Explained in 60 Seconds: What is Giant Hogweed and Why is it a Threat?
What is Giant Hogweed?
Giant hogweed (Heracleum mantegazzianum) is a non-native highly invasive plant that originated in the western Caucasus region. In the 19th century, it was introduced to Britain as an ornamental plant and has spread to Ireland since.
- Can grow to heights of 2-5m.
- Has a stout, bright green stem with extensive dark reddish-purple splotches
- Has huge leaves between 1-1.5m wide.
Although somewhat similar in appearance, our native Common Hogweed (Heracleum sphondylium) is much smaller than Giant Hogweed; typically reaching heights of only 50-120cm.
Why are Invasive Alien Species a threat?
Introductions of invasive alien species to
native habitats are known to exert significant impacts on local plant
diversity, community dynamics, and ecosystem processes (Pyšek et al., 2012). The
severity of incursions by invasive species will depend upon the invader’s
persistence, the type, magnitude and time scale of the invasion, and the
ability of native communities to buffer against the full or partial
displacement of resident species (Vilà and Weiner, 2004).
Examples of invasive species in Ireland include Japanese knotweed (Fallopia japonica), Rhododendron (Rhododendron ponticum), Himalayan balsam (Impatiens glandulifera) and Giant Hogweed (Heracleum mantegazzianum).
Why is Giant Hogweed a threat?
Due to our warmer climate and nutrient-dense
soil, Giant Hogweed germinates more readily here than in its native range. Giant
Hogweed plants can usually live for several years, flowering in the third to
fifth year and dying after they bear seeds. However, in poor conditions, they
can persist for 12 years.
They rely exclusively on reproduction from
seeds. Nevertheless, self-fertilisation is possible, making the establishment
of isolated populations likely. They flower in June and July, and each plant
produces an average of 20,000 seeds making their reproductive potential enormous.
Human activities, water and wind disperse the
seeds long distances. Subsequently, seeds can lay dormant in the soil until
conditions are favourable for up to 2 years. These seeds germinate early in
spring before native vegetation appears.
Giant Hogweed can grow to 16ft tall, standing
well above native vegetation and shading it out. As Giant hogweed spreads
particularly well along river corridors, it can destabilise banks and increase
This species also poses a threat to human
health. It secretes a watery sap, containing photosensitizing furanocoumarins, that
could easily touch off exposed skin. The issue with these compounds is that, when
exposed to the sun’s ultraviolet radiation, they cause burns and rashes (Nielsen
et al., 2005). If contact with the sap
occurs, the area should be covered and washed with soap and water immediately. Medical
attention should be sought as topical steroids may reduce the effects of the
contact (Nielsen et al., 2005).
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Pyšek, P., Jaroˇsík, V., Hulme, P., Pergl, J., Hejda, M., Schaffner, U., Vilà, M., 2012. A global assessment of invasive plant impacts on resident species, communities and ecosystems: the interaction of impact measures, invading species’ traits and environment. Glob. Change Biol. 18, 1725–1737.
Vilà, M., Weiner, J., 2004. Are invasive plant species better competitors than native plant species? Evidence from pair-wise experiments. Oikos 105, 229–238.
Nielsen, C.; Ravn, H.P.; Nentwig, W. and Wade, M. (eds.), (2005). The Giant Hogweed Best Practice Manual. Guidelines for the management and control of an invasive weed in Europe. Forest & Landscape Denmark, Hoersholm.