Japanese knotweed spreads quickly during the summer and can easily do so across house borders, under fences, under paving slabs and tarmac, and a small part of the rhizome spread about can start a whole new growth. Whilst the plant does not reproduce in the normal fashion, an extensive rhizome root system can extend up to 3 metres laterally. This means that a garden fence is little defence to an invasion from your neighbour’s garden.