ONE of America's most irritating weeds threatens to spoil the summer months of thousands of hay-fever sufferers.
Ragweed has established itself in central Europe and is spreading westwards towards Britain and Ireland, scientists warned today.
A single ragweed plant can spew out a billion, highly allergenic pollen grains in just one season and this highly oversexed, invasive US weed is now firmly over here on this side of the Atlantic, the scientists say.
An increasing number of Europeans are showing signs of ragweed allergy as the plants spread from Hungary to the fields of Italy, Austria, France and more northerly regions bordering the English Channel.
Scientists fear it might only be a matter of time before the common ragweed, Ambrosia artemisiifolia, gains a foothold in Ireland and Britain where it could become an invasive species with the help of warmer summers, milder winters and the formidable reproductive powers of the plant itself.
A scientific conference in Vienna this week has been called to address the problems posed by the plant species.
"Common ragweed is not an issue here -- yet," said Dr Clare Goodess, of the Climatic Research Unit at the University of East Anglia in the UK. But, she said, as global temperatures rise the highly invasive plant is spreading rapidly westward in Europe and it's probably "only a matter of time" before it hits us.
Warmer summers extend the pollen season and higher concentrations of carbon dioxide in the atmosphere have been shown to boost the production of ragweed pollen.
In the US, ragweed pollen is one of the most common causes of hay-fever and asthma attacks -- 75 pc of Americans who are allergic to pollen are allergic to ragweed.
It can also travel long distances, being found 400 miles out to sea and two miles up in the atmosphere.
The northern limit of ragweed in Europe is moving further north with climate change, according to Dr Jonathan Storkey, a plant ecologist at Rothamsted Research in Harpenden, Hertfordshire. "The concern about ragweed centres on the health issue rather than the problem of its invasiveness. We have invasive plant species already but this species has pollen that is highly allergenic -- it's bad news for hay-fever sufferers," he said.
Because ragweed produces its pollen between late summer and early autumn, it can significantly extend the hay-fever season.