This is the dreaded Japanese Knotweed, Fallopia japonica, introduced from Japan in 1825 and causing problems ever since. In my native Swansea it was a spectacular problem. Entering the city by train you passed through seemingly endless groves of it. It is listed by the World Conservation Union as one of the world's worst invasive species. In Australia it is actually illegal to have it growing on your property. This was growing on the edge of Amberley Brooks in West Sussex.