Nettles Urtica dioica
Early childhood memories of Nettle stings are a strong reminder to avoid contact with that plant, since the effect is painful and lasts for several hours. Although it is not known to be fatal in humans, even in severe cases, it is reported that dogs have been killed by being badly stung. It is likely that the Nettle sting evolved as a deterrent to herbivores, and certainly the Nettle is a very successful agricultural weed, growing especially well in pastures frequented and manured by animals, and in the South West it is probably the most widespread native plant.
At one time it was thought that formic acid is the active factor in the stinging hairs, but it is now known that the concentration of this is very small in the hairs. The three organic chemicals mainly responsible are – histamine, serotonin (5-hydroxytryptamine) and acetylcholine. Histamine is a well-known cause of irritation, and the other two components act as synergists for this effect. The single celled hairs are made of silica (below), which easily fragment on puncturing the skin, releasing the toxic fluid.
The only other stinging plant in the UK is the Annual
Nettle (Urtica urens). Other plants, including close relatives of the
Nettle (Mucuna, Laportea) from the tropics are known to have even
more powerful stings. The name Urtica is derived from the Latin ‘urere’
to burn, while the name Nettle may be related to the word ‘needle’. There may
be a good reason for the use of Dock 
(Rumex)
as the antidote to the nettle sting, since this plant appears to have
anti-histamine activity.
Many
animals are not deterred by the stinging hairs, most notably the larvae of
several of our common butterflies (Tortoiseshell, Red Admiral, Peacock and
Comma) that depend on the nettle as their food source. Animals differ in their
sensitivity to the stings. Rats appear to be immune, while guinea pigs and
other domesticated animals are badly affected.
Richard Mabey recommends cooked Nettle leaves in his
book ‘Food for Free’. Having tried these myself I find them rather bland, but
not unlike Spinach. They are best harvested when young, not beyond the middle
of June. After this time apparently they become laxative and rather fibrous.
The latest research suggests that Nettle roots may
provide a cure for benign prostatic hyperplasia - a common affliction in males
in later life. I imagine that Godfrey in ‘Dad’s Army’ shows the classical
symptoms of this affliction, and he has my sympathy! The Royal Society of
Medicine also reports that Nettle stings may be used to reduce the pain of
osteoarthritis. This may be related to the neurotransmitters present in the
sting.
In common with the close relative, Hemp, Nettles
have a useful fibre, which has been used extensively for the production of
cloth. The properties of this fibre are not greatly inferior to those of flax.
In the First World War the Germans used Nettle fibre to produce army shirts,
each shirt needing about 40 kg of Nettles. There is now a revival of this
source, and there are proposals to start the commercial cultivation of the
Nettle plant for this purpose. The shoots are cut in September or October, kept
in a damp atmosphere for 3-4 weeks and the fibre is then easily separated.
Nettles were also used during the Second World War
in the production of the green chlorophyll to colour camouflage drapes. Nettles
are a good source of this pigment since enzymes causing its oxidation are
virtually absent in this plant.
The specific name of the Nettle ‘dioica’ implies that this plant is dioecious, having the sexes on two separate plants. Some floras suggest that it is possible to sex Nettles by observing the gross structure of the ‘catkins’ at the internodes. From personal experience this is not a good indicator, and it is necessary to use a hand lens to view the floral structure more closely. The male flowers are almost spherical when unopened. Opening occurs suddenly with the appearance of four very pale anthers. The female flower is smaller, somewhat pear-shaped, with a brownish stigma at the tip.
Terry Smith Home
Email
t.a.smith@blueyonder.co.uk
07 September 2003