Japanese knotweed’s wider environment cost

DateNov 15, 2018
AuthorChris Butler

In this blog, Chris Butler discusses Japanese knotweed and it's range of effects on ecosystems. If you have any questions about this blog or would like to find out more, please contact info@groundsure.com.

Another year, another Japanese knotweed ‘season’ has come and gone, but why exactly does the name Japanese knotweed strikes fear into many? The World Conservation Union has listed the species as one of the world’s worst invasive species (Lowe et al. 2000). Much has been written on the plant and its impacts on the construction industry, incurring increased costs and causing completion deadlines to be missed.

Japanese knotweed is relatively easily distinguishable with its shield-shaped leaves, purple-spotted bamboo-like stems and small, creamy white flowers, which start appearing during the late summer (RHS 2017). The plant is not native to the British Isles and as such, it is not exposed to any of the controlling organisms or conditions that maintain it within its natural extent in Southeast Asia.

With the absence of controlling organisms (such as bacteria, fungi and invertebrates) and climate, and with the ability to occupy a large amount of space in invaded habitats, the plant has been able to cause significant damage to native ecosystems. This includes competitive displacement of native vegetation and associated fauna, increasing risk of soil erosion and exacerbating flooding through the impediment of water flow by its dense stands.

Below is a brief rundown of Japanese knotweed’s range of effects on ecosystems:

Biodiversity: Japanese knotweed outcompetes native vegetation, limiting species diversity, causing local displacement and a reduction in biodiversity in both flora and fauna. (Gerber et al. 2007).

Water quality and flood risk: With dense canopies of leaf in the summer, Japanese knotweed can cause shading of small water bodies and intercept rainfall, channelling it elsewhere. In the winter the canes can increase the erosion of river banks as the growth dies back, causing siltation, and can reduce the capacity of river channels by forming blockages and dams causing floods, as well as blocking vital flood infrastructure such as sluices, drains and ditches.

Recreation and amenity: Most obviously Japanese knotweed can be a blight and a bane to gardeners and allotment holders alike, but the plant can also negatively affect anglers, as it can obstruct access, shade rivers and lakes, and cause damage to banks, negatively affecting water and fishing quality. Other water users such as sailors, rowers and canoeists may find the dense stands a barrier to entering the waterways, and during the winter months the dead canes can become visually unappealing and can block views (PCA 2014).

Infrastructure: The plant's rhizomes are particularly good at exploiting cracks and gaps in concrete, and even walls and drains, causing significant damage. This can add huge costs to development and regeneration schemes.

The UK's railway infrastructure is a notorious source of Japanese knotweed, and possessing some 20,000 miles of track (Network Rail 2017) has a huge role to play in tackling and managing Japanese knotweed. As previously mentioned, during the winter, the Japanese knotweed will shed its leaves and die back, everyone remembers how wet leaves caused chaos on London's Piccadilly Line in November 2016 (BBC 2016). With its dense stands and ability to out-compete other plants, Japanese knotweed can potentially greatly exacerbate this problem. Shed leaves can potentially create operational issues for track circuitry, reducing trains’ grip, impacting the ability of a train to start from a station, accelerate and climb hills, or stop at stations and signals (Network Rail 2016). This has financial implications due to delays and disruption to services.

Network Rail also has a responsibility to its lineside neighbours and as we will see below, comply with legislation regarding the management of Japanese knotweed.

Housing devaluation: It has long been held that the presence of Japanese knotweed can have an impact on its actual and perceived value, and as a result, a number of mortgage providers refuse applications for properties with Japanese knotweed (RICS 2012).  In July 2018 the High Court set a landmark legal precedent and case law for the control of Japanese knotweed. In the case (Network Rail Infrastructure Ltd v S Williams & R Waistell) two adjoining bungalow owners brought claims against Network Rail for allowing Japanese knotweed growth to encroach on their properties. Further consolidating the opinion that the presence of Japanese knotweed can and does interfere with a property and reduce its market value, this can typically be between 5-20% of a properties value (Phlorum 2016). No small amount considering that the average UK house costs just over £231,000 according to the Land Registry price index as of July 2018 (HM Land Registry 2018), thereby potentially wiping anything from £11,550 to £46,200 from the value of the average property. Unfortunately, the reading doesn’t get much better, recent YouGov research suggests up to 5% of UK properties might be affected by Japanese knotweed, whether directly or by an adjoining property (YouGov Plc 2017). It is therefore not hard to see why the housing market is quite so wary of the plant.

Management costs: Local councils and authorities manage vast tracts of land across the UK, as a result, they incur significant costs to tackle Japanese knotweed infestations on their land, employing grounds staff, diverting resources and hiring private contractors alike in an attempt to prevent it from spreading onto neighbouring properties. In the domestic housing market, herbicide treatment plans can cost thousands of pounds. Owing to this Japanese knotweed is estimated to cost the Great British economy £165m every year (Williams et al. 2010), with the cost of a UK-wide eradication attempt cited to cost more than £1.56 billion (Williams et al. 2010). If the problem can indeed ever be fully resolved (Jones et al. 2018).


  1. British Broadcasting Corporation (BBC) (2016). Piccadilly Line delays: Wet leaves cause train shortage. www.bbc.co.uk/news/uk-england-london-38129886
  2. Gerber E, Krebs C, Murrell C, Moretti M, Rocklin R, Schaffner U (2008). Exotic invasive knotweeds (Fallopia spp.) negatively affect native plant and invertebrate assemblages in European riparian habitats. Biological Conservation 141: 646-654
  3. HM Land Registry (2018) UK House Price Index. http://landregistry.data.gov.uk/app/ukhpi
  4. Jones, D., Bruce, G., Fowler, M.S. et al. (2018) Optimising physiochemical control of invasive Japanese Lowe S, Browne M, Boudjelas S, De Poorter M (2000). 100 of the World's Worst Invasive Alien Species: A Selection from the Global Invasive Species Database. The Global Invasive Species Database knotweed. Biol Invasions 20: 2091. https://doi.org/10.1007/s10530-018-1684-5
  5. Network Rail (2016). Leaves on the line cause problems for the railway – here’s how, and what we’re doing to reduce the impact. www.networkrail.co.uk/running-the-railway/looking-after-the-railway/delays-explained/leaves/
  6. Network Rail (2017). Who We Are. www.networkrail.co.uk/who-we-are/
  7. Phlorum (2016). Japanese Knotweed Removal Cost. www.phlorum.com/services/japanese-knotweed/japanese-knotweed-removal-cost
  8. Property Care Association (PCA) (2014). Code of Practice for the Management of Japanese knotweed
  9. Royal Institution of Chartered Surveyors (RICS) (2012). Japanese Knotweed RICS Professional Information, UK and residential property 1st edition, information paper
  10. Williams F, Eschen R, Harris A, Djeddour D, Pratt C, Shaw R S, Varia S, Lamontagne-Godwin J, Thomas S E, Murphy S T, (2010). The Economic Cost of Invasive Non-Native Species on Great Britain. Centre for Agriculture and Bioscience International (CABI)
  11. YouGov plc (2017) Japanese knotweed Research Survey carried out by YouGov. https://environetuk.com/blog/Japanese_knotweed_Research_Survey_carried_out_by_YouGov
  12. The Royal Horticultural Society (2017) Japanese knotweed. www.rhs.org.uk/advice/profile?pid=218