Japanese knotweed: risks to infrastructure and engineering projects
14 March 2017
Japanese knotweed (Fallopia japonica) was first recorded growing wild in the south of Ireland in 1902 (1) and it has since spread across the country, particularly along watercourses, transport corridors and on waste ground. Today, Japanese knotweed and related knotweed species are categorised into a group of unwanted plants known as invasive alien plant species (IAPS).
Such plants are generally defined as those that are introduced outside their natural range whose presence and/or spread threatens biological diversity, the environment, ecosystem services, the economy and human health. Invasive alien species introductions are now recognised as one of the main causes of natural biodiversity loss (second only to direct habitat destruction), and the estimated annual cost of invasive species to the Irish economy is approximately €260 million (2).
Why is Japanese knotweed such a problem?
Japanese knotweed is a fast-growing, robust, perennial plant that rapidly produces dense and extensive vegetation stands. In Ireland, Japanese knotweed has no natural predators or pathogens; therefore, it has the ability to outcompete native vegetation and threaten biodiversity.
In spring, the plant produces new shoots (tinged with red/purple) from the rhizome (underground stem) on a dense underground crown. As the shoots grow, they extend to produce upright, hollow, bamboo-like green canes which can grow up to 3m high and achieve a diameter of 40mm in a single season. The plant flowers between August and October, producing clusters of small creamy-white flowers. These dense stands of vegetation that the plant forms can block sight-lines and signage, having major safety implications on our road network.
The rhizomes are thick (5-100mm diameter) with a knotty appearance and a distinctive bright orange colour beneath the bark. The plant requires elevated light and water levels, and a burst of growth in spring prevents other plants from out-shading it, which can result in the loss and displacement of native habitats and species. The root system of the plant is extensive (15–20m in length) and acts as a storage organ which allows for that rapid burst of growth in spring.
The extensive root system can cause a range of problems due to its prolific growth habit including damage to paving and structures, the erosion of riverbanks and damage to flood defence structures. If there is any weakness in a structure, the rhizomes will use this weakness as a conduit. As the rhizomes grow and expand over time, they will cause further damage to the structure by exacerbating the existing weakness.
Japanese knotweed is a plant that is often subject to some taxonomic confusion due to a combination of numerous synonyms and its capacity for hybridisation between closely related species. Giant knotweed (Fallopia sachalinensis) is also present in Ireland, although is far less extensive in its distribution than Japanese knotweed. A hybrid between Japanese knotweed and giant knotweed (Fallopia x bohemica) is currently widespread in Ireland, whilst Himalayan knotweed (Persicaria wallichii) is also increasingly recorded in parts of the country.
All of these species are recognised as aggressive colonisers and potentially highly invasive. Russian vine (Fallopia baldschuanica) is another close relative of Japanese knotweed. Although Japanese knotweed is known to hybridise with this vigorous climber, the seedlings rarely survive in the wild. Russian vine is not currently widespread across Ireland, but it is recognised as a ‘medium-impact invasive species’ by the National Biodiversity Data Centre.
Only female Japanese knotweed plants have been recorded in Ireland and while seeds are sometimes produced, these are usually hybrids with the related species mentioned previously and rarely survive. The majority of the rhizome system is confined to the top metre of soil, reflecting the fact that the rhizome is an underground stem rather than a root and will spread laterally in preference to achieving depth. Japanese knotweed rhizomes have an extremely high regenerative potential and greenhouse trials have shown that as little as 0.7 gram of rhizome material (10mm in length) can produce a new plant within 10 days.
Rhizome material may remain dormant for long periods, possibly as long as 20 years, and may regrow when disturbed. Cut fresh stems will produce shoots and roots from nodes when buried in soil or immersed in water. Dispersal typically occurs through rhizome fragments being transported in soil by humans or to a lesser extent, through passive mechanical means such as in floodwaters and large vehicles along roads.
Implications of non-native IAPS on development
A recent landmark ruling in the United Kingdom resulted in Network Rail being ordered to pay £15,000 compensation for damage caused to homes by Japanese knotweed which had spread from railway embankments to the foundations of nearby homes. The outcome of this particular case is likely to have significant implications for owners of public land across the UK and it is likely that similar cases may start to emerge here in Ireland with the escalation of the introduction and spread of IAPS.
It is vital that practical measures to control the presence and spread of IAPS are incorporated into the planning, construction practices and maintenance regimes of any development schemes due to the cost and legal implications associated with dealing with these species. The control of non-native invasive species in Ireland is governed by both European and Irish legislation, namely:
- Regulation 49(2) of the European Communities (Birds and Natural Habitats) Regulations, 2011 (3);
- Section 40(1) of the Wildlife Act, 1976 (4), as amended;
- EU Regulation No. 1143/2014 (5) of the European Parliament and of the Council of 22 October 2014 on the prevention and management of the introduction and spread of invasive alien species; and
- Use of Plant Protection Products Legislation.
