Giant Hogweed

Growth of colonies is mainly by marginal spread, due to the local area of seed dropping; though seed cast into rivers, especially in flood conditions, seed trapped in mud in the coats of animals, or in their hooves, as well as in car tyres and human clothing, can account for longer distance distribution.

A very high proportion of seeds germinate in the vicinity of the mature plant, usually earlier than native species. Although many of these die because of competition from their siblings, the rest get away and easily form a mass of first-year rosettes, about 8 metres in diameter. This mass can become occlusive in the second year. At this time, about 10% will flower and set more seed, whilst the remainder live and grow for subsequent seasons, until mature enough to flower. It is not difficult to see how quickly dominance can be achieved.

Heracleum mantegazzianum was first recorded on the seed list of the Botanic Gardens at Kew in 1817. A likely first incidence of introduction in the wild was 1828 in Cambridgeshire. The plant then spread through the greater part of Europe by 1900, mainly vectored by those most potent agents in the distribution of invasive plants; namely, gardeners and plantsmen.

It does what invasive plants do. Its height, spread and capacity for producing dense stands of plants, cuts out 80% of the light reaching lower growing species. Many of these are out competed as the plants germinate or start vegetative growth very early, and grow with alarming rapidity. This loss of plant diversity leads to subsequent loss of the other organisms that depend on that very diversity. Like Japanese knotweed, its size and alien appearance, whilst sometimes breathtaking, does not accord well with the visual congruity of the English countryside.

This enormous plant grows from the seed to build a great rosette of foliage, with leaves up to 3 metres long. When it has stored enough energy in the rootstock, it thrusts up its tall flower stem to hold multiple umbels of flowers, which may produce as many as 100,000 seeds. In exceptionally poor growing conditions it will delay the flowering stage until it is big enough to make a good job of it. This may take 12 years or more. Under more favourable conditions, flowering is from between the 3rd to the 5th year of life. The plant is usually monocarpic, and its life ends when seed has been set. It is reported that some individuals form perennating crown buds arising from the rootstock, and that subsidiary flower stems can be produced in seasons subsequent to the main flowering.

Seeds, when shed in autumn, do not immediately germinate, having immature embryos and are dormant. They form a short term seed bank in the top 5 cm of soil, which can amount to 12,000 living seeds/m2. The great majority of seeds germinate the next spring, following embryo development and winter chill, though a small proportion remain dormant for two years or more, which provide the plant's progeny with disaster insurance. Individual plants are substantially self-fertile, so an isolated plant, dropped for example, as a seed from mud in a cow's foot, is quite capable of starting a new colony, without help.

A massive biennial to short-lived perennial measuring up to 6 metres in height. Usually monocarpic, but occasionally perennating by subsidiary crown buds arising from the rootstock. The whole plant has a powerful odour.

Stems grow to 12.5cm diameter, and are hollow, ridged, blotched and stained purple, branching with height.

Leaves are immense, ternate or pinnate with each segment to 1.3 metres and pinnately lobed, covered in a coarse pubescence. Leaf margins are irregularly serrate.

Flowers grow in massive umbels to 75 cm diameter, (occasionally to 120cm) rays of 50 to 150, from 15 to 50cm in length. Involucral bracts are linear to ovate.  Petals are white to dirty pink, measuring 12mm.

Fruits are glabrous, sometimes villous, 9-11mm in diameter and bearing brown swollen resin canals.

Its main claim to notoriety is in its ability to produce a virulent contact dermatitis on the human skin, when conditions of strong sunlight and high temperatures and humidity combine with skin exposure to sap.

The chemicals causing this unpleasant syndrome are known as Furocomarins (Syn Furanocoumarins) which are also found in other members of the Apiaciae and some other plants. Symptoms include intense reddening and extensive blistering, dark pigmentation of the skin, and a discoloured scarring, which remains visible for a very long time. Sap splashed in the eyes can have particularly bad consequences.

Some Furocoumarins may be carcinogenic, or be associated with birth defects.

Giant hogweed is by no means the only cause of Phytophotodermatitis, but it seems produce very severe symptoms. In many years gardening I have seen cases, some quite unpleasant, associated with other plants, such as garden rue, spurges, ordinary hogweed, hemlock and gone-to-seed coriander.

I also suspect (without the least evidence) that the much greater seeming incidence of toxic effects reported, may indicate that the chemical content of the giant hogweed plant is increasing. It seems that the problem only gained public awareness in about 1970. The great plantsman, Graham Stuart Thomas, writing in 1976, mentions only very occasional reports, and I certainly revelled in playing with the huge stems about 45 years ago - to no ill-effect.