Combating invasive alien species in Dublin’s waterways

Our focus is primarily in the aquatic and riparian habitat associated with the main rivers, canals and standing watercourses in the urban Dublin area. Most of our watercourses have problems with invasive species, the primary culprits being riparian species. The extent of the problem was first assessed in 2008 by Dublin City Council Parks and Landscape Services by way of field surveys and mapping of watercourses. The first Biodiversity Action Plan for the city (DCC, 2008) identified threats of invasive species to biodiversity. In 2009, Dublin City Council, with support from the national Heritage Council, engaged consultants to further assess invasive plant species along the city’s main watercourses and this was mapped using GIS and a report produced (Ecoserve, 2009).  Principles among these are Giant hogweed (Heracleum mantegazzianum), Japanese knotweed (Fallopia japonica), Bohemian knotweed (Fallopia x bohemica) and Himalayan balsam (Impatiens glandulifera). Other  highly invasive species that currently present problems in our watercourses include New Zealand pigmyweed (Crassula helmsii) and Curly-leaved waterweed (Lagarosiphon major), the former in one of our major navigation waterways (the Grand Canal) and the latter in a dedicated fishery developed to cater for disadvantaged youths in a Dublin city park (Darndale Park).

The watercourses we will describe come under the control of Dublin City Council and include the River Tolka Valley and the Darndale Fishery.

The River Tolka is the second largest river in Dublin and occupies a catchment area of 14,150 hectares. It flows for much of its length through Dublin city to discharge directly to an EU Special Protection Area under the Birds Directive, the Tolka Estuary SPA. The Valley is home to a number of resident species designated under the EU Habitats and Birds Directives, including the Otter, Kingfisher, four species of Bat, Salmon (Salmo salar), Trout (Salmo trutta), as a resting area for migratory protected birds such as Brent geese, as well as seven species of Butterfly and a number of rare, nationally-protected plants and birds of national conservation importance. Tolka Valley was first settled during the Norman period and there are archaeological as well as botanical influences to this day from that period.  The Valley was somewhat industrialised, with mill races constructed and localised brick manufacturing in the 16th-19th Centuries (IAC, 2011).  It remained agricultural until the late 20th Century when, during the national construction boom, it was planned to develop the Valley, on the periurban fringe of the city, with a commuter rail station and high-density housing for a population of about 15,000 (Pelletstown Area Action Plan, 1999).  This was partially realised prior to the collapse of the national banking and construction sectors.  The river valley was planned as an amenity resource for this new community. However, habitat management plans only arose from 2005 onwards, after the main residential and commercial centre had been built.

The Darndale Fishery is an artificial lake that was built in Darndale Park in 1999 to provide an amenity feature as part of an urban regeneration project of a disadvantaged area of north Dublin. In 2008 Inland Fisheries Ireland introduced Common carp (Carpio carpio) into the lake to provide angling for the local community and to provide an angling learning facility for local youth as a means of biodiversity awareness under the IFI’s Dublin Angling Initiative .

The Giant hogweed problem in the River Tolka Valley almost certainly originated during the 19th Century in an estate garden in the upper reaches of this river and spread downstream via seed dispersal. This is believed to be the origin point for the species nationally. From the late 1970s large areas of the urban river were totally overgrown with this highly invasive and hazardous species. It is not clear how the Himalayan balsam gained access to the River Tolka Valley but may have been intentionally introduced because of its attractive flowers. The extensive public access to zones along the river has permitted transfer across sites, as well as dispersal by water of certain species.  Management regimes which were designed to clear vegetation from river banks due to antisocial behaviour allowed for further colonisation by non-native species.  Traditional grazing practices died out as the river valley became increasingly urbanised, causing changes to the management of former pastures (Ecoserve, 2009).  Since the early 2000s this plant has spread aggressively along the riparian zone in this Valley and occupied large sections of river corridor.

