The name Urtica urens is universally accepted for this common, widespread weed (Hartley, 1979). There appears to have been virtually no revision of the taxonomic treatment or nomenclature of the species since it was described by Linnaeus in 1753. Plants are diploid and have a 2n chromosome number of 26 (Woodland et al., 1976).
U. urens is an annual, growing up to 75 cm tall, branching at the base. It can be a particular nuisance because the bristles or hairs on its leaves and stems give off a substance that causes an intense burning sensation. Clusters of small, greenish-white flowers form where leaves join stems. The seed leaves are round or slightly elongated with smooth edges and a notch in the tip. First true leaves and all later leaves have distinctly toothed edges.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
U. urens is adapted to many environments, infesting a wide range of horticultural crops, especially where there is irrigation or summer rainfall. In pastures, it can become prevalent in situations rich in organic material or manure, such as stock camps, holding yards or watering points (Lazarides et al., 1997).
U. urens has been reported in many types of vegetable crops, orchards (citrus, pome and stone fruits) and vineyards. It is also a problem in nursery crops (conifers, ornamental shrubs, forest trees, fruit trees, roses, cut flowers) and gardens.
U. urens is frequently found on light-textured soils, especially those rich in organic matter. It responds well to N and entire plants contain over 5% N.
Seed can remain viable for 20-100 years in the soil. Emergence is enhanced by soil disturbance and mostly occurs from within the top 2.5 cm of soil. Only 4% viable seed remained in the soil after 6 years of cultivation, compared with 39% viable seed in undisturbed soil (Holm et al., 1997).
Seed germination is optimal at 25°C and decreases rapidly at temperatures below 20°C (Andersen, 1968). Germination is greater in darkness than in light. Most seedlings appear in spring, but emergence continues through to mid-summer. Few plants emerge from late summer to early winter. Plants flower from late spring to autumn and are killed by frost.
U. urens is light-loving and dry matter distribution is not affected by light intensity. It is most competitive in full sunlight, whereas the perennial U. dioica is better adapted to shade (Corre, 1984). When drought-stressed, U. urens flowers several days earlier than normal and the number of nodes with inflorescences drops from eight to four per plant (Boot et al., 1986). Occasionally it flowers and sets seed when 8-10 cm tall and before the cotyledonary leaves have dropped off. Several successions of inflorescences are usually found on early emerging plants. Seeds formed early in the season may produce new plants in the same year.
Pollen release in the Urtica genus is unique. Immature stamens are bent towards the centre of the flower. When the anthers mature, the stamens suddenly straighten, shooting pollen into the wind. Plants are cross-pollinated and produce 100-1300 seeds weighing 0.5 mg each. Seeds are rich in oily endosperm and do not float on water (Holm et al., 1997).
Each stinging hair on U. urens is a tapered, elongated cell, constricted just below the tip, with a bulbous base embedded in the multicellular pedestal. When hit, the tip breaks off and the hair becomes a miniature hypodermic needle that penetrates the skin and injects its irritating chemicals. The tip of the hair is high in silica, but the silica concentration decreases towards the base, where it is replaced by calcium (Thurston and Lersten, 1969). Each hair is 100 µm long and has 10 µg of fluid that contains histamine and acetylcholine. Stem hairs have 2.5 times more acetylcholine than leaf hairs, whereas upper and lower surface leaf hairs have equal concentrations. The leaf itself has nearly as much histamine and acetylcholine as the leaf hairs. Crushed leaves can also give a stinging sensation, but are not as irritating as the hairs. The stinging reaction disappears within 1-3 hours for most people, but the hairs can remain in tissue and cause pain for 24-36 hours. Plants are not considered toxic to livestock, but cause the same irritating reaction in all animals (Everist, 1974).
Damage produced on U. urens in Argentina by the fungus Septoria urticae suggests that this organism is a potential biological control agent (Dal-Bello et al., 1993; 1995). Species of Pratylenchus nematodes were found in the roots of 31 weed species surveyed in Germany, with the highest infestations on U. urens (Rossner, 1983).
