Japanese Knotweed
Fallopia japonica
Japanese knotweed was introduced into the British Isles probably around 1825 by Victorian plant hunters and is native to Japan, Taiwan, Korea and Northern China. It was bought in as an ornamental plant and soon escaped into the wild around the 1880s. It had been introduced to many European countries and the US. It is also known as Sally Rhubarb, Bambard and Japweed.
Identification
The reddish purple shoots appear in early spring and the leaves unfurl as the canes grow rapidly, up to 7.5cm a day, and it can reach 3 meters in height. The canes or stems are hollow and grow in a distinct zigzag pattern. They also have purple blotches on them. The leaves are heart shapes (Chordate) and are 5-12 cm long and 5–8 cm wide, arranged alternatively up the stem. It flowers in late summer showing small creamy white flowers in panicles or groups of 6-8. The root system is rhizomatous - they creep 
laterally and get bigger with age. The roots are orange when freshly snapped but not as flexible as other tap roots like Docks.
The plant is a perennial and dies back each autumn but the stems persist as woody upright hollow stems. In the UK Japanese knotweed is unable to set seed and reproduces vegetatively from the rhizomes. It can regenerate from cut stems in the right environment. 
In the UK it is found growing in many habitats including grassland, woodland, coastal and beach areas, riverbanks and in urban areas – it can grow through walls and tarmac! Japanese knotweed is a primary coloniser in countries of origin.
Problems caused
As the species spreads by rhizomes it can turn up accidentally or deliberately in many places. A small section, even a 1cm cube of rhizome, can grow into a new plant. Therefore, moving any contaminated soil around can cause problems even if you can’t easily see the plant. The Rhizomes can grow very deep and spread up to 7 metres from the initial plant.
It has been spread in the UK in soil, garden waste and as a garden escape. As the plant grows very well on riverbanks it causes two problems in this habitat. First, the dead stems can fall into the water and cause blockages and flooding when it dams up culverts especially in urban areas and secondly, erosion of the bank can cause the rhizomes and stem pieces to be washed downstream and re-colonise new areas. Riverbank inspection is very difficult too.
As it is a successful competitor, Japanese knotweed can form dense canopies blocking out sunlight and out-competing native plants – it can seriously damage wildlife sites.
The species can cause widespread problems, for example for anglers on riverbanks, it may reduce visibility on roads, paths and railways, and can trap litter.
Mowing an area can cause further spread as the cuttings are spread or flailed about and cuttings moved to another site can regenerate. If the cuttings end up in a composting scheme and are not heated to above 55 degrees C they can regenerate.
In the urban environment, it can push up through tarmac, pavements, car parks, through cavity walls and cause problems in graveyards. The rhizomes can push their way through concrete foundations, land drainage works and cause instability. This can incure high costs, not only to repair the damage but to eradicate the plant completely.
It can seriously reduce the value of land or even property if it is growing within the land boundaries.
It is illegal under the Wildlife and Countryside Act of 1981 to plant Japanese knotweed or introduce it in any other way into the wild – this includes fly tipping of garden waste!
Distribution
In the UK, Japanese knotweed is found in virtually all Counties and most islands. In Essex, it is found in most areas of the county.
Control measures
Once established, Japnese knotweed is very difficult to control but it is susceptible to a range of herbicides such as Glyphosate(e.g. Roundup) which translocates down to the rhizomes.
It is important to kill all parts of the plant especially the rhizomes, though application may be needed for 2-3 years to successfully kill it completely. With chemicals, caution is needed near aquatic habitats and areas of wildlife importance.
Other methods of control include cutting or mowing ( safe disposal of the cuttings is essential), pulling and grazing. You can burn the cuttings.
All these methods eventually weaken the rhizome over a few years. It can also be dug out at great expense – you need to dig 2m deep and 2m away from the last stem to ensure total removal – then you need to dispose of the contaminated soil!
If the plant is on your property, it is up to you as the landowner to control it.
More information can be obtained from the
Environment Agency click here to go to the website
They will be able to assist you with advice on control.
Actions that the public can take
Don’t Fly tip garden waste especially if it contains Japanese Knotweed stems or rhizomes. This includes over the back fence! If you see anyone fly tipping, report it immediately to the Environment Agency. (Do not confront the person - note date, time, location and car registration if possible).
Don’t contaminate your local green waste composting scheme by putting it in the bins – it can survive the composting process and someone may get Japanese Knotweed in their hanging basket instead of pretty flowers!
Don’t accept top soil unless it is guaranteed free from the Rhizomes – check it yourself if possible.
If you find Japanese knotweed - control it as soon as possible. Be cooperative with your neighbours if one or both of you has it on your land.
If you see a patch, please note where it is, how big the patch is, when you saw it and let us know.