Knotweed spreads from fragments of root and stem, and canal banks and lightly managed green corridors are two of the easiest ways for those fragments to move. Runcorn has plenty of both, from the Bridgewater Canal to the footpath network that links the new-town estates.
What it means for your garden
If your garden backs onto a canal bank, a footpath or open landscaping, walk the back boundary every spring looking for purple-red shoots and again in summer for tall hollow canes.
Who is responsible
Whoever owns the land it is growing on. A survey of your side records whether it has crossed the boundary.