Giant Hogweed

SpeciesHeracleum mantegazzianum

FamilyApiaceae

Common namesGiant hogweed, The Hog, giant cow-parsnip and cartwheel flower.

A Brief History:

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 circulation of invasive plants; namely, gardeners and plantsmen. Here is Gertrude Jekyll, (probably the most influential gardener of the twentieth century), in a couple of articles written for "Country Life". These articles begin in the 1890's, but were mainly written after 1912.

"For cool, quiet places where there is a backing of copse, there should be free planting of the giant Heracleum mantegazzianum, with its very large leaves and branches of white bloom four feet across. It is a distinct improvement on the older kind of giant Cow Parsnip, Heracleum giganteum, for the leaves are larger and a glossier green, more deeply slashed and more sharply toothed, and the massive bloom is larger altogether."

In another article:

"Prominence would be given to the fine Heracleum Mantegazzianum, a notable improvement in all ways on the older Heracleum giganteum, give it the appearance of a specially proud and sumptuous plant. It is a native of Abkhasia, a small Russian province on the Black Sea at the foot of the Caucasus." Quite correctly, Miss Jekyll ascribes the source of the new plant to the western Caucasus.

Giant hogweed does what invasive plants do. Its height, spread and capacity for producing dense stands of plants, cuts out around 80% of daylight 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 lost of plant diversity leads to subsequent loss of the other organisms that depend on that very diversity. Like Japanese knotweed, (Which I will be covering in my next blog) its size and alien appearance, whilst sometimes breathtaking, does not look well with the visual congruity in our rural areas.

This mammoth plant grows from the seed to build an enormous rosette of foliage, with leaves up to 3 metres long.

When it has enough energy stored in its rootstock, it thrusts up its tall flower stem to hold multiple umbels of flowers, which can generate as many as 100,000 seeds. In extremely 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 so. Under more sympathetic 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 ancillary flower stems can be produced in seasons succeeding to the main flowering.

Its main claim to notoriety is in its ability to produce a virulent contact dermatitis on the human skin, (Phytophotodermatitis) 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 which are also found in other members of the Apiaciae and some other plants. Symptoms consist of intense reddening and extensive blistering, dark pigmentation of the skin, and a discoloured scarring, which remains noticeable for a very long time. Sap splashed in the eyes can have particularly bad consequences.

Organic Control Methodssafety of the operators affecting the control of this plant is the first concern when instituting a control programme. Long gauntlet gloves, waterproof overalls and boots and eye protection must always be worn. Where strimmers and powered cutters employing high-speed blades are used, suitable protective masks should be worn to prevent atomised sap being inhaled.

Unless circumstances dictate that the control method is cutting of the umbels, all other control methods should be started early in the growing season.

Perhaps most surprisingly enough I have found out, sheep and cattle can be used to graze the plants, and will develop a distinct liking for it, making substantial control possible. Introduction of animals early in the season on sites where there is still a mixture of vegetation, or performing a pre-grazing cut to encourage a mixed sward, reduces the possibility of initial rejection by the animals, or of toxic effects being experienced. Breeds with dark skin on exposed areas and external mucus membranes are unlikely to suffer phytophotodermatitis. (The nostrils, eyes, anus, genitals and udders of the grazing animals should however be monitored,) and any animals adversely affected, removed to clean pasture.

Mechanical methods include cutting the root of the plant at a depth greater than 10cm below ground level, using a sharpened spade or similar implement. The above ground plant parts are then pulled and spread to dry. This is a no-argument method for small infestations.

Mowing conducted 3 times in the growing season for a number of years will exhaust the seed-bank and finally the more mature plants, and can eliminate infestations. Competing grass swards should also benefit and become denser.