Japanese knotweed

The greatest impact knotweed has on the human environment is the way it can grow through man-made structures, resulting in irreparable damage. Thin new shoots, with the vigour to grow quickly due to their access to the extensive nutrient store contained in the rhizomes and crowns of the mother plant, can exploit the smallest of weaknesses in tarmac, concrete, paving, mortar, etc. Once knotweed shoots have penetrated the structure (e.g. walls, drainage systems, hard landscapes, etc.), subsequent growth will increase the size of the structural fault and gradually push the whole edifice apart. Horizontal structures, such as paving, will be lifted by knotweed, causing similar damage to that caused by tree roots but in a much shorter period of time.

Japanese knotweed will dominate any area where it is allowed to establish itself. In the Far East it is not seen as a problematic species, since a number of plant competitors and a range of natural insect and fungi predators keep it firmly in check. In the UK, Europe, North America and Australasia it has no competition or predators at all (though this may change, see FAQs) so it is thriving. This is usually at the expense of native species, which are simply 'shaded out' by knotweed's thick leaf canopy. Knotweed's predilection for moist conditions means it can be found growing alongside rivers and streams where it restricts access to river environments and increases the risk of flooding by trapping sediment and clogging waterways with leaf and stem debris.

Japanese knotweed can regenerate from tiny fragments of itself. While stem and leaf regeneration have been recorded, rhizome and crown regeneration are more common. A fragment of rhizome the size of a paperclip can give rise to a new plant.