DCC Parks and Landscape Services commenced a chemical treatment regime in 2007-2008 for all main river corridors on lands they owned and managed, including the Tolka River.  This was refined following the 2009 report and chemical treatment is now carried out thrice annually, with mapping of progress annually.  Since 2011, DCC Parks and Landscape Services have been using the recording system devised by the National Biodiversity Data Centre (NBDC) to standardise monitoring.  The data collected so far  is  mapped in GIS by the NBDC in 10km grid squares and is publicly available on their website (see Ref.) .  It is noteworthy that no records are shown in the upper region of the catchment, and that occurrence of invasives such as Giant hogweed seem to correlate with urbanised sections of the river channel.

DCC is also including monitoring of invasive species in its reporting procedures under the Water Framework Directive and, as lead authority for the Eastern River Basin District, has held workshops for the 12 local authorities of the District on biodiversity.  These included presentations on invasive species.  IFI also hosted a workshop in Dublin in April 2013 for the EU Life+ Project RESTORE which included a site visit to Tolka Valley and presentations by Dr. Caffrey and Maryann Harris on invasive species and the projects described herein.

Assessment of the habitat of the Tolka River Valley is being undertaken as part of the project.  The first survey by DCC (with Heritage Council support) was in 1998, to assess habitats prior to major urbanisation planned for the locality.  In 2010, DCC produced habitat mapping in GIS of the entire city, as a progression of earlier mapping undertaken in 2006 for the production of the Biodiversity Action Plan.  This is used to correlate habitat changes.  Habitat surveys suggested that biodiversity potential is high but that habitat degradation had occurred and needed to be reversed (Keeley, 2010; Tubridy, 2010).


In the Darndale Fishery Curly-leaved waterweed was intentionally introduced by an angler in about 2009 in the mistaken opinion that it would grow and provide dissolved oxygen for the resident fish. He was unaware of the highly invasive capacity of this weed and did not realise that it would grow to occupy the full depth of the water column in the fishery and the full area of the watercourse – as it did within two years. This illustrates the need for biodiversity awareness-raising activities by government bodies, particularly for urban areas where there can be rapid changes occurring to ecosystems.

Problems and challenges:

In the River Tolka Valley the Giant hogweed and Himalayan balsam have caused serious problems for native biodiversity. Where these plants grow in abundance they block light from native herbaceous plants and grasses, ultimately removing them from the banksides. Not only does this reduce biodiversity among the resident flora, it exposes the bare banksides to river scour during the winter months. This has caused river bank subsidence and introduces silt and clay into the river, which can result in the clogging of trout and salmon spawning gravels. The addition of extraneous material into the river can also result in flooding problems during times of high flow and the River Tolka has flooded with very serious consequences for Dublin city in the past. In addition, Giant hogweed is a hazardous plant whose sap can cause serious burns when it comes into contact with skin, particularly in the presence of bright sunlight. Many reported incidents of children and adults being burned by this plant in the River Tolka Valley have been reported over the years.

In the Darndale Fishery Curly-leaved waterweed has totally overgrown the watercourse and rendered it unfishable by anglers. This has removed a valuable local amenity from the local community in an area where general amenities are scarce. It has also removed a prime location that was in constant use by the Dublin Angling Initiative for training young anglers. In addition, the excessive growth of weed resulted in dramatic fluctuations in dissolved oxygen levels in the water, particularly during the summer months, with the resultant death of resident fishes.

Extent of the impact:

DCC reports and monitoring since 2008 by both Parks and Landscape Services and the River Basin District indicate that all of the main river catchments in Dublin are impacted by invasive alien plant species, to the extent that in some sections of channel they have become the dominant species. The NBDC website shows for each species the extent of impact is considerable. The type of ecosystems affected include semi-natural grasslands, riparian woodlands- where regeneration is affected – amenity grasslands, and coastal habitats, including Annex 1 SAC habitats in north Dublin Bay.  The presence of invasive species in amenity grasslands can have an impact on recreation and also on protected species.  For example, the availability of amenity grasslands in parks to protected Brent geese has been considered in parks management plans where Japanese knotweed is present in South Dubln Bay.

The presence of giant hogweed is also a concern for public safety and human health in terms of its potential to cause skin burns and eye injuries.  This topic was presented by DCC at the national environmental science conference ENVIRON in January 2013.  While no evaluation of the economic worth of angling on the River Tolka has been undertaken, It is deemed to be substantial based on the large numbers of club and non-club affiliated anglers that utilise the resource each season.

