A long-term programme is under way to tackle a giant alien plant which is invading around 30 miles of bank on the lower River Usk in south Wales.
The Usk hogweed problem is one of the biggest in the UK
Environment Agency Wales said giant hogweed was causing increasing problems for landowners, walkers and fishermen.
As well as damaging the environment, hogweed is poisonous, and its sap can cause blisters which are sensitive to sunlight.
The agency said getting rid of the plant could take up to 10 years.
A special forum takes place next month to discuss dealing with the plant.
The hogweed is mainly found between Crickhowell and Newbridge-on-Usk but some is also found close to the mouth of the river in Usk.
The plant is a native of the Caucasus Mountains between Russia and Turkey and was originally introduced in Britain as an ornamental plant in the late 19th Century.
The perennial plant can grow up to five metres tall with leaves reaching one metre across. Each plant can produce more than 50,000 seeds every year, which can remain viable for 15 years.
Environment Agency Wales, in partnership with other agencies, started work last spring on ridding the Usk of the hogweed.
Project manager for the agency Stuart Craxford said the programme aimed to stop the seed production at the upper extent of the infestation, preventing the seeds entering the Usk and spreading the problem further downstream.
Mr Craxford said the Usk was believed to have one of the biggest problems with hogweed in the UK.
"We are looking at this problem and its solution in a detailed, systematic way," he said.
The plant was originally imported as an ornamental
"Every effort is being made to reduce and contain this invasive plant on the lower Usk, where it has become a growing problem.
"If the work is successful there will be benefits for conservation, recreation and flood defence work."
He said there was also a health and safety issue with the hogweed.
The edges and underside of the leaves bear small hairs, which are coated with a poisonous sap, and even the slightest touch can cause painful blistering and severe irritation.
The reaction can occur up to 48 hours after contact and, in some cases, can lead to recurrent dermatitis.
The environment agency has spent £5,000 on the first phase of the project and during 2005-6 it surveyed mapped and developed a control strategy
The Giant Hogweed Forum is to take place at Coleg Gwent's Usk campus between 1900 and 2100 GMT on 13 February.