GIANT HOGWEED

CARTWHEEL PLANT, GIANT COW PARSLEY

Heracleum mantegazzianum

Carrot Family [Apiaceae]  

month8jun month8june month8jul month8july month8Aug

status
statusZneophyte
 
flower
flower8white
 
morph
morph=HemiZygo
 
petals
petalsZ5
  (10)
type
typeZclustered
 
type
typeZumbel
 
stem
stem8round
 
stem
stem8hollow
hollow
contact
contactZsevere
 

5th May 2010, River Irwell, Prestwich Forest Pk, M/cr. Photo: © RWD
A young plant is very low and probably even more photo-toxic than older specimens.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The plant is taller than it looks, it is growing lower down the river bank upon which the photographer stands. At this stage the leaves look a little like those of an ornamental Rhubarb: large and fan-shaped with large teeth. Giant Hogweed can grow to 4 or even 5.5m in height, taller than the 2 to 3m of Hogweed.


25th June 2005, Peak Forest Canal, Whalley Bridge, Derbyshire. Photo: © RWD
Right at waters-edge. These have now been eradicated. The leaves on Giant Hogweed are a slightly brighter green than those of the similar Hogweed.


25th June 2005, Peak Forest Canal, Whalley Bridge, Derbyshire. Photo: © RWD
The large fan-like leaves have a snow-flake like symmetry.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
British Waterways have now had these removed, but they will soon be back, for the plant grows on several miles upstream, and the seeds float downstream.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The leaves are large growing up to 3 metres long by 2 metres wide, ternate or 1-pinnate, highly divided and with coarsely toothed lobes.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The umbels are triply compound, with up to 8 smaller satellite umbels around a larger central umbel at the terminus of the main stem of over 40cm across. The terminal umbel is usually larger than 40cm across and with up to 50 umbellets each with between 50 and 120 rays (Hogweed has but 10-20 rays in the terminal umbel). Satellite umbels have fewer umbellets with each umbellet having fewer flowers than the main umbel. Each pair of seed pods produces 2 seeds in total (2 per flower). So, potentially, there are up to about 100,000 seeds per plant.



21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The terminal umbel is largest and up to a metre across.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The largest petal is deeply cut and forms a V-shape


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
Five stamens with greyish-black anthers bearing cream-coloured pollen. Pedicels (flower stalks) have glandular hairs as does the nearly spherical ovary just under the flower.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
Like many umbellifers, the outer petals are much longer and more deeply cleft than inner facing ones.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
Note the fine thread-like bracts beneath each sub-umbel.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The large cupped bracts at each 'armpit'.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The blotched stem.


5th May 2010, River Irwell, Prestwich Forest Pk, M/cr. Photo: © RWD
The red-blotched stem of a young plant, with transparent hollow hairs or trichomes, full of furanocoumarins. The hairs seem to vanish from mature specimens. The base of the hairs has a conspicuous 0.6mm to 1mm wide oil-body which contains the dangerous furocoumarins (the furocoumarins are also present in the sap). It should not be touched without wearing protective clothing.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
Between five and seven flower umbels surround the single seed umbel.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
The seed pods. Each mericarp contains but one seed but a single plant can produce between 20,000 and 100,000 seeds.


19th Aug 2015, sewage works road, Clifton, Gtr M/cr. Photo: © RWD
At last your Author has found some ripe seed pods that are at head height rather than at an un-reachable 3 to 5m in the sky. Unfortunately these have been splashed with wet dirt by the passage of fast lorries along the road. You cannot have it both ways. The seed pods are larger than those of Hogweed and elliptical (rather than nearly round) along the axis of symmetry.


19th Aug 2015, sewage works road, Clifton, Gtr M/cr. Photo: © RWD
The seed pods are flat and in parallel pairs (best seen in the pod just left of dead centre)., similar to those of Hogweed. Each mericarp of a pair contains but one seed (2 per flower). The four oil ducts have very long curving teardrop shape as those of Hogweed (top left, or right middle). At 10-14mm long the fruits are longer than those of Hogweed which are about 6mm long.


19th Aug 2015, sewage works road, Clifton, Gtr M/cr. Photo: © RWD
The ripe fruits have much longer remnants of styles than do those of Hogweed, almost as long as the fruit itself and they hang down (they curve upwards in Hogweed).


