THE POISON GARDEN website Arum maculatum berries on a Cannabis leaf 


This free script provided by JavaScript Kit

Click for menu of plants in the A to Z section

Heracleum mantegazzianum, giant hogweed

giant hogweed, Heracleum mantegazzianum

Heracleum mantegazzianum,
giant hogweed


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);
Plenty of giant hogweed in Edinburgh, as usual
MailOnline invents a new name for giant hogweed to try and make it more scary
Would giant hogweed be found on an island in the Adriatic?
'Poisonous Plants 1-2-1' video about giant hogweed
Applying the Jerry Maguire Test to myself
TV programme shows that 'natural habitat' for Alaskan bears has been invaded by hogweed
New Zealand has problems with giant hogweed in gardens
Two more local news reports on burns from giant hogweed
The Portadown Times reports on teenagers severely burned by giant hogweed
As the plants in Edinburgh set seed, there is evidence that control does work (photo-blog)
Edinburgh hogweed is now in full flower (photo-blog)
A month later and still no sign of any spraying in Edinburgh; flowers are starting to form
12 days on the Edinburgh hogweed continues to thrive
Widespread giant hogweed in Edinburgh provides an identification guide
The Tweed Forum Invasives Project annual report shows there is still work to do.
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

'Poisonous Plants 1-2-1' video

This short video summarising the story of the giant hogweed is just one of a series.



Giant hogweed in flower

Giant hogweed in flower

Meaning of the Name

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.
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 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, deadly nightshade, 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.


Heracleum sphondylium

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.

In the summer of 2015, reports of a couple of children suffering very nasty burns to their hands and arms led to a number of other reports in the press. There were calls for more action by government and local authorities. Edinburgh City Council was one of those who assured the public it was doing everything it could to control the plant - all evidence to the contrary. There was a report of compensation being paid by one private landowner to someone who came into contact with the plant on his land. It must be expected that this will lead to further claims.

As always, it is impossible to know if there were genuinely more incidents in 2015 or whether the incidents that happened were more likely to be reported than in previous years. The plant's science is not well enough understood to be able to say whether 2015 was an especially bad year or, if the plants were more harmful, why that was.

After an apparently quiet 2016, reports of hogweed burns were more prolific in 2017. The press claimed, based on nothing, that it was feared that the hot weather had made the plant 'stronger'. As in 2015, there is no way to know whether increased awareness led to more reports rather than there actually being more incidents.

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 anecdotes of children using the hollow stems as peashooters and suffering burns around the mouth.

Though cattle and sheep are used to graze off the very young shoots animals can be affected by the plant. There are a few accounts of dogs suffering burns after contact so animals should be kept away from larger plants.

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

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. In 2012, the situation was as bad, if not worse, and you can follow the links above to see photo-blog entries showing plants in Edinburgh.

In July 2012, the Portadown Times reported on some teenage boys who had suffered severe burns after contact with giant hogweed. The paper kindly gave permission for photos of the boys to be reproduced on this site.

Burns from Heracleum mantegazzainum, giant hogweed

Copyright 2012, the Portadown Times. Used with permission.

Folklore and Facts

There is a theory that plants produce furocoumarins as a defence against attacks from a root fungus. Anyone suffering burns is, therefore, suffering 'collateral damage'. The theory could explain why other genera and species will, sometimes, cause similar problems but only sometimes. My thought is that, perhaps, the Heracleum mantegazzianum was not exposed to this fungus in its native territory and still over-reacts to a fungal attack resulting in the excessive output of the chemicals. That might, also, explain why the plant importers seem to have been unaware of the problems the plant could cause.

These will have to remain speculative unless anyone is willing to fund detailed research into the issue.

This hypothesis suffered a setback when, in July 2017, a correspondent reported encountering a species of Heracleum in Georgia, the former Soviet republic, and suffering burns as a result. The picture is confused though because the same correspondent provided pictures of the plant being harvested and transported in large bundles for use medicinally. The collectors appeared to have no concerns.

leaves of a young giant hogweed

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.

