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].
Hybridises 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 hospitalised 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.