The invasive plant can even grow through concrete and tarmac
An invasive plant is choking water courses in the Isle of Man, according to The Manx Rivers Improvement Society.
The countryside group has said the Japanese knotweed is spreading rapidly and is in danger of causing problems.
In summer, the plant can grow up to 3m in height and it effectively chokes out native plants and river life.
Society secretary Brian Craig has said there is a real danger that parts of the Douglas River through Tromode and Baldwin will be blocked by knotweed.
He says urgent action needs to be taken.
"The association would like to see something done.
"If somebody was to start at the top of the river and work their way down, you could make an impact on it.
He added: "Recent reports in the UK have put costs of eradicating the weed at millions of pounds.
"If the Isle of Man government was to do something about it now, they could save themselves all that money."