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Japanese knotweed - the indestructible alien

Posted: Friday 14th July 2017 by RiverSearch

Pic: Japanese Knotweed (Snowdonia National Park Authority).

One of the hardest to eradicate of the invasive species, Japanese knotweed is a huge headache for councils, homeowners and conservationists.

Japanese knotweed is a tall herbaceous perennial with bamboo like stems, often growing in dense thickets. It first appeared in the UK at the Royal Botanical Gardens Kew in August 1850. Much like the persistent bindweed that gardeners battle with on a daily basis, Japanese knotweed is able to grow vegetatively from the smallest of fragments. This has enabled it to easily move around the UK, most commonly through construction and road building schemes.

Thankfully it does not produce viable seed in the UK but can hybridise with some other plants from the same family. Negative impacts include outcompeting native flora, contributing to river bank erosion and increasing the likelihood of flooding. Japanese Knotweed is listed under Schedule 9 to the Wildlife and Countryside Act 1981. As such it is an offence to plant or otherwise cause Japanese knotweed to grow in the wild.

Japanese Knotweed in Europe occupies two main types of habitat. It is particularly well-suited to growth along riversides where it is able to spread naturally by water-borne rhizome or stem fragments. The other main habitat is in areas such as roadsides, railways, derelict industrial land and anywhere else it has been discarded. RiverSearch volunteers have found it to be the second most common invasive species along river banks in Surrey with Himalayan balsam being in the top spot.

Where the plant grows on development sites it can cause potential risks such as damaging hard structures and surfaces
There are no definitive industry wide figures for how much Japanese Knotweed costs, but even on relatively small sites the cost of control can run into hundreds of thousands of pounds. It was estimated the cost of removing Japanese Knotweed during the construction of the Olympic Park in London over 10 acres was £70 million!

It is extremely difficult to eradicate due to their massive network of underground woody rhizomes however the most effective method appears to be injecting a herbicide into the stem of the plant during Autumn on an annual basis.
There is currently research being carried out on a biological control for Japanese knotweed in the form of a Japanese pysillid beetle. The beetle has been found to have no detrimental effects to any of our native species and will happily munch its way through the plant. Trials are ongoing but if successful it could help us to turn the tables on this thuggish weed. 


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