Introduction:
Japanese knotweed is a significant alien plant species found across mostly economically developed countries. Invasive species are well-fitted to the UK’s environment and, having limited to no control methods, multiply quickly and cause social, environmental, or economic issues in many places. They spread very easily due to human and natural disturbances, such as floods or something as simple as a person breaking part of the plant and dropping it into a river where it then gets washed up and starts growing at another riverbank. The spreading can also be sped up by low-standard control methods and the dumping of soil polluted with knotweed rhizome (Jones et al, 2018). Japanese knotweed causes massive disturbance to natural and managed areas and the effects of the failure to control it are huge, yearly costing hundreds of millions of pounds in the UK alone.
It is a perennial plant that outcompetes other plants, forming dense tall stands that can harbor vermin, collect litter, block drainage channels, and die back to an unpleasant mass of dead stems in winter. Invading shoots displace channel protection systems and erosion control materials. They can penetrate footpaths and paved surfaces with their strong roots (Llywodraeth Cymru Welsh Government, 2014).
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Why biological control is needed:
The aim is not to remove Japanese Knotweed completely from the UK but to lower the overall population to an appropriate point. Biological control will not be a solution for sites that are completely overrun by knotweed. The forceful behavior of knotweed means that it can revive from rhizome fragments as tiny as 0.7 g, demonstrating the sheer trickiness it can to get rid of it entirely from an overrun site (Kurose et al, 2006 cited in Payne, 2012).
The thick weed presents a big danger to flood management for two reasons. It can harm the river sides when its shoots start to die off in autumn time and they enter the waterway which in consequence, lowers its ability to sustain the amount of flood water. It can also be destroyed by spreading into concrete or other construction materials which is attributed to flood defences. This creates the hassle of having to make sure that the land is 100% free of weeds before the construction of flood defense schemes is begun (Shaw,2009). This comes with a large cost however the cost would be much greater if the weed is left to expand over time.
Possible control methods:
Most of the control methods aren’t 100% effective as there is still a chance the weed could come back however; they do reduce the chances considerably. The lack of natural enemies in the UK makes it almost impossible to eradicate the weed naturally, compared to Japan where the populations are kept in check. Currently, samples of these enemies (e.g. fungi and insects) are under detainment being examined for use, however, if they were introduced into the UK then they could pose a threat to native plants as well as Japanese Knotweed (Shaw, 2009).
Japanese knotweed is one of the many non-native species that cannot be composted as it is very persistent, will survive the composting course, and most likely infiltrate the area where the compost is situated. To stop the weed from spreading to surrounding areas, herbicides can be used to spray directly onto it however this is a long process and generally takes 3 years for the underground rhizomes to become inactive. Usually, herbicides are used along with mechanical control as it is much more effective and reduces the chances of the weed returning to the area once removed. This method is expensive as a lot of herbicides are required along with people needed to use the machinery. If a faster method is required, you can bury Japanese Knotweed at a depth of at least 5 meters if the plant remains are covered with a substance that does not allow the plant to grow (identified as a root barrier membrane layer) and other types of waste are not buried with it. The soil that is contaminated by the weed mustn't be dumped/sold somewhere else unless it has been treated in the location where they were produced (Environment Agency,2018).
Burning the weed is another option however this comes with environmental impacts such as releasing carbon dioxide into the atmosphere. The fire will not just burn Japanese Knotweed though, most of the biodiversity in the area will be affected so is a very destructive method. It is not completely effective either as some Japanese Knotweed crowns and rhizomes could survive the burning and then continue to develop again which will make the burning process never-ending. To prevent this the site should be inspected thoroughly after burning and any remaining plant material should be disposed of. Even the smallest piece of plant material not removed could lead to the weed taking over the site again, they may be hard to spot as they are underground, but the area could be dug up afterward as a secondary precaution.
