outlets. Seeds of U. dioica also seem to be widely available, and plants are often raised and traded locally for their useful food, medicine and butterfly-rearing properties, especially in countries like New Zealand where it is not so widely distributed (Monarch Butterfly New Zealand Trust, 2015).

Social Benefit

People in many parts of the world still use freshly collected young leaves and stems of U. dioica as a source of food (cooking destroys the effects of the stinging hairs) and of medicinal cures (Royal Botanic Gardens Kew, 2015). The tender young leaves and shoots are used fresh, for example in soup, which is reputed to clean the blood, or as a vegetable. The mature leaves are used in the production of cheese (notably Cornish Yarg) and in pesto, cordials and herbal tea. In modern Turkey, although nettle tea is a popular commercial herbal tea, its ingestion is not without risks. Sahin et al. (2007) report two patients, a man with one enlarged breast (gynaecomastia) and a woman exhibiting galactorrhoea; both had regularly consumed nettle tea before symptoms appeared, and when they stopped drinking it the symptoms disappeared. Uslu et al. (2011) report a case of a 17-day-old breast-fed infant with an urticarial rash brought about by her mother using a stinging nettle concoction to ease her cracked nipples.

Traditionally U. dioica has been used medicinally as a haemostatic, antirheumatic and as a remedy for urinary infections and stones. It is also claimed to have antiasthmatic, antidandruff, astringent, depurative, diuretic, galactogogic, haemostatic and hypoglycaemic properties (PFAF, 2015). A traditional remedy for rheumatism involves stinging the affected area with fresh nettle leaves, an effect apparently due to the anti-inflammatory properties of the stinging hair toxins (Royal Botanic Gardens Kew, 2015). Nencu et al. (2013) found that total polyphenol and ascorbic acid contents were highest in young leaves and declined as the plants approached the flowering stage. Further information on the medicinal and other uses to which U. dioica has been or is being put can be found in Randall (2003).

Environmental Services

Nettles sustain a rich and diverse invertebrate fauna, with around 100 insect species benefitting from the plant (Davis, 1991). They are a food source for many butterfly and moth larvae, and ladybirds consume the aphids that thrive on the plants. The numerous seeds produced are an important food source for birds (Royal Botanic Gardens Kew, 2015).

Uses List

Environmental > Wildlife habitat

Materials > Cosmetics

Materials > Dyestuffs

Materials > Fertilizer

Materials > Fibre

Materials > Miscellaneous materials

Medicinal, pharmaceutical > Source of medicine/pharmaceutical

Medicinal, pharmaceutical > Traditional/folklore

Human food and beverage > Food additive

Human food and beverage > Leaves (for beverage)

Human food and beverage > Vegetable

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

To prevent nettle infestations of grasslands, Natural England (1999) recommend avoiding the creation of bare ground and avoiding nutrient enrichment.

Eradication

According to Bayer CropScience (2015), eradicating an established stinging nettle colony is difficult because the extensive root system expands annually and cannot be suppressed by mowing, although cutting when the new shoots reach a height of 15-30 cm tall was recommended as a means of eradication by the UK Ministry of Agriculture and Fisheries in 1939.

Control

Non-chemical control measures for U. dioica are summarized by Bond et al. (2007).

Cultural Control and Sanitary Measures

Bayer CropScience (2015) claims that some control of nettles may be obtained through repeated tillage and cultivation over several years. Bassett et al. (1977), quoting Muenscher (1955), indicate that American stinging nettle will not survive repeated ploughing and mechanical cultivation; several years of such practices will effectively reduce infestations by destroying the extensive root systems.

Physical/Mechanical Control

Carey (1995) reported that U. dioica regenerates from its buried rhizomes and seeds relatively quickly after a fire, although its top growth is presumably severely damaged. Greig-Smith (1948) reported that the species will not withstand repeated cutting. Machines for pulling out grassland weeds have been developed and are seen as environmentally friendly alternatives to herbicide use. One such, developed in the late 1990s, is the Eco-Puller, a tractor-trailed and PTO-driven machine that pulls weeds from the ground in a way that mimics hand pulling. This is achieved by feeding tall plants between a pair of gripping rollers which then provide a good firm vertical pull which is necessary to get the root out of the ground. The weed gripping height is adjustable, but weeds should be at least 30 cm tall to be pulled effectively. Stinging nettle should be pulled early in the season as soon as the stems are robust (Natural England, 1999).

Biological Control

No biological control agents have been identified for this species and, since it is considered native to several continents, a search for such agents would probably not be practical.

Chemical Control

Bassett et al. (1977) suggest that, in Canada, since there are no specifically recommended chemical control measures for the species, general control recommendations for perennial weeds in non-crop land will probably eradicate this plant. However, Popay et al. (1982) tested a number of herbicides commonly used for weed control and found that few gave complete control after a single application. Only 2,4-D + picloram or picloram granules were effective and, after a single application, nettle clumps were either completely killed or very few shoots emerged later and were killed by a repeat application a year after the first. Natural England (1999) also recommends clopyralid + triclopyr, 2,4-D + dicamba + mecoprop, fluroxypyr, mecoprop and triclopyr for effective nettle control.

IPM

In their evaluation of cutting as a control measure for U. dioica in a field used to corral cattle in Slovakia, Vozár et al. (2009) found that cutting every 5 weeks with or without removal of plant biomass reduced the dominance of nettles over the study period (2004-08). However, cutting every 5 weeks together with reseeding with the strongly competitive species Dactylis glomerata and Trifolium repens gave the best control.

Control by Utilization

Although U. dioica is very nutritious, containing as it does 21-23% crude protein, 3-5% crude fats and 9-21% crude fibre, it is rarely grazed by livestock or wild mammals, presumably because of its stinging hairs or because of its fine, dense hairiness in non-stinging subspecies. However, although livestock usually avoid mature plants, they will often eat them as seedlings or when cut and wilted (Natural England, 1999). Pollard and Briggs (1984b) reviewed literature on the subject and found that some breeds of cattle, rabbits and American bison (Bison bison) consumed nettles at least on some occasions. Taylor (2009) notes fallow deer (Dama dama), roe deer (Capreolus capreolus) and red deer (Cervus elaphus) as feeding on nettles.

When Pollard and Briggs (1984b) tested U. dioica unpalatability by observing interactions between plants with differing densities of stinging hairs and grazing sheep (Ovis aries) and rabbits (Oryctolagus cuniculus), the amount of nettles consumed by both animals declined as density of stinging hairs increased. Both grazing species sometimes showed signs of immediate pain response. In further experiments in an area populated by wild rabbits, the more or less stingless nettle plants were much more heavily grazed than those with more dense stinging hairs.

Many invertebrates are obviously not deterred by the plant’s hairs and defoliation by slugs, snails and caterpillars can sometimes be nearly complete, but plants later recover by regrowing from their extensive root systems.

Gaps in Knowledge/Research Needs

The taxonomy of Urtica dioica clearly needs further scrutiny and, since the diploid and tetraploid North American subspecies do not normally interbreed (Basset et al., 1974), separation of the two subspecies into species may well be warranted, as suggested by Henning et al. (2014).

Links to Websites

Name	URL	Comment
GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gateway	https://doi.org/10.5061/dryad.m93f6	Data source for updated system data added to species habitat list.

References

Anon, 1982. Stinging nettle (Urtica sp.) and dogs. Veterinary and Human Toxicology, 24(4):247.

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