The riparian ecosystems of Dublin City have been given greater focus in the recent City Development Plan (DCC, 2011), whereby a planned network of green infrastructure is proposed. This will increase connectivity of green corridors and increase mobility of pedestrians and cyclists.  Such policies are in line with the EU’s recent statement on Green Infrastructure in May 2013 (EC, 2013).  However, with such increased connectivity planned, the potential for invasive species to mobilise too must be considered and addressed.  Loss of indigenous species has not been assessed in Dublin City.  However, percent of channel for presence of 4 key species has been analysed (Ecoserve, 2009).

DCC Parks and Landscape Services is undertaking Species Action Plans for a number of key sites of highest biodiversity and individual parks.  It is planned to then prepare a communications strategy which will include training for the public in conjunction with NGO’s to assist with recording/eradication of certain key species.  Challenges include public safety issues for volunteers due to the riparian terrain, urban landownership complexities and the need to exercise care when dealing with giant hogweed.  There are 1400 hectares of public parks and open spaces, so a prioritisation of sites is being prepared.  This is being informed by national biodiversity research under the SIMBIOSYS Project coordinated by Trinity College Dublin Biodiversity Research Centre.  This includes a strategy for core areas of invasives as well as for ‘outlier’s which could be future agents for dispersal.

Approach and activities:

Inland Fisheries Ireland (IFI) has assumed responsibility for aquatic invasive species in watercourses under its control, nationwide, and has developed an extensive ’war chest’ of practical information materials for stakeholders, community groups and the public in general. IFI also has staff that are trained in both identifying and physically tackling invasive species that encroach into watercourses. IFI staff regularly conduct training sessions on invasive species identification and provide information on how to make accurate and timely reports if potentially harmful invasive species are found. These informal training sessions also alert participants to species that have not yet been recorded in Ireland or in Dublin city but which could be introduced. These activities lay the groundwork for good prevention, early detection and rapid reaction.

Efforts to control Himalayan balsam have focused on the use of herbicides and physical removal by pulling (a method that has become known as ‘balsam bashing’). To date, chemical control has been operated by Dublin City Council (DCC) staff and is proving to be quite effective. However, work conducted elsewhere in Ireland has shown that involvement of the local communities and committed stakeholders in coordinated ‘balsam bashes’ can effectively clear large areas of infested river corridor. It is intended to establish community groups along the River Tolka Valley to conduct extensive ‘balsam bashes’, under the supervision of DCC and/or IFI staff.

Because of the hazardous nature of Giant hogweed, control has been conducted solely by trained operatives. Spraying or stem injecting with the herbicide glyphosate is permitted ‘in or near rivers’ and is one of the most effective methods to control this invasive species. Large scale spraying of Giant hogweed stands along the Valley has been conducted by DCC since 2007.

The construction of a new public park by DCC along a section of the River Tolka afforded an opportunity to pilot a new approach under Renew4GPP, an EU LIFE Project (LIFE09 ENV/BE/000406).  The objective is of the project  to enhance green public procurement (GPP) by demonstrating to municipal authorities the ecological advantages of high-quality, innovative landscaping products based on 100% renewable resources.  These are weed control mats for the landscape industry, but their use for control of invasive species is novel to the project in Europe and Ireland.  Previously, it has been used for highway embankment stabilisation and to control common herbaceous weeds. The product characteristics are:

  • Made of 100% bio-polymer and natural fiber
  • 100% biodegradable and compostable and based on 100% renewable resources. It was developed by Dam De Saedeleir in 2007 – introduced on the market in 2008.
  • Also serves as an erosion control mat .
  • By using biodegradable weed control mats the use of synthetic mats or herbicides can be avoided.
  • UV-stable, no bark or mulch is necessary to cover and/or protect the mat.
  • It blocks the growth of weeds for at least 3 years and then fully degrades into compost for native riparian plants which installed to establish vegetation cover.

