Giant Hogweed and why it is a problem

Giant Hogweed (Heracleum Mantegazzianum) is an invasive alien species (IAS) which comes from the Caucasus and was originally introduced into Ireland over 50 years ago as an ornamental. It grows huge, up to 5 metres tall, with rough serrated leaves one and a half metres wide, and large umbrella-like flowers that can yield many tens of thousands of viable seeds.

  • Giant Hogweed - Riverbank

Giant Hogweed is easy to identify when mature, but more difficult when young, click here. For tips on how to identify it at all stages of growth, and how to distinguish it from our native hogweed (Heracleum Sphondylium) and other similar native species, click here.

Ireland’s equable climate suits Giant Hogweed perfectly. It favours the damp, open margins of river banks where its seeds can readily disperse downstream but, increasingly, it is found at field boundaries, along road verges and in people’s gardens.

Giant Hogweed is regarded as one of the world’s worst invasive alien plants for the following reasons:

  • It produces a toxic sap in all parts of the plant which can cause a very serious burn that is known as a phytophotodermatitis because the sap is activated by light. Check out ways to stay safe, click here.
  • Its sheer size shades out smaller native plant species and eventually kills them. Because Giant Hogweed seeds are relatively heavy, they tend to fall within a few metres of the parent plant, and this leads to dense infestations of hundreds of plants across a small area.
  • There is increasing evidence that Giant Hogweed changes the chemical and metabolic status of soil it is growing in, making it inhospitable to native species.
  • In the winter it dies back. Because there is no native vegetation, soil is exposed and river banks become vulnerable to erosion.
  • One plant can produce a large number of seeds which can disperse downstream if the parent plant is near water. There is also evidence that Giant Hogweed seeds are being accidentally harvested with fodder crops, and may germinate at the location the fodder is taken to. This means that Giant Hogweed can be found in any location.

Giant Hogweed is controlled by two pieces of European Union legislation:

  • Invasive Species of European Concern (EU Regulation 1143/2014)
  • EC (Birds and Natural Habitats) regulations S.I.477 (2011)

The legislation puts the onus on national governments and regional councils to devise and implement methods that control the dispersal of species listed. For more information visit the National Biodiversity Data Centre website here.

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