Japanese Knotweed
Species and Names
Species/ Fallopia japonica
Common Names/ Japanese Bamboo, Pysen saethwr(Welsh), Polygonum cuspidatum, Reynoutria japonica
| Height | To 3m |
| Stem | Zig-zag, reddish, turning green |
| Leaves | Alternate rounded triangular to sharp point, to 200mm long |
| Flower | Creamy white in loose, branched spikes |
| Seeds | To 5mm heart shaped winged. (sterile) |
Japanese knotweed is regarded by many as one of the most invasive plants in the United Kingdom. Its resistance to traditional weedkillers and lack of natural enemies in this country allow it to out-compete most native plants.
Why is it in the UK?
As the name suggests the plant is a native of Japan. It was introduced to the UK in the 1830s and soon became a very popular garden plant. By the end of the Victorian era, it was growing in gardens throughout England. It did not take long before Japanese knotweed was found growing in the rest of the UK. It soon spread from gardens and into the wild, colonising riverbanks and large areas of disused ground.
How does it spread?
As most of the Japanese knotweed outside Japan is functionally female, it does not spread through seed germination. The underground network of rhizomes is capable of producing new plants which are genetically identical to the parent plant. A piece of rhizome as small as a thumb nail can produce a viable plant in a very short time.
Fly-tipping of Japanese knotweed has led to its presence on many brownfield sites across the UK. Japanese knotweed is common on many riverbanks because the rhizomes are very hardy and can remain dormant in water, taking root when washed up.
Construction sites are often plagued by Japanese knotweed due to it being spread by tracked vehicles. Also, movement of soil which contains tiny fragments of rhizome can cause its spread.
What damage can it cause?
Japanese knotweed can cause damage to structures, roads, and ecosystems. The plant exploits weaknesses in structures and road surfaces and is often be found growing through tarmac. The damage to Britain’s ecosystems is considerable. By dominating the native plants there is an impact on the animals which populate the area.
Often Mistaken for...
Dogwood (Cornus sanguinea), the Dock Family (Polygonaceae), and Rosebay Willowherb (Chamaenerion angustifolium).
