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The Giant Hogweed

Giant hogweed makes triffids look like pansies (from Marianne Leitch in London)

John Wyndham's distinctly nasty but purely imaginary triffids wouldn't stand a chance against the latest alien plant which is taking over Britain's waterways.

Giant hogweed (heracleum mantegazzianum) has British botanists and day trippers worried. At 15 foot, its pungent-smelling white flower heads tower over all other riverside vegetation. An average sized specimen is twice the height of most human beings. More importantly, giant hogweed displays blatantly anti-social characteristics, as picknickers and angry farmers are painfully finding out.

"Anyone who touches the plant and is then exposed to sunlight can get a very nasty burn indeed," says hogweed expert, Dr Jim Forbes.

"The stem and leaf stalks of giant hogweed contain large amounts of sap and copious quantities are released if a stem or leaf is broken or cut. The sap contains furocoumarins, substances which photosensitise the skin on contact."

Victims can suffer anything from a mild rash to painful, watery blisters, which in severe cases can require treatment in hospital. For some super-sensitive types, contact with giant hogweed results in a severe case of recurrent dermatitis, with the rash sometimes reappearing months after the original contact with the plant.

Children are particularly at risk, as they are attracted to the plants by their spectacular height and large flowers.

The invasion of the giant hogweed is puzzling British agriculturists and scientists. The plant was introduced from the Russian Caucasus at the end of the 19th century as a decorative plant for the gardens of great houses.

Its effective reproduction system - the massive flower head on a single plant produces 5000 or more seeds - caused its immediate spread to neighbouring riverbanks and wild areas. But it is only over the past few years that giant hogweed has spread so rapidly as to give farmers and agriculturists grave cause for concern.

"There must be hundreds of miles of riverbanks overrun by now" said Dr Forbes glumly. "The Tweed is bad, but so are the rivers Deveron, Lossie, Findhorn, Nairn and South Esk. Over the past few years it has grown like wildfire."

The giant hogweed's extraterrestial characteristics are highlighted by its fantastic growth rate. In two months it can grow from a pretty harmless-looking six inch weed into a 15-foot monster. "You can almost hear it growing," said a harrassed county council official.

An Edinburgh professor believes the plant sends on average 10 Scots per week to hospital. Many people who are unaware of the plant's hostility are drawn to it by its stature and color - the reddish tinged stem and leaf stalks covered with bristles support a huge white flower head.

Cattle and sheep love it, and it doesn't seem to do them any harm. "Animals seem to relish giant hogweed, and they can graze on the young plants, but most grow too high," said Dr Forbes.

Children try to use the long, hollow stems as peashooters or "telescopes" - with dire results.

Dr Forbes describes the smell of giant hogweed as "unpleasant - a mixture of parsnips, celery, parsley and carrots." In fact, giant hogweed is related to the parsnip family but the family connections are loose, and don't extend to the nutritious aspects of its milder and more congenial cousins. Experts advise in the strongest possible terms against eating the plant.

They attribute its harmful effects on the skin to chemicals contained in the sap which are similar to those used in several leading tanning lotions. "It effects the skin in the same way as suntan preparations," said Dr Forbes. "The rash and blisters can be likened to a severe case of sunburn."

Giant hogweed poses a serious threat along British waterways and in areas of uncultivated land.

The only way to eradicate it for good is to cut the plant below ground level with a spade - a laborious and impractical method where thousands of plants have taken over whole tracts of land.

Dr Forbes and his colleagues are experimenting with herbicides, hoping to find one which will kill giant hogweed without harming surrounding vegetation.

"I don't want to be alarmist," he added, "it's still only a minor problem. But we ought to look out now before it becomes a major one. It's already taken over miles and miles of river banks, and its already at the stage where I think its going to be very difficult to control."