How to eliminate or control Japanese Knotweed

In this post we discuss Japanese Knotweed and the issues that this plant can have on land and property decisions. 

Japanese Knotweed (Fallopia japonica) is a plant hailing from East Asia, where it does not cause any major problems. However, the situation is different in the UK.

Japanese Knotweed was introduced to the Europe in 1848 by Philip Franz von Siebold who brought a specimen from Japan to Europe (1). After this, the plant was introduced in many botanical gardens in the UK because of its characteristic of being fast growing and nicely decorative. Since then it largely spread in every corner of the country, thus becoming highly invasive.

This species is not of concern in Japan thanks to many natural controllers (such as invertebrates, pathogens and many plant competitors) that co-evolved with Japanese Knotweed within their native ecosystem. However, when easily adaptable species, such Japanese Knotweed, are brought out of native balanced ecosystems, they may become invasive in the new ecosystems, given the lack of controllers, competitors or diseases able to reduce their population.

Invasion of Japanese Knotweed in the urban environment can be very dangerous. In fact, this species can easily grow through fractures or weaknesses of buildings, construction materials and foundations, causing damage that can affect both the stability and the value of the property (2). Furthermore, this plant has a great capacity of vegetative propagation, which allows it to spread very rapidly reproducing vital plants from tiny clusters of root. This is why intrusive work, associated with new development or property extensions, can be the main reason of new Japanese Knotweed explosions caused by dormant clusters of roots brought back to the surface during the ground working phase (3).

The removal of this species can be very complicated. Plants of Japanese Knotweed have a strong disproportion in biomass between the above and the underground, in fact a 1m tall plant can have a 7m across and 2m deep root system (2,3).

The main strategies to control or eliminate Japanese Knotweed and the best situations where they can be used are explained below:

1) Physical removal: This entails the removal of the plant and of all the ground that might contain any part of the root system. This strategy implies the use of heavy machinery and intrusive works and it is a preferable solution when intrusive works are already required at the property i.e. if a site is to be redeveloped. This technique however,  is expensive and time demanding. Furthermore, the soil excavated needs to be treated and disposed of as special waste (4). Alternatively, the soil can be treated ex. situ with steam, in order to kill dormant plants hidden into the ground, and then replaced. However, this can be even more expensive than to dispose of it as special waste and if just a tiny cluster of the root remains alive it will cause the failure of the whole remediation (2,3).

2) Herbicides: This is the quickest solution when handling a site where intrusive works are not required. However, it is just a control strategy, because pesticides cannot fix the problem. The majority of them are glyphosate-based products, which often just kill the above ground portion of the plant, causing the dormancy of the rest of it – Japanese Knotweed can stay in dormancy for decades (3). Furthermore, herbicides are not targeted to a single species, they may also kill or damage surrounding plants, which means they cannot be used in sensitive areas. Besides, plants, especially strong plants like this, are quick in evolving tolerance to herbicides, which need to be always stronger or used in higher quantity to be effective, becoming dangerous for the environment (3).

3) Biological control: This is the most controversial, most sustainable but less used technique. It involves the purposeful introduction of a species able to control Japanese Knotweed in the environment. That can be tricky considering that the species used as ‘controller’ might not be able to survive in the new ecosystem (3). Moreover, this option can be dangerous because the introduction of an exotic species to limit the adverse effect of another exotic species might cause a reiteration of the same problem and a new invasion. Recently, in few test areas of the country a natural controller of Japanese Knotweed has been introduced in affected areas. This controller species is an insect, called Aphalara itadori, which is similar to an aphid and able to inhibit the growth of Japanese Knotweed. So far, there are no evidence of this insect causing harm to others plants and animals, however, further research will be needed before a large scale release of a new alien species in the nature (5).

4) Last but not least, not many people know that Japanese Knotweed can be eaten, it tastes like rhubarb! (3)

In conclusion, the best treatment to eliminate or control Japanese Knotweed invasion should be evaluated case by case. If you have questions about Japanese Knotweed at a property there is government guidance available here . For residential properties we still recommend that a RICS HomeBuyers Report is undertaken by a RICS accredited surveyor, or specifically a Japanese Knotweed Survey would be the most appropriate means of identifying Japanese Knotweed (although please note that as it is a seasonal plant, it is harder to confirm its presence in winter). This will also outline potential remediation solutions for the property.  We believe that Japanese knotweed is a very relevant subject for property professionals, and Groundsure will still provide CPDs on Japanese Knotweed should there be a demand for it.

Furthermore, land and property owners need to keep in mind that causing the spread of it is illegal under the Wildlife and Countryside Act 1981 (for further guidance regarding how to identify, control and dispose of Japanese Knotweed in compliance with governmental guidance please click here).

Resources

1. Japanese Knotweed removal by Phlorum – http://www.phlorum.com/services/japanese-knotweed/
2. Invasive Species Ireland – http://invasivespeciesireland.com/toolkit/invasive-plant-management/terrestrial-plants/japanese-knotweed/ (link no longer available)
3. How to Control Japanese Knotweed – https://www.youtube.com/watch?v=4oecHwMAl6M
4. Prevent Japanese knotweed from spreading – https://www.gov.uk/guidance/prevent-japanese-knotweed-from-spreading
5. Japanese knotweed: Tiny insect could finally tame Britain’s most invasive plant – http://www.independent.co.uk/news/uk/home-news/japanese-knotweed-tiny-insect-could-finally-tame-britains-most-invasive-plant-9804698.html