The park site had a latent seed bank of Giant hogweed and Himalayan balsam.  The disturbance of soils during construction had exacerbated the problem already described above. The use of the matting for disturbed areas was piloted to determine if non-chemical methods would be effective.  Two types of matting were used:  Type 1 = planted through with increased density of native riparian species and Type 2 = for seeding with native wildflower/grass mixes. Biodegradable weed control matting was installed in 2012 in areas of exposed soil. Both types are fire-retardant to resist vandalism and secured to prevent loss through flooding. Monitoring by the Parks and Landscape Services Division will continue until 2015. Preliminary results indicate that it is preventing soil loss during flooding, resisting burning and reducing maintenance requirements. Some uplifting has occurred of matting by latent Himalayan balsam underneath, and this is being monitored to inform the LIFE project.

A new and innovative method to control of large infestations of Curly-leaved waterweed was developed by IFI scientific staff in a large lake in the west of Ireland. This method involves covering the target weed with large mats of jute (also known as hessian or burlap), which block incident light from reaching the vegetation. This material has proved to be easy to handle, relatively cost-effective, environmentally safe and highly efficient at killing Curly-leaved waterweed. An added benefit is that it permits local seed banks of native plant species to germinate and grow though the jute pores once the offending invasive species has been killed. This method was applied to treat the Curly-leaved waterweed in the Darndale Fishery. The money for the jute matting was secured by a local community group from the Irish Heritage Council and the jute was put in place by a large team of personnel from the local Darndale community, local anglers, IFI and DCC. This operation was conducted in early 2013.


In 2011 Regulations that strengthen the controls on the introduction and dispersal of invasive species into Ireland were signed into Irish law.  However, at the time of writing, the Regulation that prohibits the importation, sale or distribution of invasive species has not been commenced. This means that harmful invasive species, such as Curly-leaved waterweed, are still being sold in this country. It is hoped and anticipated that this latter Regulation will be commenced before the end of 2013.

Funding to effectively tackle invasive species in Ireland is relatively limited, although Government agencies are applying this limited resource to address significant invasive species problems or problem areas. DCC has invested considerable funding into the redevelopment of the River Tolka Valley, where tackling the invasive species problem was central to the redevelopment programme.

Lack of awareness of the problems posed by invasive species among stakeholders (e.g. anglers, boaters) and the general public can be a major obstacle to progress in the battle against these species. This is currently being addressed, however, through active engagement at meetings and on site for practical demonstrations. Another problem relates to Himalayan balsam because it has such an attractive flower. It is not uncommon for people to collect seeds from this plant and sow them in their gardens or adjacent habitats. This is one of the main causes of spread of this plant in Ireland in recent years. A further problem relating to this species is the fact that it is a favourite of bee keepers because of its attraction for bees. It will be important to deliver the message to this group that the benefits by way of honey are far outweighed by the negative impacts that the plant has on biodiversity and ecosystem services.

 Social Dimension- the role of citizens:

Communicating the negative effects of invasive species, particularly those that are perceived to have some appeal (e.g. attractive) or functional value (e.g. oxygenate ponds for fish, provide a nectar source for bees), is vital to the success of any campaign to effectively tackle invasive species. IFI has been to the fore in communicating this message, whether through the national or local media, through information delivery, erection of bankside signage, production of free-to-download invasive species apps for smart ‘phones, etc..

Attendance at major social and stakeholder events in Dublin city (e.g. the Bloom festival that attracts over 50,000 plant enthusiasts, Irish Angling Expo that attracts over 9,000 angling enthusiasts, etc.) has served well to pass the message on to interest groups, and the results have been very positive. More people are now contacting IFI, DCC and other agencies to report sightings of invasive species and community groups are offering their services to help control these pernicious species.

Work conducted in Dublin city to tackle invasive species has largely been exceptionally well received. The hazard that Giant hogweed represented throughout the River Tolka Valley, particularly for children that played there during the summer months was significant and many were burned by the sap from this plant. In the early days of this infestation people did not know of its dangers and often the cause of the burns went undetermined. This has changed and most people who live adjacent to the Valley or who use it to fish, boat, walk or relax are now informed of the human health that the plant represents. The reaction to DCC’s coordinated Giant hogweed control programme was thus greeted with praise and gratitude.


