Giant hogweedGiant hogweed

Name: Giant hogweed
( cartwheel flower, common hemlock, giant cow parsnip (Pictures thanks to ©Peter L Herring) )

Latin name: Heracleum mantegazzianum Sommier & Levier, (H. asperum, H. giganteum, H. villosum)

Occurrence: An introduced biennial to monocarpic perennial naturalised on roadsides, hedgerows, waste places and rough grassland. Giant hogweed is scattered throughout the British Isles and is locally common especially along riverbanks. A native of the Caucasus mountains of south-west Asia, it was introduced in the late 19th century as an ornamental plant. Spread of the weed increased greatly between 1930 and 1982. In Britain, giant hogweed is mainly a lowland plant. It is resistant to frost.

Giant hogweed is the largest herbaceous plant in the European flora. Naturalised plants are very variable in perennation, size and shape. Hybrids occur between the giant hogweed and the common hogweed (H. sphondylium).

The stem and leaf stalks contain a sap that is released by handling, cutting or just brushing against the plant. The sap contains furanocoumarins that photosensitise the skin so that exposure to sunlight results in painful blistering. Some injury may be seen in stock that graze the plant but usually there are no long-term effects. Higher levels of furanocoumarins are found in plants growing in open sites than in plants growing in the shade.

Giant hogweed is a host of carrot fly (Psila rosae).


Biology: Giant hogweed flowers in June and July. It can take 4 years for a giant hogweed to flower after which the whole plant normally dies. The flowers are insect-pollinated and self-compatible. A single flower head may have over 5,000 seeds and a plant may produce 50,000 to 80,000 seeds. Seeds are shed from late August to mid-October.

Fresh seed may have an after-ripening requirement. Most seeds germinate in the year after shedding and following a period of natural stratification over the winter. Seed mixed into the surface soil and stirred periodically emerged from January to June. The main peak of emergence was in April. Other studies suggest that a few seeds germinate in autumn and in mild spells in winter and early spring, with most germination from January to March. In the wild, seeds germinate well in surface organic matter and detritus if adequate moisture is present. A sufficient depth of soil is then required to allow the taproot to develop.

In the first year, plants can develop leaves over 1 m wide. The dense rosette of leaves suppresses other vegetation. The plant dies down in winter then regrows in March-April from the large fleshy taproot. In the second, third or fourth year it sends up huge flat topped flower heads on a hollow stem up to 4 m tall. Plants may need to develop taproots of a minimum size before flowering can occur.


Giant hogweedPersistence and Spread: Seeds can remain viable in soil for 15 years. Seeds in dry-storage remain viable for up to 7 years.

Giant hogweed out-competes other vegetation due to its size and forms pure stands that expand from year to year if not controlled. The flat seeds are dispersed by the wind, water and by man. The removal and transport of seedheads for use in flower arrangements has aided dispersal. Normally, most seeds fall around the parent plant. Seeds from plants growing on riversides are carried downstream and deposited further along the riverbank. Infestations often begin as a single plant that sets seed and soon forms a small colony.


Management: Control must begin with the prevention of seed spread but seed already present in the soil seedbank will continue to emerge for 3-4 years. Seedlings and young plants can be hand pulled, larger plants can be cut down to ground level or may be dug out but full protective clothing must be worn to prevent any contact with the sap. The taproot should be cut 8-12 cm below ground level. Cutting plants at or above ground level encourages vigorous regrowth. Plants should be cut every 2 weeks in spring. It is not advisable to attempt cutting plants that are taller than 1.5 m. Bare areas of soil left after removal of the weed should be sown or planted with native vegetation to help prevent recolonisation by the giant hogweed.

On agricultural land ploughing will destroy seedlings, however, the roots of large plants may regrow.

Cattle, sheep, pigs and goats can graze the plant with no apparent ill effects. Grazing should begin in mid-March and continue through the growing season. Growth is suppressed but rotational grazing does not kill the plants. Pig foraging should eradicate the plant through root damage.

Giant hogweed is attacked by a number of insect species but none do sufficient damage to be useful as potential biological control agents. Fungal pathogens also infect the giant hogweed, including powdery mildew (Erysiphe heraclei) and various soft rots. Snails graze the plant.

Updated October 2007.

Further Information / Links:

For more information on this weed

»UK farmers' case studies

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Comments

  1. We have a terrible infestation of Giant Hogweed and no way of controlling it as it is growing along and in the banks of the river that intersects our farm!The seed has washed down and lodged and the E.A cannot come up with a cohesive stratedgy for dealing with this Giant of Giants!The river banks are very steep and sandy and fenced from stock and so destruction of individual plants is our only ploy to date.
    - GRANT ROFFEY 2---1-2005

  2. I have found Hogeed in Minnesota what should I do .
    - John Kimmel 6---0-2006

  3. I have found Hogweed in southern Minnesota what should I do ? Call DNR ?
    - Ben John 6---0-2006

  4. For the last 3 yrs I have had an outbreak of the sores that you explained and have not been able to identify what the sourse was. We had an ariticle in the news paper about it and it said that you could go blind from it. I looked this site up and know that I have this on our bank where Iweed. I live in the area of Elmira NY.
    - Mae 7---0-2007

  5. Call your local Environmental Conservation group. They will know what to do. Whatever you do, do not handle the plant!
    - Rachel 7---0-2007

  6. we have a few hogweed plants in peterborough (uk) a dog got killed by this plant and a man went blind
    - kerryann 9---0-2007

