We have the dreaded knotweed in our garden and around it. The best way to kill it off is to let it grow to a good size so that you can cut the stem and pour glycophosphates directly down it, the branches then die very quickly and it takes a while for more to come back in the same location. To control smaller new growth spray directly on the leaves but it is not effective.

We are still waiting to see how much comes back next year but at least our garden has not be completely overtaken by the weed.