Biological Flora of the British Isles: Urtica dioica L

1. This account presents information on all aspects of the biology of Urtica dioica that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. 2. Urtica dioica is a tall, usually dioecious, rhizomatous, perennial herb with numerous stinging hairs, probably native in fens and semi-natural ancient woodlands, but widely naturalized in a range of habitats and abundant throughout the British Isles. British material is mainly ssp. dioica which also extends throughout Europe, and locally ssp. galeopsifolia (without stinging hairs) which is also found in western, central and eastern Europe. 3. Urtica dioica is a moderately shade-tolerant species, which occurs on most moist or damp, weakly acid or weakly basic, richly fertile soils. 4. A highly competitive ruderal species, Urtica dioica often forms monospecific stands which are not infrequently the product of a single individual that has spread by means of horizontal rhizomes. 5. Urtica dioica has frequently been described as a nitrophile, but there are many soils in which the supply of inorganic nitrogen is adequate for growth. However, there are other soils in which growth is checked with symptoms of severe phosphorus deficiency unless soluble phosphate is added to the soil. This is especially so in the long-established deciduous woodlands, and where there has been no addition of fertilizers to the soil. The growth responses of U. dioica to the availability, source and utilization of nitrogen and phosphorus have been examined experimentally in some detail. 6. Urtica dioica with its stems and leaves densely covered with stinging hairs, which release potential pain-inducing toxins when brushing contact is made with them, is rarely eaten by cattle and rabbits, but is palatable to some species of snail. It is the food plant of the larvae of a number of attractive butterflies and other phytophagous insects. 7. Male clones of U. dioica flower in advance of female clones. The pollen grains are extremely small. It is usually wind-pollinated, but occasionally insect-pollinated. 8. The low seed mass of U. dioica enables the production of vast numbers of seeds. Few of the seeds germinate in the period immediately following dispersal, and the species maintains a seed bank that changes little in size with season and is large in relation to annual seed production.