  7. my boy jordan age 7 was playing next to a burn and got mud on his legs as he was wearing shorts he grabbed a leaf of a plant and rubbed the mud of both legs then carried on playing about 24 hours later a red rash appeared on both legs we thought it was sunburn but later on on the day his legs started to swell we took him to hospital where they were quite confused to what it might be and sent us home with some medicine but the next day we went back to hospital small blisters started to appear he got admitted and for 3 days he was in agony on parasetimal when in fact he should have been on morphine eventually a skin specialist recognised it was the the sap off a plant called hogweed by this time the blisters where the size of tennis balls after another 10 days he was released thats been 2 weeks now were still dressing his wounds 3 times a day and he will be scarred for life i have pictures but to imagine in your head its as if someones poured acid over his legs from the knee down jordan has survived 3 open heart operations the last one being in april this year 07 had this plants sap got into his bloodstream we could have lost him it can blind you permanently if it comes into contact with eyes it is a very dangerous weed and is illegal to plant we are currently talking to our mp and the media to highlight the dangers of this highly toxix plant a lot of authorities are sticking there heads in the sand because it would cost to much to search and destroy but they dont realise is the problem is going to get much worse and it will cost alot more in cash and injuries i think it should be introduced in education to primary schools before anyone else gets badly burnt 10/08/2007
    - david mccullagh 9---0-2007

  8. my boy jordan age 7 was playing next to a burn and got mud on his legs as he was wearing shorts he grabbed a leaf of a plant and rubbed the mud of both legs then carried on playing about 24 hours later a red rash appeared on both legs we thought it was sunburn but later on on the day his legs started to swell we took him to hospital where they were quite confused to what it might be and sent us home with some medicine but the next day we went back to hospital small blisters started to appear he got admitted and for 3 days he was in agony on parasetimal when in fact he should have been on morphine eventually a skin specialist recognised it was the the sap off a plant called hogweed by this time the blisters where the size of tennis balls after another 10 days he was released thats been 2 weeks now were still dressing his wounds 3 times a day and he will be scarred for life i have pictures but to imagine in your head its as if someones poured acid over his legs from the knee down jordan has survived 3 open heart operations the last one being in april this year 07 had this plants sap got into his bloodstream we could have lost him it can blind you permanently if it comes into contact with eyes it is a very dangerous weed and is illegal to plant we are currently talking to our mp and the media to highlight the dangers of this highly toxix plant a lot of authorities are sticking there heads in the sand because it would cost to much to search and destroy but they dont realise is the problem is going to get much worse and it will cost alot more in cash and injuries i think it should be introduced in education to primary schools before anyone else gets badly burnt 10/08/2007
    - david mccullagh 9---0-2007

  9. Genesis have a brilliant song describing this scurge.
    - jim lockey 9---0-2007

  10. I was fishing a few Months ago, and My mate Slapped me across the back with one of the flower heads, A few weeks later i had Blisters on my back and he had em on his hands and arms. I think they are similar to Ragweed,
    If u have to handle them, do it at night or wear long sleeves
    - Cathal mc kinney,, Ireland 0---1-2007

  11. This is a very dangerous plant to anyone not aware of the injuries it can cause if touched with bare skin. Early detection is crucial if one wants to dig it up, but always wear protective clothing and gloves. Tall plants should be avoided at all cost unless one's body is fully protected from the sap.
    - Mohawk52, Basingstoke, England 3---0-2008

  12. swollen thumb due to weeding this plant. still painful after 4 days i think i cut myself so poss tiny bit in bloodstream. no burning or blisters. Maybe different types of hogweed ahve different effects (mine, i think from giant hogweed). seeing doctor tomorrw. I will let you know. Will wear gloves in future or use round up. sorry to hear about the lad.
    - steve 7---0-2008

  13. i was walking in hackney marshes with my wife n 2 daughters when i noticed a masive giant hogweed plant and if i hadnt of known about the dangers of this my 3 year old n 1 year old would have been free to rome around and even come in to contact with it. because to the untrained eye its just a harmless plant. i think it should be taught in schools.also i think the park rangers should have full knolage of the plant as well as the danger it poses and carry the duty to report it and restrict acess!!!FROM PUBLIC
    - soner koroglu london hackney 7---0-2008

  14. hi could any one tell me if hogweed can kill shetland ponys
    - critchell 8---0-2008

  15. Hogweed is infact the most effective way to kill a Shetland pony. It is also useful again rogue moles, and hairlice affecting dolphins. Clement Atlee once used the hogweed juice to paint his drawing room.
    - Hercules Mantegazzer 9---0-2008

  16. I was out with some friends, quite recently infact, one of my friends took the liberty of picking one of these plants and chasing me with it, I explained to him what it would do, he was rather shocked.

    A few days later, blistering. He was licky, no scarring, it was a halfbreed.
    - Scott Dobbie 9---0-2008

  17. I owned a river fishery for salmon and sea trout until recently with natural alderwood margins such as are environmentally advised for river habitat. Yet ironically
    these proved the perfect habitat for Giant Hogweed
    which remained totally ineradicable (except maybe after
    ghastly side effects and unacceptable expense). We noted no damage to wildlife nor to flora, no inconvenience to anglers. I suspect the anathema exercised against this interesting plant is simply yet another example of iurban do gooders working up a new enthusiasm for making life difficult for countryide managers and their white feathered apologists.
    - Martin Stransfeld 2---0-2009

  18. Just thought pepole might want to know the toxic compound of this plant, from the photogenic oil/sap which that compound is most likely furanocoumarius?, (whatever the chemistry of the compound is, I do not know as of yet).
    - Edward 3---0-2009

  19. I know of several fields where loads of giant hogweed existed side by side with Shetland ponies, in all seasons for some years, no ill effects were seen, as is confirmed by the owner.
    - Jules the progenator 1---1-2009

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