Norfolk Biodiversity

‘Stemming’ the tide of Japanese knotweed

Japanese knotweed was widely planted in ornamental gardens in the late 18th century, but has spread to almost every part of the UK, dominating countryside and wasteland thousands of miles from its native East Asian habitat. A series of well-publicised knotweed infestations and the introduction of ‘Asbos’ for failure to control its spread (see here for more details) have built momentum against this invader – could 2015 be the year to stop Japanese knotweed in its tracks?

Japanese knotweed by RHS

Japanese knotweed looking very innocent (Credited to the RHS)

Getting to the ‘root’ of the problem

There are many characteristics which make Japanese knotweed one of the worst invasive plants in Europe. With vigorous growth, knotweed can quickly dominate an area with a dense thicket of tall bamboo-like stems. Stem growth is renewed each year from deeply penetrating and widespread root networks.  Although knotweed doesn’t produce seeds in Europe, the plant can be spread very easily through fragments of the root or stem.

A new plant can sprout from a fragment of root as small as 1 cm in less than 10 days! This makes control and eradication of Japanese knotweed very difficult.

Spread of Japanese knotweed

Japanese knotweed really can spread when given the chance (Credited to Mike Thomas)

Japanese knotweed is not easy to tame, as we found out in 2012, when the cost of clearing it from the East London Olympic site was estimated at £70 million (see here). This ‘superweed’ struck again in 2013, when a couple were told it would be cheaper to demolish their £300,000 house and rebuild than try to eradicate the Japanese knotweed growing under their home (more information here).

"An unwelcome visitor to a London apartment" (Credited to the BBC)

“An unwelcome visitor to a London apartment” (Credited to the BBC)

What makes Japanese knotweed so destructive?

Not only Japanese knotweed out-grow, out-shade and out-compete its native competitors – it also uses a form chemical warfare – allelopathy.

Allelopathy is a mechanism whereby plants produce and release compounds which directly inhibit and suppress the growth of other plant species around them.

An experiment by Murrell et al. (2011), clearly demonstrated this.  Activated carbon (AC), known to have a high capacity to absorb organic compounds (such as those produced by knotweed), was added to native plant communities both with and without Japanese knotweed. The addition of AC greatly improved the growth of native plants where knotweed was present, but had no effect when knotweed was absent.

Allelopathy is a relatively common phenomenon in the battle for resources amongst plants. However, the impact of the allelopathic compounds produced by Japanese knotweed appears to be much greater in Europe than in it’s in native range, further increasing the plants  destructive capabilities.

Native European plants have had no time to ‘co-evolve’ with Japanese knotweed, making them particularly susceptible to its allelopathic compounds having never encountered them before. Combine this chemical weaponry with its ability to sprout from small fragments of root, its problematic removal and its vigorous growth and you have an invasive that is well and truly determined to stay put.

How can you help?

You can help by reporting any Japanese knotweed that you encounter. This can be done through the RINSE smartphone App, which is available for iPhone and Android handsets. For more details please visit:

Japanese knotweed in winter Credited to Devon County Council

Japanese knotweed in winter (Credited to Devon County Council)


About Norfolk Biodiversity

The Environment Team at Norfolk County Council

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This entry was posted on January 9, 2015 by in Invasive Species, NNNSI and tagged , , , , .

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