Yes, Japanese Knotweed can grow within cavity walls. We have experienced stems and healthy leaves emanating through vents and air bricks located 2m above ground level. When knotweed grows in cavity walls it may grow for some time without being noticed, and has the capacity to force the two skins of the wall apart if left untreated.

We have also seen Japanese Knotweed growing from chimneys on roofs, where it has grown with vigor inside the cavity walls and eventually found its way into roof space and out of the chimney.