THE POISON GARDEN website       

Heracleum mantegazzianum,
giant hogweed

Summary

An example of the law of unforeseen consequences. This plant was brought to the UK to beautify large gardens but the burning it causes scars many people.

Blog Entries

You can read more about giant hogweed in these blog entries (most recent first);
How many people do get burns from Heracleum mantegazzianum?
An audience member tells me about children using giant hogweed as blowpipes
A giant hogweed plants starts growing in October. How far will it get?
Deciding what is giant and what is common hogweed and other plants that burn
How should we try and deal with giant hogweed on private land?
The determination of plants to reproduce
Giant hogweed in Edinburgh
The many things we don't know about giant hogweed

Family

Apiaceae

Giant hogweed in flower

Meaning of the Name

Heracleum
Named for Hercules. Literally, ‘belonging to Hercules’. Some sources say this is simply because of its great size but others suggest that Hercules derived his healing abilities from the plant. There are about sixty species in the genus but none is especially known for having medicinal properties.
 
mantegazzianum
Named for Paolo Mantegazza, 19th century Italian anthropologist and ethnographist who is credited, by some, with being the first person to extract cocaine from coca leaves.

Common Names and Synonyms

giant hogweed, giant cow parsnip, cartwheel flower, efwr (in Wales), wild rhubarb

How Poisonous, How Harmful?

The speckled stem of a giant hogweed

The plant contains furocoumarins (psoralens) which produce changes in the cell structure of the skin reducing its protection against the effects of UV radiation. These can be released from the plant simply by brushing against it. Exposure to sunlight after contact causes severe skin rashes and/or blistering and burns but the effects may not start until about twenty four hours after contact. It may take several years for the skin to return to normal during which time any renewed exposure to even quite dull daylight will produce new burns.

Depending on the extent of the contact, the victim may suffer a reddening of the skin, blisters or burns requiring hospital treatment.

In some cases, a permanent change in skin pigmentation occurs.

Whenever Heracleum mantegazzianum is being discussed there will be those who claim that the case against it is over-stated and that many other plants are more dangerous. That point of view seems to be opposed to the findings of a 1996 Swiss study of 29 years of plant poisoning reports. Though only 18 reports in that time concerned giant hogweed producing 'serious' consequences that made it the second most dangerous plant with only Atropa belladonna, at 42 cases, exceeding it.

Video Clip

Click to watch a short YouTube video produced and narrated by John Robertson.

Heracleum mantegazzianum - the Sequel

The video above was made in June 2008.

This video contains footage from May 2009 and shows just how hard eradication of giant hogweed is.

Incidents

Heracleum sphondylium, cow parsnip,
common hogweed.
A common and much less troublesome
relative of giant hogweed

Typical contact comes from brushing through a stand of plants when found on a riverbank, strimming a patch of rough ground in early spring without realising that the young plant is present and even contact with pets which have had contact with the plant.

A man who had stripped to the waist whilst strimming a patch of rough ground suffered numerous small spot burns where he was hit by spray from the strimmed plant. He reported suffering the burns again two years later after removing his shirt on a hot day.

There are numerous reports of children using the hollow stems as peashooters and suffering burns around the mouth.

Though cattle and sheep have been used to graze off the very young shoots animals are affected by the plant.

A woman, in her fifties, showed pigmentation scars resulting from contact with giant hogweed when she was seven.

A man still had visible scars fourteen years after hogweed burns.

On the walk where I saw the plants featured in the second video, above, a friend slashed at a plant with his walking pole. I was struck, just above the mouth, by a single drop of juice but that was enough to give a small red spot which reappears on sunny days.

The confusion which can be caused by common names was ably illustrated by an online discussion following a news story about Heracleum mantegazzianum in part of Canada. Some commenters claimed the plant was not as harmful as people said, apparently confusing the sphondylium species with the giant hogweed. One even wrote about its many medicinal properties before it became clear he was talking about an entirely different genus.

There was also a lot of discussion about whether this was a new problem and how bad its effects might be. Both points were answered by one poster who remembered, about 30 years ago, playing with his brother, in a field of giant hogweed. The effects started to appear the following day and, within a week, they 'looked liked two mini elephant men...[and] had seeping water sacks hanging off of us for a long time.' He clearly remembered repeated burning for several years after and said his skin is still discoloured.

Heracleum mantegazzianum growing in Edinburgh.
In the top right corner is a McDonald's sign.

In spring 2010, I spotted a piece of derelict land, within the boundaries of the Fort Kinnaird Shopping Centre, in Edinburgh, which was almost completely covered in giant hogweed. The land is just across the road from a large DIY superstore, B & Q, and within sight of the fast food restaurants serving the area.

With no fencing around this piece of land, there is a real risk of someone suffering serious harm if they do not know what the plant is. As the picture shows, the plants are already overgrowing the pavement so someone just walking along the road could brush against it.

Edinburgh City Council say that all they can do is offer the landowner advice on removal, that is assuming they can find out who owns the land. They do not have the power to order the land to be fenced off.

A visit to Edinburgh, in July 2010, revealed why I failed to get Edinburgh City Council interested in this small plot of land. The whole city was blanketed in giant hogweed. Without stopping the car or turning off one of the main feeder roads into the city centre, we spotted at least six sites where giant hogweed was thriving and, in many cases already setting seed.

Folklore and Facts

Heracleum mantegazzianum, giant hogweed

It was introduced to the UK by the Victorians who thought its size would make a dramatic statement in large gardens. It escaped and has spread rapidly to be a major problem on river banks in some areas. The Wildlife and Countryside Act 1981 makes it an offence to plant or cause giant hogweed to grow in the wild. In July 2003, the Daily Telegraph reported that an EU funded programme was looking at at bringing fungi from Russia to attack the plant. No further information has been found on this project but, in a 2010 publication, the Northern Ireland Department of Agriculture and Rural Development says that a range of fungi has been found to be associated with the plant but that they had to found to have insignificant effects on its growth. This leaflet simply says that no biological control is available.

Each giant hogweed plant is capable of producing about 50,000 seeds and, though they only drop close to the plant, they can be transported on shoe soles to other areas. The seeds remain viable for seven years meaning that eradication is a long and expensive process. After spending £250,000 in two years it was claimed that there was no flowering giant hogweed on the river Tweed in 2005. Since then, annual spraying in the spring is undertaken to keep the plant down. Sadly, the plants shown in the two videos above were filmed on the River Tweed network in May 2008 and 2009.

One of many hundreds of giant hogweed
plants in Edinburgh

The only alternative to a seven year programme of spraying is to completely remove the topsoil which may contain the seeds. Complete removal with a guarantee that it won’t return cost £20,000 for an area described as not much bigger than a small town garden.

Though the Heracleum sphondylium, cow parsnip or common hogweed, is very less frequently a cause of problems it does produce the same chemicals as giant hogweed. Not enough is known about the conditions required to increase the concentration of the furocoumarins to the point where harm can occur but, it would appear, that strong sunlight is required for the ordinary hogweed to produce burning.

The same seems to be true of Pastinaca sativa, parsnip - both wild and cultivated, and a page about this plant has been added to this site.