10th May 2015, nr River Irwell, Ringley, Bolton, Lancs.. Photo: © RWD
The giant fractal-like fan-shaped leaves on a low young plant.


21st June 2007, M/cr Bolton & Bury River Lock, M/cr. Photo: © RWD
Overlapping leaves.


10th May 2015, nr River Irwell, Ringley, Bolton, Lancs.. Photo: © RWD
Unusually some stems are square, not the more usual round.


10th May 2015, nr River Irwell, Ringley, Bolton, Lancs.. Photo: © RWD
The stems have deep grooves along the length whether they are square or round. Note the long white hairs.


10th May 2015, nr River Irwell, Ringley, Bolton, Lancs.. Photo: © RWD
Your Author was certainly not going to break the brittle stem of this plant with his bare hands! He instead broke the stem of a 1-foot high young plant by kicking it hard. It has a small hollow channel running the length of the stalks surrounded by a light-green area which in turn is surrounded by a white annulus with yet more vascular filaments running through it. This, at just 2cm across, is just a thin stem of a very young plant; those 4m high can be up to 10cm across.


19th Aug 2015, sewage works road, Clifton, Gtr M/cr. Photo: © RWD
Cut by a roadside hedge-slayer. Older stems are very hollow but structurally rigid.


Not to be confused with Giant Knotweed nor with Giant Montbretia (Crocosmia masoniorum), Giant Scabious (Cephalaria gigantea), Giant Butterbur (Petasites japonicus), Giant Viper's Bugloss (Echium pininana), Giant Herb Robert (Geranium maderense), Giant Onion (Allium giganteum), Giant Redwood (Sequoiadendron giganteum), Giant Fescue (Festuca gigantea), Giant Himalayan Bramble (Rubus armeniacus), [plants with similar names belonging to differing Families].

Hybridizes with : Hogweed (Heracleum sphondylium) to produce Heracleum sphondylium × mantegazzianum and is intermediate in size, hairiness, leaf shape and fruit character between the two but has a very low fertility.

Some similarities to : Hogweed but Giant Hogweed is up to twice as tall at 4 or 5.5 metres, the flower umbels of larger diameter up to 1m across, and the fruits are narrower. The leaves of Giant Hogweed are not only larger, up to a metre long, but differ in form being more fan-shaped, rather than having great chunks missing from the edges which is what Hogweed looks like.

When it has half-grown, but its flowers are still wrapped up in a cream-coloured bract it can look a little like an ornamental Rhubarb.

The large leaves have some similarities in shape to those of Chilean Giant Rhubarb and Brazilian Giant Rhubarb and for which it may be mistaken when young and flower-less, but the stems of Giant Hogweeed lack the short stubby curved spines of these two Giant Rhubarbs. Uniquely identifiable characteristics : There is no other umbellifer as tall and large as this one!

Distinguishing Feature : It's is very tall, over 1.5m (up to 5.5m).

It is a neophyte, originally from SW Asia having been introduced and naturalised in the UK, and which is proving troublesome to control let alone eradicate.

The Flowers are white or off-white. Like as in many umbellifers, the outer petals are longer than those on the inner-facing side of flowers, especially those flowers on the outer edge of the umbel.

Giant Hogweed is a troublesome spreading weed, that proliferates especially along water-courses such as canals and especially rivers. With up to 100,000 seeds per plant, and the high ability to self pollinate and high-seed viability, one plant can produce a whole colony miles down the water-course. Great efforts are being expended in trying to eradicate any out-breaks. The two clusters where the above photographs were taken have now been eliminated. Its extermination is being pursued vigorously because not only does it spread un-controllably but it shades other plants from growing.

Furanocoumarins are exploited medically for the treatment of psoriasis, which is somewhat surprising as you might have thought they would cause psoriasis.

Because the stems are very stiff, long and hollow, children have a propensity to play swords with them or reportedly use them as pea-shooters (although this seems unlikely given their size and girth!). Many were hospitalized in the 1980's as a result of the severe photodermatitis that ensues on subsequent exposure of the skin to sunlight. Extensive skin contact with the sap and subsequent sun-exposure can lead to death in severe cases. The sap of Hogweed and Wild Parsnip also contain much the same photosensitizing furocoumarins, and should also be handled with care (using strimmers to hack them down is not recommended!).

