Mr Roberts said the Environment Agency and Dorset Council did not have the resources to tackle the problem, leaving conservation groups and volunteers to step up.
A decade-long clearance project on the Lymington River and its tributaries in Hampshire saw 661 volunteers pull Himalayan balsam along 53km (33 miles) of riverbank.
Hampshire and Isle of Wight Wildlife Trust said areas that had once been dense with balsam had been “restored to a carpet of native bluebells, stitchwort and pink campion”.
Himalayan balsam is also the most invasive plant affecting Isle of Wight’s watercourses, where the trust leads a similar project on the Eastern Yar river.
Mr Roberts said: “The Lim is a small river, about 5km (3 miles) long, so I’m hoping we can get rid of it.
“The problem is that seeds can stay dormant for up to three years, which is why we’ve said five years.
“We can clear the surface balsam then, three years on, they’ve come back a massive amount.”
River Lim Action is being supported by Lyme Regis Town Council, which has committed five years of funding for gloves, bins, secateurs, and tools to get into the bramble where the balsam is growing.
“People say it’s too big a problem but someone has got to grab it and say it’s really important,” said Mr Roberts.
“If we’re not careful, the whole of England will be covered in Himalayan balsam.”