Control of Giant Hogweed on the River Loobagh

Giant Hogweed (Heracleum mantegazzianum) is an invasive alien plant (IAS) which spreads by seed and favours river corridors. It produces a toxic sap that can cause a nasty, persistent burn on contact with skin, and its sheer size and rate of spread causes substantial biodiversity loss and degradation of habitats. 

  • Control of Giant Hogweed on River Loobagh

Giant Hogweed can be confused with some native plants such as native hogweed (Heracleum sphondylium) and angelica (Angelica sylvestris) which are harmless. Click here for information on how to distinguish between these plants.

In July 2016, Ballyhoura Development CLG, in partnership with Limerick City and County Council, began a programme of giant hogweed control on the River Loobagh in Kilmallock, but it soon became evident that seeds were travelling downstream and Kilmallock would continue to become infested until the infestation in the upper reaches was contained.

In 2018 Limerick City and County Council secured funding from The Heritage Council to survey and map giant hogweed along the entire length of the River Loobagh corridor and to develop a management plan for its control. Based on this plan, funding was subsequently secured from the Department of Culture Heritage and the Gaeltact under the National Biodiversity Action Plan to deliver the strategy to undertake a three-year programme to clear the River Loobagh corridor (54 Km of river bank) of giant hogweed and monitor the subsequent re-establishment of native species in the riparian margin.

    Giant Hogweed Project

    The aims of the project are to:

    • Achieve a high level of giant hogweed control on the R. Loobagh corridor (54 km river bank; ‘corridor’ = 15 metres out from both banks)
    • Employ manual control methods, with minimal  the use of chemicals
    • Simultaneously, take actions to restore riparian margin to prevent recolonization by giant hogweed & secondary invaders
    • Engage landowners and the local community
    • Raise awareness of giant hogweed and invasive alien species (IAS) in general
    • Encourage the public to report sitings of giant hogweed and other IAS, there the Report Invasive Plants smart phone App.

    The project began with:

    • A survey of the banks of the River Loobagh conducted between April and May 2019, which involved walking the entire river to the intersection with the River Maigue, including the R. Loobagh’s upper catchment tributaries
    • Communication with landowners in the area
    • Liaison with Ballyhoura Development CLG, the members of which have three-years’ experience of controlling giant hogweed in the Kilmallock area
    • Desk-top research to identify recent best practice

    From the outset, the programme adopted a manual approach to eradication with minimal use of chemicals. The rationale for this approach is as follows:

    • Glyphosate is the most widely used systemic herbicide against giant hogweed and other IAS but it may be banned within the next few years and, currently, there are no alternatives which can be used safely near water. Therefore, it is prudent to proactively develop an approach which is not reliant on herbicides.
    • Ireland is experiencing a catastrophic loss of biodiversity due to the compounding factors of climate change, invasive species, tree diseases, and land management practices. A key abatement measure to help halt this loss is to reduce chemical loading in the environment, particularly the use of herbicides and pesticides. The National Biodiversity Action Plan and the All Ireland Pollinator Plan both recommend reduced use wherever feasible.
       
    • Although there are very few specific studies published, the use of herbicides in riparian margins almost certainly damages ecological integrity and it is likely to make the habitat more vulnerable to colonisation by new invasive species. Therefore, it is anticipated that a manual approach will have less of an ecological impact that chemical treatment and will allow more rapid and more complete restoration of the riverside habitats.
       
    • Previous best practice guidance (Pysek, 2007) highlights the benefits of an integrated approach and emphasises the importance of working systematically downstream.
    Giant Hogweed along river bank in Kilmallock
    Vigorously growing giant hogweed along the river bank in Kilmallock, May 2020.

    A key feature of the project is partnership with Ballyhoura Development CLG and the involvement of Rural Social Scheme members in the control effort.

    Work schedule

    • The 2020 work schedule started in January 2020 because the mild winter meant that giant hogweed did not stop growing. Dig-out of plants continued from January throughout spring and summer:
    • The upper, middle and part of the lower catchment were cleared of all visible giant hogweed plants
    • Very few visible giant hogweed plants matured to produce flowering heads
    • Systematic clearance allowed for more straightforward monitoring

    Project outputs to date:

    2019

    2020

    Approx 700 flowering heads cut and macerated

    More than 35,000 giant hogweed plants dug out and roots and stems macerated

    1 tonne seed bagged and disposed of securely

    Thousands of newly emerging seedlings removed

    Japanese knotweed control programme implemented in Loobagh corridor

    Very few visible plants matured to flowering. 5 kg seed removed and disposed of

    Raised awareness through radio & TV interviews

    Entire corridor cleared of visible giant hogweed plants (as of end Sept)

    Significant support secured from landowners

    Areas beyond the Loobagh corridor were included in the budget, including Elton village, areas of Mountcoote townland, a property in Kilfinane and several properties in Kilmallock

    RSS workers engaged through Ballyhoura Development and trained in giant hogweed dig-out methods

    Safety guidance for working with giant hogweed circulated to landowners in the Loobagh corridor

     

    Web page posted onto LCCC website

     

    Project logo designed

    Unfortunately, training of local landowners and other interested individuals, which had been planned for October 2020 had to be deferred because of Covid-19 restrictions. This will now take place in Spring 2021.

    Conclusions

    A substantially higher than expected level of control has been achieved throughout the R. Loobagh corridor and several large, infested areas in the Loobagh catchment outside of the R. Loobagh corridor have been dealt with. In addition, a programme to control Japanese knotweed in the R. Loobagh corridor is now in its second year, and it is anticipated that eradication is achievable within the next two years.

    The results are very encouraging. The literature provides evidence that giant hogweed seeds remain viable in soil for no more than two to three years. This means that the seedbank in the R. Loobagh corridor should be substantially reduced already because of the work programme to date, and it may be exhausted by the end of 2021. An ongoing programme of monitoring will be essential combined with constant surveillance for new infestations which could develop in the river corridor from seeds brought in by machinery, people and animals. Nevertheless, a very high level of control and possible eradication of giant hogweed from the R. Loobagh corridor appears achievable by the end of 2021.

    You can find a map of invasives in Limerick including Giant Hogweed here.

    Ballyhoura Development CLG I Dept Housing Local Government and Heritage

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    Environment Strategy

    Phone +353 61 556000