Stinging Nettle Urtica dioica
Introduction and Biology
Nettles are a common weed found growing all over the world, apart from the Arctic and Antartic. The Latin name Urtica dioica means “burning” and “uro” means “I burn”. Nettles are from the plant family of Urticaceae.
Urtica dioica is a tall, rhizomatous, perennial herb with numerous stinging hairs, they can grow up to heights of a metre or more. They have dark green, heart shaped, serrated edged, leaves covered in small stinging hairs, as are the large square stems. The flowers are green and hang in catkins from the plant.
It thrives on nutrient rich soils and so probably originated in fens on silty soils and semi-natural ancient woodlands, but widely naturalized in a range of habitats and abundant throughout the British Isles. Urtica dioica is a moderately shade-tolerant species, which occurs on most moist or damp, weakly acid or weakly basic, richly fertile soils. A highly competitive ruderal species, Urtica dioica often forms monospecific stands which can increase in diameter by 60cm a year.
These stands are not infrequently the product of a single individual that has spread by means of horizontal yellow rhizomes each of which can exceed 50cm in length.
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.
Plants are usually dioecious so male and female flowers are found on separate plants. Male clones of U. dioica flower in advance of female clones. Male flowers appear quite yellow, whilst female flowers have a silvery furry appearance.
Male Nettle Flowers
Female Nettle Flowers
Some insect species favour feeding on the developing seeds on female flowers whilst others feed on the pollen of male flowers (Davis, 1991). The pollen grains are extremely small elliptical in shape (1 to 1.2mm length 0.4mm diameter) and yellow/grey in colour. U. dioica are usually wind-pollinated, but occasionally insect-pollinated. The low seed mass of U. dioica enables the production of vast numbers of seeds. Over 70% of the seeds germinate in the period immediately following dispersal, the species maintains a seed bank that changes little in size with season and is large in relation to annual seed production.
The sting of nettles contains the chemicals Histamine, which irritates the skin, Acetylcholine that causes a burning sensation and Serotonin, which causes the other two chemicals to react. This is the main side effect from using nettles.
The stinging hairs when examined under a microscope exhibit a swollen base containing the irritant fluid and bearing a tapering stiff tube that ends in a bent portion with a narrow constriction. The walls of this tube are impregnated with silica and therefore they are brittle. When brushed the tip is broken and the fractured microscopic tube enters the skin. The consequent pressure on the sac gives us a hypodermic injection of the fluid. Grasping the nettle has been recommended as less painful. The rationale for this argument is that when handled roughly the hairs on this plant are broken lower down the tube and do not penetrate the skin. This mechanism is thought to offer some measure of protection against browsing as it can cause irritation to skin around and inside the mouthparts (Salisbury 1961).