Giant Hogweed (Heracleum mantegazzianum) makes an impressive sight when fully grown but giant hogweed is invasive and potentially harmful to people. Chemicals in the sap cause photosensitivity, where the skin becomes very sensitive to sunlight. A painful rash, which then blisters, develops sometimes hours after the contact but can last for years and leave a scar. If a gardener or walker touches the plant it is usually on the hands or arms – the photo shows where a gardener did not wear long enough gloves so was affected on the wrist.
Several photos have appeared in the media recently where children have played among the plants leading to very painful blistering around shoulders and other parts of the body.
Heracleum mantegazzianum was introduced into Britain from the Caucasus in the nineteenth century as a striking garden plant. It is now widely naturalised throughout much of Britain and Europe. Research by the RHS indicates there are four types of giant hogweed in Britain some of which are biennial and others perennial. All have high levels of furanocoumarins (the chemicals which make the skin sensitive to sunlight).
The very common native hogweed, Heracleum sphondylium, is also in flower just now. It is a much smaller plant than giant hogweed with darker, less pointed leaves. As the name implies it was used as animal fodder and sometimes by people usually by cooking the young shoots. It occasionally causes rashes but reactions tend not to be as severe as with the larger species. It is not known why this sometimes happens; the plant may be under some stress which causes it to produce furanocoumarin. So best to us gloves if dealing with it especially on bright days.
Giant hogweed is usually biennial, sending up a flower spike in the second year and then setting seed. A big plant can produce 50,000 seeds which can remain viable for at least ten years.
There is no legal requirement for landowners to eliminate giant hogweed on their ground, but some do while many local authorities remove infestations in public areas. It is an offence to plant or otherwise spread giant hogweed. However the plant can rapidly spread often along water courses as the seed readily floats.
When controlling giant hogweed always wear gloves, cover your arms and legs, and ideally wear a face mask. If practical avoid sunny days. Wash any skin that comes in contact with the plant immediately. In a garden pull up young plants in spring before they have produced flowering spikes. Thickets are best left to the professionals who have full protective clothing. Strimmers send sap and fragments flying so face protection is essential. Systemic weedkillers based on glyphosate e.g. Roundup are usually the best choice as these kill roots also. However if the plant has set seed young plants are likely to emerge in future years. (Note: weedkiller must never be allowed to run off into waterways.) Like Japanese knotweed giant hogweed is classed as a controlled waste so, if it is taken off site, can only be disposed of in a licensed landfill site.