Factors used to determine which control method to use:
The control method that is used for a specific site is dependent on several matters. It needs to be suitable for the site so that it can be as effective as possible. Other issues concerning the owner of the land such as budget will also affect which method can be used as herbicides and mechanical action cannot be used if they do not have the funds. The first factor to take into consideration is whether the Japanese Knotweed is near a watercourse or other vegetation that needs to be protected or is precious in one way or another. This is because some methods such as burning would be destructive to the surroundings as well as the weed and so wouldn’t be suitable for a significant surrounding area.
Another factor would be whether the weed is well-established in the area or if it has recently arrived (Environment Agency,2018). This would determine the extent of the invasion and so how much effort needs to be put into removing it. If construction is planned for the land containing Japanese Knotweed, then this will determine the timescale in which the method is needed to eradicate the weed. It also needs to make sure the weed is completely gone if some is still left after construction begins then it could cause serious expense to the construction process. Lastly, whether it is intended to establish grass, trees, or shrubs in the area once the weed is removed. This requires there to still be fertile soil once the process is finished and so would require a less destructive method such as mechanical control.
Conclusion:
To conclude, several solutions can be implemented at a local level in the UK which could demonstrate make a positive difference in successfully controlling the Japanese Knotweed population. It is already recognized as an issue and operations are taking place to keep the problem from getting any more critical, however, more could be done in terms of incentives to encourage people to help rid their local communities of this relentless weed.
References:
- CENTRE FOR AGRICULTURAL AND BIOSCIENCE INTERNATIONAL (CABI)., 2010. Japanese knotweed is one of the most high-profile and damaging invasive weeds in Europe and North America. CABI. Accessed on: 9/11/19. Available from: https://www.cabi.org/japaneseknotweedalliance/
- COTTET, M., PIOLA, F., LE LAY, Y., ROUIFED, S. & RIVIERE-HONEGGER, A. 2015, 'How environmental managers perceive and approach the issue of invasive species: the case of Japanese knotweed (Rhone River, France)', Biological Invasions, vol. 17, no. 12, pp. 3433-3453. Accessed on: 9/11/19. Available from: https://search-proquest-com.gcu.idm.oclc.org/docview/1732837947?accountid=15977&rfr_id=info%3Axri%2Fsid%3Aprimo
- ENVIRONMENT AGENCY, DEPARTMENT FOR ENVIRONMENT, FOOD & RURAL AFFAIRS, AND NATURAL ENGLAND., 2018. How to identify, prevent spread, and dispose of Japanese knotweed. Environment agency. Accessed on: 9/11/19. Available from: https://www.gov.uk/guidance/prevent-japanese-knotweed-from-spreading#contents
- JONES, D., BRUCE, G., FOWLER, M., LAW-COOPER, R., GRAHAM, I., ABEL, A., STREET-PERROTT, F. & EASTWOOD, D., 2018. Optimizing physiochemical control of invasive Japanese knotweed. Biological Invasions. 20(8), pp.2091-2105. Accessed on: 9/11/19. Available from: https://doi-org.gcu.idm.oclc.org/10.1007/s10530-018-1684-5
- JONES, D. & EASTWOOD, D., 2019. Sustainable Control of Japanese Knotweed. Outlooks on Pest Management. 30(5), pp.195-200. Accessed on: 9/11/19. Available from: https://www.gov.uk/government/publications/the-biological-control-of-japanese-knotweed
- PAYNE, T., HOXLEY, M., 2014. UK Japanese knotweed in the UK-built environment. Volume 30, pages 24-42. Accessed on: 9/11/19. Available from: www.emeraldinsight.com/0263-080X.htm
- SHAW, R.H., BRYNER, S. & TANNER, R. The life history and host range of the Japanese knotweed psyllid, Aphalara itadori Shinji: Potentially the first classical biological weed control agent for the European Union., 2009. Biological Control. Accessed on: 9/11/19. Available from: https://www-sciencedirect-com.gcu.idm.oclc.org/science/article/pii/S1049964409000371?via%3Dihub
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