What is Japanese Knotweed

What is Japanese Knotweed

Japanese Knotweed, Fallopia japonica, is a large herbaceous plant native to Eastern Asia. It is a strong-growing, clump-forming perennial with tall, dense annual stems but also has an extensive underground woody rhizome system, which is responsible for its spread and persistence. Stem growth is renewed each year from the stout, deeply-penetrating rhizomes and it is spread when small pieces of the plant or rhizomes are broken off. One piece of rhizome or stem the size of a fingernail can produce a new plant and rhizome segments can remain dormant in soil for twenty years before producing new plants.

In early spring red/purple shoots appear from the ground. The new stems elongate rapidly and, in a matter of weeks, they have put out a lush green canopy of large, heart or spade-shaped leaves. The plants are fully grown by early summer and the mature stems are hollow with a distinctive purple speckle and distinct raised nodes that give it the appearance of bamboo, although it is not closely related. The stems may reach a maximum height of 3–4 m each growing season but it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The plant flowers in late summer/early autumn and produces clusters of spiky stems/tassels covered in tiny creamy-white flowers. The above-ground growth is very frost sensitive and dies back during the first autumn frost leaving behind the characteristic reddish-brown stems, which persist throughout the winter, and provide some protection for the emergent shoots in the following spring.

Japanese Knotweed is very successful in North America and Europe and has been classified as an invasive species in many countries. The thick, dense colonies completely crowd out any other herbaceous species and it is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35 °C and can extend 7 metres horizontally and 3 metres deep, making removal by excavation extremely difficult. Japanese Knotweed was introduced to Britain in the 19th century as an ornamental plant and over time it has become widespread. 

Japanese Knotweed in Europe occupies two main types of habitat, one natural and one created or altered by us.  It is particularly well-suited to growth along riverbanks, where it gives every appearance of being native, and along which it is able to spread naturally by water-borne rhizome or stem fragments.  The other main habitat is in man-managed areas such as roadsides, railways, derelict industrial land and anywhere else it has been discarded. In Britain, it is classed as “controlled waste” under part 2 of the Environmental Protection Act 1990. This requires its disposal at licensed landfill sites.

Did you know?

“At the Olympic site in East London the RHS reports the cost of removing and disposing of Japanese Knotweed to be estimated at £70 million.”
The Environment Agency calls Japanese Knotweed “indisputably the UK’s most aggressive, destructive and invasive plant”

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