U. urens is susceptible to arabis mosaic nepovirus and hop mosaic carlavirus (Brunt et al., 1996).
Holm et al. (1997) rank U. urens amongst the 200 worst weeds of the world. As with most weeds, however, quantitative data on the economic impact of the species is extremely limited.
U. urens is a weed of 27 crops in 50 countries and is a frequently reported weed of vegetables and orchards (Holm et al., 1997). Once U. urens appears in vegetable fields, populations can increase rapidly. In locations where U. urens was one of the dominant weeds in unweeded potato crops in Egypt, tuber yield was reduced by 40% (Abusteit and Shehata, 1993). Where it was one of the dominant weeds of faba beans in Portugal, yield losses were 29-34% (Fernandes, 1989).
U. urens is included in a catalogue of problem plants in southern Africa (Wells et al., 1986), where its impacts are listed as competition, replacement of preferred vegetation (indigenous), skin irritation, seed contamination and obstruction of access.
In Morocco, U. urens is an alternative host for Leveillula taurica, the causal agent of tomato powdery mildew (Besri and Hormattallah, 1985). Carnation ringspot dianthovirus and tomato bushy stunt tombusvirus were found on apple, pear, cherry, sweet cherry and plum in East German orchards and were also isolated from U. urens (Kegler et al., 1983).
Herbal uses of U. urens have been known for centuries. Fresh plants have a painful, but beneficial effect on rheumatism. Leaves and flowers are reportedly powerful diuretics (Holm et al., 1997). In spite of the stinging hairs, young stems and leaves are edible and can be boiled as a green vegetable or in soup (Lazarides et al., 1997). Nettles have also been used to make beer and tea. According to Szabo et al. (1973), U. urens has a crude protein content of about 25% of dry matter. Zulu peoples in Africa regarded the plant as an aphrodisiac (Watt and Breyer-Brandwijk, 1932).
Leaf extracts of U. urens showed nematicidal properties against the citrus nematode, Tylenchulus semipenetrans (Mohammad et al., 1981).
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.
Cultural control is effective, but handweeding is not recommended because of the plant's irritant properties (Whibley and Christensen, 1982). Selective mechanical weed control methods for several weeds, including U. urens, was investigated by Fogelberg and Gustavsson (1998). Emergence is enhanced by soil disturbance and the resultant seedlings can be controlled by follow-up cultivation or herbicides.
Vegetable cropping favours weed species which require only a comparatively short interval between emergence and the start of seed production, and whose seeds can germinate over a wide temperature range. Urtica urens is one of several weed species that can build up large seed banks during intensive vegetable production (Roberts, 1983).
U. urens is susceptible to control by flame-weeding (Ascard, 1995).
Herbicide recommendations are available for most crops and situations where U. urens is a problem (Parsons, 1992), summarized below:
Agamalian (1991) used N fertilizer solutions (liquid ammonium nitrate and ammonium thiosulfate) for selective control of broadleaved weeds resistant to soil-applied herbicides, including U. urens. Application of the N fertilizer solutions was most effective when weeds were at the 1-4-leaf stage.
Apart from the studies by Dal-Bello et al. (1993, 1995), there has been little consideration of the biological control of U. urens.
Wagner WL; Herbst DR; Sohmer SH, 1990. Manual of Flowering Plants of Hawaii. Bernice Pauahi Bishop Museum Special Publication 83. Honolulu, Hawaii, USA: University of Hawaii.
Watt JM; Breyer-Brandwijk MG, 1962. The Medicinal and Poisonous Plants of Southern and Eastern Africa. Edinburgh and London, UK: E & S Livingstone Ltd.
Wells MJ; Balsinhas AA; Joffe H; Engelbrecht VM; Harding G; Stirton CH, 1986. A catalogue of problem plants in South Africa. Memoirs of the botanical survey of South Africa No 53. Pretoria, South Africa: Botanical Research Institute.