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Alien Species

Giant Hogweed

Giant Hogweed (Heracleum mantegazzianum) is a non-native plant species that grows abundantly along the banks of rivers and streams. It is a cause for concern in that it is both a human health hazard and has a negative ecological impact on infested river corridors. It can reach 2-5 metres in height, and up to 7m. It is further distinguished by a stout, dark reddish-purple stem and spotted leaf stalks that are hollow and produce sturdy bristles. Stems vary from 3-8 cm in diameter, occasionally up to 10 cm. The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk. The plant has deeply incised compound leaves which grow up to 1-1.7 m in width


Giant Hogweed produces a sap that can induce painful blisters on contact with human skin. The sap can also permanently impair the skins ability to filter ultraviolet radiation. Like other invasive plants hogweed can form dense stands outcompeting native vegetation which is used by native fauna. Left unchecked dense stands of hogweed will deteriorate the natural environment.

Japanese Knotweed

Japanese knotweed (Fallopia japonica) was intentionally introduced as an ornamental plant. It is widely considered an invasive species or weed. It is listed by the World Conservation Union as one of the world’s 100 worst invasive species.

It is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive plants. Knotweed is known to out-compete native riparian vegetation which normally provide food and cover for fish and other aquatic species. The plant dies back in the autumn period exposing soil to erosion which is damaging to spawning fish (lamprey and salmon). The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35C and can extend 7 metres horizontally and 3 metres deep, making removal by excavation extremely difficult. The plant is also resilient to cutting, vigorously re-sprouting from the roots. The most effective method of control is by herbicide application close to the flowering stage in late summer or autumn.

Himalayan Balsam

Himalayan balsam (Impatiens glandulifera) is a large annual plant, native to the Himalaya mountain region in Asia. This annual plant grows to 3 metres high and has pink flowers, with a hooded shape, 3–4 cm tall and 2 cm broad. After flowering between June and October, the plant forms seed pods 2–3 cm long and 8 mm broad, which explode when disturbed, scattering the seeds up to 7 metres from the parent plant. The aggressive seed dispersal, coupled with high nectar production which attracts pollinators, often allows the Himalayan Balsam to outcompete native plants. The plants grow in dense stands along the banks of rivers and effectively suppress any native grasses and herbaceous plants. The balsam dies back in autumn, exposing the now bare bank-sides to erosive winter flows. Soil erosion from winter flows is damaging to spawning fish (lamprey and salmon).

Project aim:

A trial study in the Mulkear catchment, 1998-2001, showed that giant hogweed can be controlled using herbicide. The expansion of giant hogweed (49.5 km) and Japanese Knotweed (17.2km) along the river will be controlled and the natural vegetation allowed to recover. Outlying farms will also need to be treated where there is regular traffic between the river and the farms in question. These invasive plants will be treated by trained staff of the Office of Public Works and Limerick County Council with herbicide as approved by the National Parks & Wildlife Service. The Project Team and Inland Fisheries Ireland will monitor key sites for the duration of the MulkearLIFE project.

Mulkear LIFE, Inland Fisheries Ireland, Ashbourne Business Park, Dock Road, Limerick, Ireland
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