This has been a year of astonishing growth: more grass than we have ever seen, nettles at head height, trees and shrubs sagging under the weight of blossom. Our common hogweed (Heracleum sphondylium) is standing seven feet tall in places with flower-heads the size of dinner plates.
We have been asked several times if it is, in fact, giant hogweed and the answer is: no. Giant hogweed, or Heracleum mantegazzianum, is a different species entirely, a huge invasive noxious weed, native to the western Caucasus. Like so many of our ecological problems (grey squirrels, Japanese knotweed, Himalayan balsam) it was introduced as a fashionable exotic species by wealthy Victorians in the nineteenth Century.
The sap of giant hogweed is phototoxic; if there is sap on your skin and it is exposed to UVA light, chemicals in the sap absorb energy from the light and release it into the skin causing cellular damage. The result is phytophotodermatitis: burns, blisters and scars. If you find giant hogweed anywhere, treat it with great care.
The two species are sometimes confused firstly because common hogweed in ideal circumstances can grow very tall and secondly because it too can cause skin irritations and blisters. All the plants in the Apiaceae family, which include both of the hogweeds and celery, manufacture chemicals called furocoumarins in order to protect themselves against fungal infections; these are the chemicals that are responsible, for phytotoxicity.
Flower heads as big as dinner plates
The concentration of these chemicals in any individual plant depends on many factors that aren’t really understood yet. In common hogweed the concentration is usually too low to do much damage but, in the right circumstances, the concentration can increase to the point where it can cause rashes and blisters.
Even though there is no legal obligation to remove or treat giant hogweed, if it were found in the park, we would remove it. It is a very dramatic and interesting plant but it is not native and it has the potential to be dangerous.
More invasive species: