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Japanese Knotweed

Know your Japanese Knotweed or Pay the Price

Japanese Knotweed is classified as one of the top 100 worst invasive species worldwide, and it can cause serious damage to houses and buildings. Knotweed’s capacity to grow from fragments has important implications for control, with movement of knotweed-infested soil being one of the main reasons for its rapid spread across Ireland.

The number-one rule when faced with an infestation is: do not disturb it until a plan of action is devised and implemented. Ignoring this rule may lead to infestations that are impossible to eradicate.

Infestations on neighbours’ property may also be problematic. Landowners are not legally obliged to remove knotweed from their land but, if the landowner causes it to spread across the boundary, then it may constitute ‘knowing dispersal’.  Awareness raising with the neighbour and reaching an amicable agreement is the recommended way forward following up with legal advice on potential civil proceedings should the problem continue.

Action Plan

Don’t disturb infested area, particularly soil & other ground materials.

Do engage expert Environmental Consultants to:

  • Conduct a site assessment to establish: 1) the severity of the infestation and 2) site variables which may influence options for treatment.
  • Advise on the best approach to eradication given site variables. A phased approach using a combination of methods can be most effective. Be fully aware of the consequences of different methods. Japanese knotweed is rarely eradicated in one attempt and this must be factored into works schedules and budgets.
  • Prepare an invasive species management plan which details the agreed eradication programme, the methods to be used, the rationale for the choice of method, a schedule with milestones, a plan for contingency, and an assessment of the risk of re-colonisation of the site from other infestations in the locality. This is an important document to provide evidence of good practice in the event of future litigation.
Consequence – Legal, Financial, Time

  • Builders – finding Japanese Knotweed on sites can mean significant delays and substantial costs, jeopardising the delivery of projects on time and on budget.

An invasive species survey prior to purchase is an investment worth making, particularly if there is evidence of knotweed infestations in the locality.

  • Developers –  may face sharply increased costs if Japanese knotweed or other INNS (invasive, non-native species) are present on a site, combined with a potential delay to scheduled works and a loss in site value if the infestation is not contained.

If the infestation is not effectively eradicated prior to development, the costs of remediation may run into hundreds of thousands of euros.

  • Land Owners – whose land is invaded by knotweed can be liable for damages that relate to the decrease in value of a neighbouring property as well as the cost of treating the plant.


  • Residential Site, Dublin –  a small knotweed infestation was identified in the corner of a garden. The infestation was treated with herbicide, but it was assumed in error that the plant would be killed immediately. Ground levelling proceeded, which included the movement of soil from the knotweed-infested area across the garden. The result was massive re-growth with thousands of new plants densely infesting the entire area. Remediation was challenging and very expensive.
  • Social Housing Site, Cork – eradication works were required to treat Japanese knotweed where 56 housing units were being built. Fine Gael Senator Tom Lombard called for a national task force to deal with the situation.
  • Network Rail, UK – had costs awarded against them in a case highlighting the civil risks associated with Japanese knotweed. It involved two land owners whose properties backed onto land owned by the rail network. The case confirmed that a landowner whose land is invaded by knotweed can be liable for damages that relate to the decrease in value of a neighbouring property as well as the cost of treating the plant. Landowners must consider the risks associated with stands of knotweed on their land and whether they should try to eradicate them before they face any claims.  Owners of adjoining sites should now consider whether they ought to engage with the landowners on whose sites knotweed can be found and request action to remove the plant.  They may also consider seeking damages for any resulting decrease in the value of their own land.

In the Press

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