Japanese knotweed is a tall ornamental shrub with bamboo-like stems and pretty white flowers. It is also a serious problem for our native biodiversity, shading out its competition, and poisoning the soil for other plants. Evidence for it destroying buildings might be a little bit overstated, but it is nevertheless a troublesome invader that must be dealt with carefully lest it spread even further.
Japanese knotweed (Fallopia japonica) is a perennial shrub with bamboo-like stems that can grow up to 3m tall. It has distinctive shield-shaped leaves with flat bases. The leaves emerge in a “zig-zag” pattern, and are arranged on green stems which are mottled with red or purple. In late summer, the plant produces small creamy white flowers, but does not produce seeds in Ireland or the UK. Underground, it grows from specialised stems called rhizomes that can reach several metres in length – if these are unearthed, they can be recognised by their knotted, woody appearance, white shoots, and bright orange insides (however, you should never break apart any part of a Japanese knotweed plant unless under expert advice).

Japanese knotweed leaves and flowers – note the zig-zag stems and flat leaf bases

Japanese knotweed was first introduced to the UK and Ireland as an ornamental plant in the 19th century. Its introduction and spread has been traced back to a single introduction event, so they individual plants in the UK and Ireland are actually mostly clones of this original plant – this means the plant rarely sets seed, and spreads entirely vegetatively. It can regenerate into a full plant from even tiny fragments, so great care must be taken in removing it from an area.

Japanese knotweed is a big problem for biodiversity and the health of riparian (riverside) habitats. It is a highly competitive plant in the areas in which it grows, growing tall and fast, and ultimately shading out its neighbours. Disturbed ground is the preferred habitat for Japanese knotweed, where plant fragments can find purchase and space to start growing. In this way, it quickly dominates areas it gets a chance to invade. In addition, it can secrete chemicals into soil that make it unsuitable for the growth of other plants (this phenomenon is called allelopathy). Along rivers, it crowds out other plants, and becomes the dominant plant holding the banks together. However, it has a relatively fragile root system, and dies back in the winter, meaning that the banks become much more susceptible to erosion. This is a problem not only for the biodiversity of the river and its banks, but also for humans, as a widening river may start to threaten our property or even our lives.

An identification guide for Japanese knotweed

Japanese knotweed has also been found in some cases to grow through concrete or asphalt, prompting concerns that it could be a threat to construction works. However, a study in 2018 by Fennell et al. showed that Japanese knotweed was rarely involved in damage to buildings or other constructions, and that other plants caused about as much or more damage in this way. In fact, the most damaging ways the authors found plants can affect buildings are through sucking up water from soil, resulting in subsidence of the ground under building foundations, and through trees falling and striking buildings or power lines. It is most likely that when Japanese knotweed is found growing through concrete or asphalt, it is growing through pre-existing faults in these materials rather than creating them itself.

So how can you deal with this invader if it gets into your garden or local area? If you find the plant growing in public property, you should notify your local city or county council, and the landowner if you find it growing in private property. It is illegal to allow the spread of Japanese knotweed, and so it the obligation of the landowner to remove it. Never cut, strim, mow, or hedge-cut Japanese knotweed, as you risk creating fragments that will grow into new plants. If you find rhizomes while digging, avoid breaking them up for the same reason. In addition, do not compost knotweed, as it can easily grow from here. You may want to consult with a licensed contractor or specialist if you need Japanese knotweed removed.


Written by Tom Murphy











“Where do Camels Belong?: The story and science of invasive species” – Ken Thompson, Profile Books Ltd., 2014