Knotweed spreads from pieces of root and stem, and brooks and old railway embankments are two of the easiest ways for those pieces to travel. On the Wirral, the Birket valley, other lowland brooks and the line of the old railway all give it a route into gardens.
What it means for your garden
If your garden backs onto a brook, a verge or the old railway, walk the back boundary every spring 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.