Machinery and construction activities can introduce and disperse IAPS to new areas; therefore, it is vital that adequate biosecurity measures are put in place at all construction sites. Biosecurity essentially refers to the tools and techniques that will help prevent the introduction, establishment and spread of IAPS. The most common ways that IAPS are introduced to a site are contaminated topsoil, vehicle and tool contamination and fly-tipping.
The early identification and management of IAPS can significantly reduce the resources needed to minimise the spread of these species on any development site. The approach to the control of any IAPS will depend on several factors including the scale of the infestation, the topography and terrain of the site, the proximity of watercourses or other sensitive receptors (such as protected flora), the funds available, etc. The most cost-effective means of treating a site infested with IAPS is by way of advance treatment.
Herbicide treatment is generally the most practical and cost-effective means of control when mitigating the risks of IAPS at a development site in the long term where no works are currently planned, as follow up treatments will be necessary (repeated treatments needed over 3–4 years). Physical treatment options are generally only suitable where construction work is scheduled on a site infested with IAPS. Some examples of physical treatment methods may include:
- In-situ burial;
- Temporary bunding and herbicide treatment;
- Soil removal and landfilling; and
- Soil sieving and landfilling/incineration.
When engaging in any control or eradication of IAPS on a development site, it is important to consider the implications of the current waste legislation. All contractors need to be aware of the biosecurity issues, the appropriate legislation pertaining to IAPS and must document all activities when working on sites infested with IAPS.
Implications for TII development and control strategy
IAPS are a major concern for Transport Infrastructure Ireland (TII) as they have many implications for TII infrastructure. The risks they pose to TII include:
- Implications for all major and minor works;
- Potential structural damage to light rail and road foundations, bridges etc;
- Blocking of sight lines and signage, compromising safety; and
- Implications for contracts in terms of costs, delays etc.
TII is currently engaged in a long-term treatment approach of tackling IAPS with a view to mitigating the risks on construction projects in the future. The Management of Invasive Alien Plant Species Project, led by TII, is a €5.5 million project aimed at managing invasive knotweed and other non-native invasive plant species on the national road network and its interactions with regional roads. The key objective of this project is to develop, implement and monitor a comprehensive national approach to the control and treatment of IAPS on the road network.
Initial works commenced in 2016 with 11 priority counties being targeted for a rolling three- to four-year treatment programme. This year will see the rolling out of a second round of treatment contracts to additional counties affected by IAPS on their national roads.
Dr Sarah-Jane Phelan is an ecologist with the environmental policy and compliance section in Transport Infrastructure Ireland. She is a botany graduate from Trinity College Dublin and obtained her PhD in geomorphology from the University of Exeter in the UK. Over the past 10 years, she has worked in academic research and the public sector in Ireland, the UK and the Far East. Phelan is currently a member of the Programme Executive Board for the Conference of European Directors of Roads 2016 Transnational Research Call on Invasive Species and Biodiversity.
Dr Vincent O’Malley is head of environmental policy and compliance with Transport Infrastructure Ireland. He obtained his PhD in atmospheric chemistry from Memorial University in Canada. Over the past 30 years he has worked as an environmental specialist in private consultancy, academic research and the public sector in the UK, US and Middle East. O’Malley recently chaired the Conference of European Directors of Roads Noise Research Group and is the current chair of the 2015-2018 transnational research group in climate change. He is also a member of the EU Noise Technical Group and a member of the Infra Eco European Wildlife Group.
(1) Reynolds, S., 2002. A catalogue of alien plants in Ireland. National Botanic Gardens. Glasnevin, Dublin
(2) Kelly, J., Tosh, D., Dale, K. and Jackson, A. (2013). The economic cost of invasive and non-native species in Ireland and Northern Ireland. A report prepared for the Northern Ireland Environment Agency and National Parks and Wildlife Service as part of Invasive Species Ireland.
(3) Regulation 49(2) of the European Communities (Birds and Natural Habitats) Regulations, 2011 (S.I. No. 477 of 2011).
(4) Section 40(1) of the Wildlife Act, 1976, as substituted by Section 46(a) of the Wildlife (Amendment) Act, 2000.
(5) Regulation (EU) No. 1143/2014 of the European Parliament and of the Council of 22 October 2014 on the prevention and management of the introduction and spread of invasive alien species  OJ L 317/35