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The Story of Invasive Weed Species in the Mulkear Catchment

Invasive weed species (Giant Hogweed, Himalayan Balsam and knotweed species) are problematic along the entire course of the River Mulkear and indeed throughout the entire Mulkear catchment. Considerable progress has been made in recent months by Limerick County Council (LCC) and by the Office of Public Works (OPW) in addressing the extent of the problem with targeted programmes of control and removal. These invasive plants are a major concern to MulkearLIFE as they outcompete native vegetation, leading to a loss of biodiversity, they also reduce light available for primary producers (dense knotweed canopy) and destabilise riverbanks. In addition, Giant Hogweed is a phototoxic plant and its sap can cause severe blisters when the skin is exposed to sunlight after contacting the sap.

A comprehensive programme of work was initiated in April of 2010 by the OPW and LCC using an approved chemical herbicide to treat Giant Hogweed. Due to the levels of infestation in some areas, several treatments were necessary. Spot checking by project staff revealed outstanding kill-out rates which is evidence of the hard work, skill and professionalism of the work teams of the OPW & LCC. However, several years of treatment will be required to deplete the seed reserves.


Giant Hogweed (pre-treatment)


Giant Hogweed (two weeks post treatment)


Giant Hogweed (showing levels of recolonisation by native riparian vegetation – 4 month post-treatment)

Himalayan Balsam is widespread throughout the Catchment but Himalayan Knotweed, not to be confused with Japanese Knotweed, is one of several foreign knotweeds introduced for garden purposes in the nineteenth century from the Himalayas and now established in many locations along the Newport River and is spreading rapidly. A detailed work programme was initiated using a permitted chemical spray (Round-Up Bioactive) and excellent levels of kill-off were evident. However, the plant has significant energy stores in its rhizomes and consequently several years of treatment will be needed to bring the plant under control. Work by MulkearLIFE Work will continue again in the spring of 2011 to address the problem of invasive plants. This work will be co-ordinated by MulkearLIFE’s project partners OPW & LCC. Their dedication to controlling these plants is greatly appreciated.


Aerial view of Himalayan Knotweed infestation – reddish growth along river. (image taken Nov. 2007, courtesy Irish Air Corps – note the powerline tower is same location in all 3 photos)

Himalayan Knotweed (pre-treatment)

Himalayan Knotweed (1 month post treatment)

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Mulkear LIFE, Inland Fisheries Ireland, Ashbourne Business Park, Dock Road, Limerick, Ireland
Phone: +353 (0)61 300 238   Email: limerick@fisheriesireland.ie