Ecological Impacts
Giant hogweed is not yet common or widespread enough in North America for its environmental impacts to be fully apparent. But its huge size and rapid growth, combined with its tolerance of a wide variety of habitats, makes this plant one to be concerned about. Hogweed can form dense stands that overtop and shade out competing plants. When the plants die back in the fall, bare ground is left, accelerating soil erosion (Pysek 1991). Siltation of rivers and streams in turn clogs small spaces in gravel bottoms where fish eggs are deposited, thus degrading spawning habitat for trout and other fish. Hogweed is well-adapted to streambank and river floodplain habitats, where it often becomes abundant. In Ireland dense giant hogweed stands have caused a significant loss of plant and animal diversity along rivers and streams (Caffrey 1994).
In southern Scotland, giant hogweed had been shown to reduce plant diversity and greatly reduce cover in riparian woodlands (Gibson et al.1995, in Tiley at al. 1996). Stands of giant hogweed also reduced cover of most species in riparian grasslands, though diversity was slightly higher than in uninfested grasslands, apparently because shading allowed the survival of species that would not be competitive in the unshaded grassland community.
Because it grows so large, one flowering hogweed plant often will dominate 1-2 square meters (10.8-21.5 sq ft) of space, sometimes with an "understory" of immature hogweed plants (Tiley et al. 1996). In hogweed stands in southern Scotland, Gibson et al.(1995, in Tiley at al. 1996) measured 4-7 flowering stalks and 5-19 immature plants within 1 square meter (10.8 sq ft).
Giant hogweed is a host for the the carrot fly Psila rosae as well as the plant pathogen Sclerotinia sclerotiorum, both of which attack a number of crop plants (Neiland 1986, in Dodd et al. 1994).
Phototoxic Effects
The sap of giant hogweed, along with wild parsnip and certain other members of the parsley family (including common garden plants such as parsley and celery), contains chemicals called linear furanocoumarins. When these chemicals (especially psoralin, bergapten, xanthotoxin, and marmesin) are absorbed into the skin, they cause the skin to become photosensitive (Tiley et al. 1996). Subsequent exposure to the sunlight quickly results in skin damage (photodermatitis). Burning can take place after as little as 10 minutes in the sun, although the damage may take a day or two to show up. These burns range from mild reddening and itching to severe reddening and blistering, depending on how much sap has infiltrated the skin, the sun's intensity, and the length of time the affected area is exposed. Even after the burns heal, the affected area can remain darkened and discolored for months or even years. This condition is called hyperpigmentation.
The photochemical burns resulting from contact with the sap of giant hogweed and certain other parsley family plants is different than the rash caused by contact with poison ivy or animikiibag (Toxicodendron radicans) and its relatives. While the rash from poison ivy is due to an immune reaction to the oils it produces, and is independent of sunlight, the burning and blistering from giant hogweed and certain other parsley family members is caused by a light-induced reaction of linear furanocoumarins with the DNA of skin cells, causing cell death (Towers 1980). While some people may be "resistant" to poison ivy, no one is resistant to burns from giant hogweed and its relatives.
Interestingly, by measuring the amounts of furanocoumarins produced by various parsley family plants, researchers have found that as a rule only parley family members that typically grow in open habitats such as prairies and wetlands produce furanocoumarins. [Queen Anne's lace and its cultivated form, the carrot (both Daucus carota) are an exception to this rule in that they typically grow in open habitats but do NOT produce furanocoumarins.] Furanocoumarins are toxic to insects that might eat these plants, but only in sunlight. Therefore woodland species generally don't produce furanocoumarins, because doing so wouldn't give them an advantage against insects. Animals may also be susceptible to the effects of linear furanocoumarins.
While there appears to be no direct research into the relative phototoxicity of giant hogweed as compared to other phototoxic members of this family, the scarcity of reports of photochemical burns from native relatives such as cow parsnip and angelica suggest that the phototoxic effects of giant hogweed are much more severe than these other plants. This effect may be due to the the stiff hairs on the stem of giant hogweed that exude sap and that also tend to break off on the skin, as opposed to the soft, scattered hairs of cow parsnip, which apparently don't break off (Morton 1978). Cut or broken giant hogweed plants also exude large amounts of sap (Morton 1978).
If your skin has been exposed to the juice of giant hogweed, you should take immediate action. Washing the exposed area with soap and water should help. But because they are are lipid-soluble, the furanocoumarins are quickly absorbed into the skin and so are not completely removed. Therefore the affected area should be protected from the sun's rays with clothing or by other means for 48 to 72 hours (Tiley et al. 1996). The photosensitizing effect of linear furanocoumarins peaks after about ½ to 2 hours, and then diminishes fairly rapidly. Previously burned skin heals very slowly, and may remain hypersensitive to sunlight for months or even years.
Status in Area
Giant hogweed is locally established at least as plantings in the Upper Great Lakes region, including sites in lower and western Upper Michigan, and adjacent northeast Wisconsin. Giant hogweed is listed as a Prohibited Noxious Weed in Minnesota. Giant hogweed is also listed as a federally-listed noxious weed, meaning that its sale and possession are illegal within the US. It is one of only two plants whose cultivation is prohibited by law in Britain (CABI, no date).