Heracleum mantegazzianum is considered to be one of the most problematic invasive plants in Europe (Pysek et al, 1998). It produces a toxic sap that causes a painful and problematic phototoxic reaction. It establishes dense monocultures that threaten natural ecosystems. It is also known to increase erosion of river and stream banks and to be a problematic weed in both agricultural and urban environments.
The sap of H. mantegazzianum causes a phytotoxic reaction when in contact with the skin and exposed to sunlight (Klingenstein, 2007). Toxic furanocoumarins or psoralens are stored as biologically active aglycones in sap in the oil channels or ducts in the leaves, stems, roots, flowers and seeds. When they come in contact with the skin they cause an extreme sensitivity to sunlight called phytophotodermatitis (CEH, 2004). The phototoxic reaction is can be activated by ultraviolet radiation only 15 minutes after contact, with a sensitivity peak between 30 min and two hours (Klingenstein, 2007). It can lead to severe slow healing burns or scarring (EIAS, 2003). Blistering occur s 24-48 hours after exposure to sunlight and dense post inflammatory hyper-pigmentation is visible after 3-5 days and may persist for up to 6 years (CEH, 2004; Klingenstein, 2007). Gardeners, landscape workers, and children are at particular risk. Since the plant itself is painless workers or children in contact with the plant may continue exposure to the sap for hours (Klingenstein, 2007). Its hazard to human health causes H. mantegazzianum to lower the recreational value of invaded lands (Pergl & Perglova, 2006).
Giant hogweed changes species composition and reduces species diversity of native plant communities (Neilson et al, 2005). It establishes dense stands that displace and suppress the growth of native flora, especially in disturbed areas and riparian zones (CEH, 2004; Neilson et al, 2005; Page, 2006). H. mantegazzianum outcompetes native plants by shading them out, growing leaves above resident herbs and grasses (Thiele & Otte, 2007). It may also have allelopathic properties (Page, 2006).
Its replacement of native vegetation results in other effects to ecosystems and likely causes far reaching impacts. It displaces native riparian vegetation and then causes bank erosion in the winter it dies back (Page, 2000). Such instability of river banks caused by giant hogweed poses a serious threat to salmon spawning habits in Ireland (Caffrey, 1999). H. mantegazzianum is also known to hybridize with European native Heracleum sphondylium (Klingenstein, 2007), and be a problematic weed to agricultural and urban environments (Page , 2006).
Location Specific Impacts:
Ecosystem change: Heracleum mantegazzianum increases the erosion of river banks after plants dieback in the winter (Pysek, 1991).
Reduction in native biodiversity: Heracleum mantegazzianum forms monospecific stands which reduce species diversity in the Czech Republic (Hejda et al, 2009; Nehrbass et al, 2006; Pysek, 1991).
Reduction in native biodiversity: Heracleum mantegazzianum displaces native plant species and reduces species diversity in Denmark (Anderson & Calov, 1996).
Habitat alteration: Heracleum mantegazzianum establishes stands of up to almost 100% cover that greatly decrease species diversity and significantly alter habitats in Germany (Thiele & Otte, 2007).
Hybridisation: Heracleum mantegazzianum hybridizes with European native Heracleum sphondylium (Klingenstein, 2007).
Habitat alteration: The instability of river banks dominated by Heracleum mantegazzianum poses a serious threat to salmon spawning habits in Ireland (Caffrey, 1999).
United Kingdom (UK)
Habitat alteration: The instability of river banks dominated by Heracleum mantegazzianum poses a serious threat to salmon spawning habits in Great Britain (Caffrey, 1999).
Hybridisation: Heracleum mantegazzianum hybridizies with European native Heracleum sphondylium (Klingenstein, 2007).