Andy Cawley, sales and marketing manager at Dow AgroSciences, said the plant was so prolific it could grow up to 10cm a day.
"If it gets too big then the advice is to cut back the weed and treat the re-growth when it is a more manageable size," such as 1m high, he said.
"There is never a silver bullet when it comes to controlling Japanese knotweed," he said. But he claimed his firm's product, Tordon 22k, "scores over other treatments".
Cawley said the chemical, absorbed by leaves but safe to grasses, beat rivals which could need up to five or more applications over several years to achieve a comparable level of control.
He added that Dow AgroSciences had a product in the pipeline that was "effective" against Japanese knotweed, hogweed and Buddleja and was seeking its registration.