Japanese knotweed spreads from pieces of root and stem, and water is one of the best ways for those pieces to travel. That is why banks along the Leeds and Liverpool Canal and the River Douglas are such common places to find it.
What it means for your garden
If your garden or yard backs onto the canal, the river or a brook feeding them, check the back boundary every spring for red shoots.
Who is responsible
The owner of the land where it grows is responsible for it. A survey of your side records whether it has crossed over.