Giant Hogweed has but a tap root and does not reproduce vegetatively. Each plant produces up to 5000 seeds which are able to float, and being next to flowing water-courses and rivers, spread far along the banks. Eradication is not easy; it is largely un-affected by weed-killers unless the plant is very young.

The Giant Hogweed found in the UK is very variable, differing in leaf and fruit, and it is thought that more than one species may be extant. The available suspects are H. grossheimii, H. lehmannianum, H. persicum, H. trachyloma and H. mantegazzianum. The latter is widely reported but in actual fact may not be present as an invasive species in the UK. Instead H. trachyloma, H. grossheimii and H. lehmannianum have been reported by some researchers which have previously been assigned to H. mantegazzianum. H. persicum is known to have been introduced into the UK in the early 19th Century. But the question remains whether any H. mantegazzianum grows naturalised in the UK (or not). The exact species naturalised in the UK is still open to much debate. Research is ongoing. This is important because it is only H. mantegazzianum that is illegal to plant or import in the UK; none of the other species are. And yet more species are extant in Europe, none of which are illegal to import! However, all species contain both Coumarins and FuroCoumarins, the exact identity of which varies according to species as do the proportions. All are capable of causing photosensitization.

The Author thinks that, instead of procrastinating for many years over exactly which species are present or not, the government should just ban all giant species of Heracleum. This would help prevent further infestation by other possibly even more virulent (in the UK) species. When the research is finished, they could then, and only then, allow benign species in, once they are certain that the benign species will not actually turn rampant once they are within the UK (allowing for climate change in the UK too). Common sense rules here.

FUROCOUMARINS (aka FURANOCOUMARINS)

Giant Hogweed contains exceedingly irritant chemicals in its sap. These chemicals are called Furocoumarins (aka Furanocoumarins), and cause photo-sensitisation of skin. When sap on the skin is exposed to sunlight, chemical reactions occur which cause photo-dermatitis, about 40 to 50 hours later resulting in the itching and intense reddening and formation of enormous wheals on the skin which can be troublesome to heal. The resulting darkening pigmentation of the skin and scarring lasting several years. Even skin once exposed to the sap, can remain photosensitive for 20 years or so, with the subject having to keep the sun off that part of the skin which was once in contact with the sap. Exposure of the skin to greater amounts of these furocoumarins can damage the kidneys and liver and can be fatal. Furanocoumarins are very dangerous substances, and, being phototoxic, cross-link with both strands of DNA in the presence of strong sunlight. This action leads to cell death by apoptosis, and also to mutation; Furocoumarins are cytotoxic. Blindness can result if any sap should enter the eye. Needless to say, these compounds are even more dangerous if ingested orally.

The Furocoumarins in Giant Hogweed are Heraclenin, Heraclenol and Xanthotoxin (aka 8-methoxypsoralen, or 8-MOP), upon which Imperatorin, Heraclenin and Heraclenol are all based. All three are photo-toxins and carcinogens. See the Hogweed page for the toxic modus operandi of FuranoCoumarins.

Imperatorin is another Furanocoumarin, possibly an intermediate stage between 8-methoxy Psoralen and Heraclenol and Heraclenin, shown below.


Heraclenin is identical to Imperatorin except that the double-bond between carbon atoms has been broken and replaced by an epoxy linkage using an additional oxygen atom.

Heraclenol is identical to Heraclenin except that the epoxy linkage has been hydrolysed with an extra molecule of water, creating two hydroxy groups.

p-Cymene (para-Cymene), also in Giant Hogweed, is a monoterpene that is toxic to animals, and yet present in a number of essential oils such as oil of Thyme and oil of Cumin. Ortho- and meta-Cymene can be synthesized in the laboratory, but the isomer para-Cymene is the only naturally occurring isomer of Cymene. It forms complexes with ruthenium and osmium.


  Heracleum mantegazzianum  ⇐ Global Aspect ⇒ Apiaceae  

Distribution
family8carrot family8Umbelliferae  family8Apiaceae

 BSBI maps
genus8heracleum
Heracleum
(Hogweeds)

GIANT HOGWEED

CARTWHEEL PLANT, GIANT COW PARSLEY

Heracleum mantegazzianum

Carrot Family [Apiaceae]  

WildFlowerFinder Homepage