Urtica dioica, often known as common nettle, burn nettle, stinging nettle (although not all plants of this species sting) or nettle leaf, or just a nettle or stinger, is a herbaceous perennial flowering plant in the family Urticaceae. Originally native to Europe, much of temperate Asia and western North Africa, it is now found worldwide, including New Zealand and  North America. The species is divided into six subspecies, five of which have many hollow stinging hairs called trichomes on the leaves and stems, which act like hypodermic needles, injecting histamine and other chemicals that produce a stinging sensation upon contact ("contact urticaria", a form of contact dermatitis).The plant has a long history of use as a source for traditional medicine, food, tea, and textile raw material in ancient (such as Saxon) and modern societies. Description
Urtica dioica is a dioecious, herbaceous, perennial plant, 3 to 7 feet (0.9 to 2 metres) tall in the summer and dying down to the ground in winter. It has widely spreading rhizomes and stolons, which are bright yellow, as are the roots. The soft, green leaves are 1 to 6 inches (30 to 200 mm) long and are borne oppositely on an erect, wiry, green stem. The leaves have a strongly serrated margin, a cordate base, and an acuminate tip with a terminal leaf tooth longer than adjacent laterals. It bears small, greenish or brownish, numerous flowers in dense axillary inflorescences.
The leaves and stems are very hairy with non-stinging hairs, and in most subspecies, also bear many stinging hairs (trichomes or spicules), whose tips come off when touched, transforming the hair into a needle that can inject several chemicals causing a painful sting or paresthesia, giving the species its common names: stinging nettle, burn-nettle, burn-weed, or burn-hazel. Taxonomy
The taxonomy of Urtica species has been confused, and older sources are likely to use a variety of systematic names for these plants. Formerly, more species were recognised than are now accepted. However, at least six clear subspecies of U. dioica are described, some formerly classified as separate species:

U. dioica subsp. dioica (European stinging nettle), from Europe, Asia, and northern Africa, has stinging hairs. U. dioica subsp. galeopsifolia (fen nettle or stingless nettle), from Europe, does not have stinging hairs. U. dioica subsp. afghanica, from southwestern and central Asia, sometimes has stinging hairs or is sometimes hairless. U. dioica subsp. gansuensis, from eastern Asia (China), has stinging hairs. U. dioica subsp. gracilis (Ait.) Selander (American stinging nettle), from North America, has stinging hairs and is monoecious. U. dioica subsp. holosericea (Nutt.) Thorne (hoary stinging nettle), from North America, has stinging hairs and is monoecious.Other species names formerly accepted as distinct by some authors but now regarded as synonyms of one or other subspecies include U. breweri, U. californica, U. cardiophylla, U. lyalli, U. major, U. procera, U. serra, U. strigosissima, U. trachycarpa, and U. viridis.