Plant family of the week – 6th July 2020 – Urticaceae, the Nettles.

Urtica, Parietaria and Soleirolia
Urtica dioica (Stinging nettle)

The stinging nettle, Urtica dioica, is a perennial native flowering plant that can grow to two metres or so in height. It has a high demand for nitrogen and phosphorus and commonly grows in the vicinity of human habitation or animal husbandry on soils enhanced by animal dung. Its specific name “dioica” means that the male and female flowers are borne on separate plants. Dioecy only occurs in about 20% flowering plant species.

Above: male flowering stem and detail of male flowers of Urtica dioica

Above: female flowering stem and detail of female flowers of Urtica dioica

The plants are hairy all over with simple hairs among which are much larger stinging hairs, tapered to a long point that ends in a minute spherical bulb. The walls of the tips of these hairs are silicified and brittle, so that when touched they break off diagonally to reveal a sharp point like a hypodermic needle, through which a cocktail of irritant chemicals including histamine and acetylcholine is injected into the skin causing a rash that is named ‘urticaria’ after the Latin name of the plant.

Above: Urtica dioica stem and petiole with stinging hairs and stipule scars
Right: Scanning electron micrograph of Urtica dioica leaf with stinging and simple hairs
Leaves of Urtica dioica grown in shade (left) and in the open (right)

The leaves of Urtica dioica are strongly toothed, the terminal tooth longer than those adjacent. They are borne in opposite pairs on short petioles and stipulate, although the stipules may fall. The leaves can vary considerably in shape, becoming longer and narrow under woodland shade, more nearly cordate when open grown.
Dense linear inflorescences of tiny flowers emerge from their axils.  The number of stinging hairs also varies and there is a recognized stingless subspecies, Urtica dioica galeopsifolia, that inhabits wet places such as fens and carr and has notably long and narrow leaves.

Urtica urens stem with ripening achenes

A second species in the genus, the small nettle, Urtica urens is an archaeophyte (an ancient introduction). Unlike U. dioica, it is annual and monoecious, and generally much shorter. It can be distinguished by its more ovoid leaves with coarser, deeper toothing with the terminal tooth about as long as the adjacent teeth. The lower leaves are shorter than their longer petioles and with stinging hairs only.

The family Urticaceae contains two other genera that occur in our area – Parietaria and Soleirolia.

Pellitory of the wall, Parietaria judaica is recognisably nettle-like in appearance, softly hairy all over, with untoothed leaves in alternate arrangement and without stipules, round reddish rather than square stems, smaller, softly hairy and stingless. The plants are monoecious but the short inflorescences either side of the leaf petioles have separate male and female flowers. It is uncertain whether it is native, or an introduction.

Left: Parietaria judaica with Urtica dioica on river bank at Musselburgh. Centre: close up of flowering stem. Right: details of P. judaica flowers: upper – female flowers above unopened male flowers, and lower: male flowers open and in bud.

Soleirolia soleirolii, commonly known as ‘Mind-your-own-business‘ is a prostrate, mat-forming perennial with tiny rounded alternate leaves and stems rooting at the nodes. The tiny pink flowers, 1-1.5mm, are borne singly in leaf axils, the females much commoner than the males. A native of Mediterranean islands, it was introduced to the UK in the early 1900’s as a garden rock-plant.

Left: Soleirolia soleirolii growing in a pavement crack. Right above: Female flowers and below: Male flowers.

The mutual relationship of these widely differing plants is shown in the structure of their flowers. The stigmas of the female flowers have long feathery tufts of papillae designed to capture the wind-borne pollen. The males have four stamens in a tightly-furled cross that straighten explosively, hurling the pollen into the air.

Nettles are among our commonest plants, occurring in almost every 10km square of the British Isles, and consequently they are overlooked, dismissed as weeds by most gardeners, plants to be eradicated because they insist on growing in the wrong place. Yet they have been valued in the past as a source of food and of a valuable fibre that, like flax was obtained by retting, and used to weave strong textiles.  German army uniforms were predominantly made from nettle fibre during WW1 because of the shortage of cotton, world production of which at the time was controlled by Britain.

The contribution of nettle to human cultural life is reflected in words and phrases in the English language –Aesop’s fable The boy and the nettle may be the origin of the figure of speech “grasp the nettle”. The word “nettled” meaning angry or in a prickly mood. Urticaria, is used to describe rashes of the kind produced by nettle stings. The name Urtica derives from the Latin uro, to burn or inflame.

Although nettles are handsome rather than beautiful, they can be appreciated and encouraged for their value as a wildlife resource, and many gardeners deliberately create space for them. They produce copious seed for birds, shelter and food for many species of insects that feed small birds and bats and they are the preferred larval food plant of several of our most valued species of Nymphalid butterflies.

The Small Tortoiseshell butterfly, Aglais urticae, gets its scientific name from this plant.  The larvae of the Peacock butterfly Inachis io are gregarious, the eggs laid in masses of several hundred, almost invariably on nettle, with silk webbing forming a tent in which to live and feed. The Red Admiral Vanessa atalanta prefers Urtica dioica as its food plant, but the family relatives small nettle, Urtica urens and pellitory-of-the-wall Parietaria judaica are also used. The Comma, Polygonia c-album will feed on nettle but prefers the closely-related hops. Nettles are also used by a wide range of moth species with eclectic tastes and exotic names, such as Angle shades, Mottled rustic, Burnished brass, Beautiful golden y and Gold spangle, but a tighter preference for the family is shown by the Spectacle and the Snout, which prefer Urtica and Parietaria but will also feed on hops.
Nettles and their close relatives are hosts to the aecial phase of the group of rust fungi known collectively as Puccinia urticae that complete their lifecycle on species of Carex.

Left: larvae of the Peacock butterfly feeding on U. dioica
Above: Puccinia urticata rust on U. dioica.

Text and photos © Chris Jeffree

Sources of information

J. Thomas and R. Lewington 2010 The butterflies of Britain and Ireland. British Wildlife Publishing Ltd., ISBN 978 0 9564902 0 9

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