Areas of Cornwall are among the worst affected by what conservationists are calling a tough army of foreign "superweeds".

Giant hogweed and water hyacinth are among the species causing problems for up to 80% of the nation's gardeners.

One of the worst offenders is Japanese knotweed which can even grow through concrete and tarmac.

The plant was brought to the UK by Victorians to be used as an ornamental plant but in Cornwall it has invaded vast areas of land.

On verges we would expect bluebells, ragged robins and buttercups but Winter Heliotrope is shading them out James MacFarlane Cornwall County Council

It can grow up to four inches a day.

Giant hogweed, which has toxic sap, grows up to 16 feet high while water hyacinth covers ponds and lakes, sucking out all the oxygen.

One Cornish gardener Rodney Hodges described the problem as a "nightmare".

Japanese knotweed has taken over his entire garden.

"In the summer it grows to about 10 feet high and you've got a job to push your way through it then," he said

Invasive plants

Conservationists say that global warming means superweeds are growing faster than ever.

And elsewhere in the county Asian laurel and winter heliotrope, originally from Africa, are smothering native species.

"On verges we would expect bluebells, ragged robins and buttercups," said James MacFarlane of Cornwall County Council.

"But it (winter heliotrope) is shading them out and basically excluding them."

Experts now want all invasive plants banned from garden centres and 'hit squads' to travel the country to slash back the superweeds.