This is part one of a two-part article. Read the second part here: 'Mechanical methods to contain and eradicate Japanese knotweed'. Japanese knotweed is an invasive, non-native plant with a remarkable capacity to spread rapidly and a tenacious resistance to control. Introduced into Ireland in the late 1800s, it is causing major problems for engineers across a wide range of disciplines, particularly for those involved in the construction sector and State networks (road, rail and waterways) An invasive, non-native species (INNS) is any introduced animal or plant that has the ability to spread causing damage to the environment, the economy, our health and the way we live. In Europe, approximately ten new species become established each year. In 2010, it was estimated that the total annual cost of Japanese knotweed to the British economy runs at £166 million; the total loss to the world economy as a result of INNS has been estimated at 5% of annual production. Globally, INNS have contributed to 40% of the animal extinctions that have occurred in the last 400 years and there is evidence that invasive species are a more significant cause of biodiversity loss than climate change . In 2011, EU legislation was introduced in an attempt to curb the spread of the most damaging INNS and the legislation was enacted in Ireland in 2015. Of the plant invasives, Japanese knotweed and its relatives are perhaps the most serious . Regulation 49, EC Birds and Habitats Directive, SI 477, 2011 states that it is not against the law to own land with Japanese knotweed present (or other regulated INNS) but it is an offence to knowingly allow their dispersal or escape . Invasive species’ capacity for dispersal is the main reason for their success and preventing dispersal is a major challenge in eradication programmes. Japanese knotweed is an opportunist: it can adapt to a wide range of environmental conditions and it has evolved a brilliant mechanism of dispersal. The rhizomes and shoots are brittle and break easily, and even tiny fragments (700mg) have the capacity to regenerate into new plants. 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. Indeed, a 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. The consequences of not understanding Japanese knotweed’s growth habit can be illustrated by a site in a residential area of 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.

Impact on residential and commercial property market

[caption id="attachment_28959" align="alignright" width="225"]JKW colonising river banks Japanese knotweed colonising river banks[/caption] Increasingly, buyers are finding they have invested in a property in which Japanese knotweed was concealed or not identified prior to purchase. Because Japanese knotweed dies down in winter, only an experienced eye may spot it. If the infestation is growing within seven metres of a property, it is likely to cause damage which may be challenging to rectify. Knotweed rhizomes can penetrate through gaps in stone walls, fissures in concrete and weaknesses in bituminous materials and over time it may de-stabilise walls and structures. It may exploit cavity walls, drains and water pipes. Once rhizomes have penetrated beneath a structure it may be extremely costly to eradicate. Japanese knotweed may destroy a garden or landscaping plan. If infested soil is disturbed, knotweed will re-colonise exponentially. If knotweed is cut, strimmed or mown, the resulting fragments have the potential to grow again. One farmer in the west of Ireland reported that he is having panic attacks in the mornings when he awakes to see the advance of knotweed across his land – the result of trying to control the infestation based on wrong advice. 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. Developers may face sharply increased costs if Japanese knotweed or other INNS 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. An invasive species survey prior to purchase is an investment that may be worth making, particularly if there is evidence of knotweed infestations in the locality. A second golden rule when faced with INNS is that prevention is better than cure. A young infestation can be effectively eradicated with minimum cost. If left for five years, the bill may be considerable and the task horribly challenging.

Impact of Japanese knotweed on networks

Networks, such as road and rail routes, provide an easy means of linear spread for plants that can reproduce from small fragments carried on tyres, feet, water and wind. The recession led to considerable acreage neglected and abandoned along roads, offering bare ground for opportunistic invasive species to colonise. Rivers, streams and other waterways are a major source of spread. Plants growing on river banks become dislodged during heavy rain and travel downstream where they re-colonise. The photo shows a tributary of one of Connemara’s famous fishing rivers, its banks entirely colonised with Japanese knotweed which stretches continuously for over one mile. In winter, the plants die back, leaving bare and exposed soil on banks. This becomes increasingly vulnerable to erosion and collapse, with the rhizomes exposed and readily transported downstream. Road/river bridges are high-risk areas for infestations, with significant implications for both the road and waterway sectors. There are several recent instances of county engineers appointing invasive species control teams to treat knotweed infestations at bridges prior to construction work. Unfortunately, construction work proceeded before the infestations had been killed. The consequences are intense re-colonisation and associated dispersal downstream.

Guidelines for action

[caption id="attachment_28958" align="alignright" width="300"]Japanese-knotweed Japanese knotweed re-colonising infested soil that has been disturbed[/caption] An appraisal of the methods used to contain and eradicate Japanese knotweed and its relatives will be the subject of the next article in this series. Regardless of the method(s) chosen, the following steps are advised to ensure treatment and eradication will be effective, cost effective, compliant with good practice and cause no lasting harm to the environment.
  • Do not disturb the infested area, particularly soil and other ground materials. Cordon off the infestation with a seven-metre buffer zone from outlying plants. Erect signage warning operatives and the public to stay away.
  • Conduct a site assessment to establish: 1) the severity of the infestation and 2) site variables which may influence options for treatment.
  • Seek professional advice 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.
Frances Giaquinto is a PhD botanist, applied ecologist and invasive non-native plant species specialist who lives in County Clare. To contact her, email: or telephone: 061 924 287 (mobile: 087 966 2935). See