The programmes set in place to control Giant hogweed and Himalayan balsam on the River Tolka Valley have been a great success to date. The previously common vista of long continuous stands of Giant hogweed along both sides of the rive are gone and, while occasional plants and small stands do occur throughout the Valley, these are registered by ongoing monitoring surveys or reported by concerned members of the public, and effectively treated. Large areas of riparian zone that were occupied by Himalayan balsam have been cleared and plans are in progress to engage local communities in ‘balsam bashing’ campaigns in coming years. The result of these actions has been to make the Tolka Valley safer for users and has been witnessed in the natural recovery of native bankside herbaceous species and grasses where these light-occluding stands have been removed.

In the Darndale Fishery early indications are that the jute matting has effectively killed the majority of the Curly-leaved waterweed population in the lake, as no major stands of this canopy-forming plant have been observed since the operation was completed in early 2013. Local community personnel continue to monitor progress and ensure that the jute is not vandalised. Potted aquatic plant species have been introduced in selected areas of the fishery to provide shelter for the wildfowl population on this watercourse. In the coming weeks IFI will introduce a population of fish into the lake and allow angling to resume.

 Lessons learned:

For engaging the public and relevant stakeholder groups, practical demonstrations are more effective than passing out literature or writing articles in local papers or journals. The practical, preferably on-site demonstration affords the participants the opportunity to view the invasive species at first hand and to engage the tutor or expert directly relating to key ID features, habitat preferences and control methods. Also, the enthusiasm of some of the participants can be ‘infectious’ and encourage other members of the group to become more actively involved. At such demonstrations, also, it is often possible to point out how the invasive species adversely impact on the native biota or on the functioning of the infested area (e.g. scoured and eroded banksides, clogged instream gravels, deposits of soil inchannel).

The use of technology to make information dissemination, ID and reporting more easy is very important. IFI have recently produced a free-to-download invasive species app for smart ‘phones that assists the user in identifying the species and provides summary information on its habitat and impacts. It also provides a facility to take a geo-referenced photograph which will be automatically uploaded onto the IFI server for identification by an expert. Feedback is given to the sender and, if the notification requires it, immediate action will be taken against the invasive species.

When encouraging community involvement in control operations, such as ‘balsam bashing’, it is important to give the participants as much practical and useful information as possible. Towards that end, IFI has produced a simple “How to Balsam Bash” brochure that provides information as to why ‘balsam bashing’ can be successful and how and when to engage in the exercise for maximum effect.  In 2011, IFI and DCC launched Fisheries Awareness Week in the national media with a ‘Balsam Bash’ on the river Dodder in Dublin city to raise awareness and this was carried out by local anglers, the Irish Wildlife Trust and the Green Communities coordinator of An Taisce, Ireland’s ‘National Trust’. This in turn has led to further projects such as the current City Otter Survey by the Irish Wildlife Trust supported by DCC.

Cost and funding sources:

Within IFI, support for activities relating to invasive species is financed from the core exchequer budget. Funding to develop practical invasive species control methods has been provided as part of EU funded programmes (e.g. Life+ CAISIE, Interreg IVA CIRB). These projects have also provided assistance in producing materials for education and stakeholder engagement programmes.

Dublin City Council does not have a specific budget for invasive species management, thus it is difficult to determine a precise figure.  However, DCC Parks and Landscape Services spent over €30,000 in 2011 on chemical treatment by contract and direct labour for riparian zones.  They spent a further 35,000 per annum on chemical application by injection for control of one species in Annex 1 SAC habitat at one site.  These figures don’t include the costs borne by the Roads Department who have their own chemical treatment programme, nor do they include the costs of human resources to assess and monitor.  In 2012, €20,000 was spent on re-planting of certain riparian sites with indigenous species to try to protect riverside banks in public parks with native species of willow, alder and birch (Betula pubescens).

For more information:

Caffrey, J.M. et al. (2010) Aquatic Invasions 5 (2), 123 – 129

CAISIE Layman’s Report (2013). Inland Fisheries Ireland IFI/2013/1-4114 National Biodiversity Data Centre Invasive Species Database for details of Trinity College Dublin national research project

Google Play or App Store to download the free Habitats-Invasive Species App

How to Balsam Bash. Join the Fight to Stamp Out Himalayan Balsam (2012) Inland Fisheries Ireland. ( City Council (2008) Biodiversity Action Plan

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