Heracleum mantegazzianum, giant hogweed

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. 


The POISON GARDEN website is not connected with Alnwick Garden Enterprises Ltd and/or The Alnwick Garden Trust.


Site Update

All the pages in the A to Z section are regularly updated.

If you've had a personal encounter with a poisonous plant please use the contact form to tell us about it.

A to Z Links

Not familiar with botanical names? Try this common name A to Z converter

Introduction to the A to Z section
Abrus precatorius, rosary pea
Aconitum lycoctonum, wolfsbane
Aconitum napellus, monkshood
Actaea racemosa, black cohosh
Actaea spicata, baneberry
Aesculus hippocastanum, horse chestnut
Amanita muscaria, fly agaric
Aquilegia atrata, columbine
Aristolochia clematitis, birthwort
Artemisia absinthium, wormwood
Arum italicum, Italian cuckoopint
Arum maculatum, cuckoopint
Aspergillus fumigatus
Atropa belladonna, deadly nightshade
Brugmansia suaveolens, angel's trumpet
Bryonia dioica, bryony
Buxus sempervirens, common box
Camellia sinensis, tea
Cannabis sativa, marijuana
Catha edulis, khat
Chelidonium majus, greater celandine
Cimicifuga racemosa, black cohosh
Claviceps purpurea, ergot
Clematis vitalba, old man's beard
Colchicum autumnale, naked ladies
Conium maculatum, poison hemlock
Convallaria majalis, lily of the valley
Cynoglossum officinale, hound’s tongue
Daphne mezereon, spurge olive
Datura stramonium, thorn apple, jimsonweed
Datura suaveolens, angel's trumpet
Delphinium, larkspur
Digitalis spp., foxglove
Dracunculus vulgaris, dragon arum
Echium vulgare, viper’s bugloss
Eranthis hyemalis, winter aconite
Erythroxylum coca, cocaine
Euonymus europaeus, spindle tree
Euphorbia x martinii, red spurge
Euphorbia pulcherrima, poinsettia
Fritillaria spp., fritillary
Galanthus nivalis, snowdrop
Hedera helix, common ivy
Helleborus spp., hellebore
Heracleum mantegazzianum, giant hogweed
Hyacinthoides non-scripta, bluebell
Hyoscyamus niger, black henbane
Ilex aquifolium, holly
Jacobaea vulgaris, ragwort
Juniperus communis, common juniper
Laburnum anagyroides, laburnum
Lactuca serriola, prickly lettuce
Leucojum aestivum, snowflake
Lithospermum officinale, gromwell
Lolium temulentum, darnel
Malus 'John Downie', crab apple
Mandragora officinarum, mandrake
Mercurialis perennis, dog’s mercury
Narcissus, daffodil
Nepeta faassenii, catmint
Nerium oleander, oleander
Nicotiana sylvestris, tobacco
Oenanthe crocata, hemlock water dropwort
Papaver somniferum, opium poppy
Pastinaca sativa, parsnip
Polygonatum odoratum, angular Solomon's seal
Prunus laurocerasus, cherry laurel
Pulsatilla vulgaris, pasque flower
Ranunculus acris, meadow buttercup
Rheum x hybridum, rhubarb
Rhododendron spp.
Rhus radicans, poison ivy
Ricinus communis, castor oil plant
Rosmarinus officinalis, rosemary
Rumex obtusifolius, broad-leaved dock
Ruta graveolens, rue
Salix alba, white willow
Salvia divinorum, sage
Scutellaria laterifolia, Virginian skullcap
Senecio jacobaea, ragwort
Solanum dulcamara, woody nightshade
Solanum melongena, aubergine
Strychnos nux-vomica, poison nut
Symphoricarpos albus, snowberry
Symphytum spp., comfrey
Taxus baccata, yew
Toxicodendron radicans, poison ivy
Thevetia peruviana, yellow oleander
Urtica dioica, stinging nettle
Veratrum album, white hellebore
Verbascum olympicum, Greek mullein
Vinca major, greater periwinkle
Viscum album, mistletoe
Vitex agnus-castus, chaste tree