Giant hogweed

Giant hogweed

Giant hogweed

(Heracleum mantegazzianum)

Giant hogweed

This statuesque plant can grow to a height of 4 metres and was introduced to Ireland as an ornamental in the late 1880s.  It’s preferred habitat is wasteland and the banks of many rivers in the country. The plant produces a sap that is hazardous to humans, particularly in the presence of direct sunlight. The large leaves of the plant create sufficient shade to suppress indigenous herbaceous understorey plants along banksides.
Giant Hogweed is a non-native plant species that grows abundantly along the banks of rivers and streams. This highly invasive species is a cause for concern in that it is a human health hazard and it exerts a significantly negative ecological impact on infested river corridors.
Between 1998 and 2001 the Office of Public Works commissioned the Central Fisheries Board (now IFI) to conduct a four-year control/eradication programme in the Mulkear River catchment where Giant Hogweed was widespread and restricting angler access.  Colonies often presented continuous linear stands that extended up to 2km along the river bank.  The herbicide Glyphosate was applied to plants in a catchment-wide approach in March, May, July and September of each year. 
The control programme almost completely eradicated Giant Hogweed from the treated sites by the end of the four year study.  No seeding occurred after Glyphosate was applied.  Native plants such as Butterbur (Petasiteshybridus) were able to re-establish in the riparian zone of the Mulkear River with the removal of this dangerous invasive species.
Resulting from the study a protocol for treating Giant Hogweed along river corridors was developed. This is summarised below. This protocol is currently being operated in many areas of the country where Giant Hogweed is being treated.

  • Compile maps which accurately detail the distribution of H. mantegazzianum in each affected catchment throughout the country.
  • A catchment approach to treatment with glyphosate must be taken and spraying must commence at the farthest upstream site from which the plant is recorded.
  • Spraying should commence in March or early April, when the plants have expanded their leaves and reached a height of >15cm.  Experience in Ireland has shown that young seedlings and small plants (<15cm) are less susceptible to glyphosate than their larger counterparts.
  • A dose rate of 5 l ha-1 is sufficient to kill treated vegetation.
  • Sections treated in March/April should be retreated in May, or later, if required.
  • The whole catchment must again be surveyed in July and any plants that have flowered, or are likely to flower, should be deheaded or chopped down before seeds are produced.  The cut umbels must be removed from the area and destroyed. Any regrowth should be sprayed with glyphosate as the plant will again attempt to flower and set seed.
  • A further glyphosate treatment in September throughout the catchment will kill late-developing plants or seedlings that survive earlier treatments.

(Taken from: Caffrey J.M. 1999.  Phenology and long-term control of Heracleum mantegazzianum.  Hydrobiologia 415: 223-228.)

A number of other peer reviewed scientific publications were produced as a result of this research and include the following:

  • Wade M., Darby E.J., Courtney, A.D. and Caffrey J.M. 1997.  Heracleum mantegazzianum: A problem for river managers in the Republic of Ireland and the United Kingdom. In: Plant Invasions: Studies from North America and Europe (Eds.:J.H. Brock et al.).  Backhuys Publishers, Leiden, The Netherlands pp. 139-152.
  • Caffrey J.M.  2001  The management of Giant Hogweed in an Irish river catchment. Journal of Aquatic Plant Management 39:28-33.
  • Boylen C.W. and Caffrey J.M. 2002. The introduction of Heracleum mantegazzianum, Giant Hogweed, into North America: phenology and prognosis for spread.