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Giant Hogweed - Weed KillerGiant Hogweed:

Heracleum mantegazzianum


Common names
Giant hogweed, cartwheel flower, The Hog, giant cow-parsnip.

Introductory History
Investigations of the introduction of any plant, especially if it was a long time ago, throw up interesting questions of taxonomy and nomenclature. Giant hogweed, Heracleum mantegazzianum, is no exception. Here is Gertrude Jekyll, probably the most influential gardener of the twentieth century, in a couple of articles written for "Country Life". These articles began 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 mien 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."

Correctly, Miss Jekyll ascribes the origin of the new plant to the western Caucasus. I have no doubt that she is describing Heracleum mantegazzianum. What then, was the Heracleum giganteum Mis. Jekyll put in her bog gardens before she became aware of classic giant hogweed?

If Miss Jekyll had referred to ‘Sanders’ Encyclopaedia of Gardening" of 1895, she would have seen Heracleum giganteum as synonymous with another now obsolete description, Heracleum villosum. This she would have read, grows even taller than Heracleum Mantegazzianum, which is described as having very large leaves. The information given in the "RHS New Dictionary of Gardening" of 1992, gives the modern understanding that Heracleum's giganteum and villosum are synonymous with stevenii. This plant is stated to grow to, by the same book, to a disappointing height of 1metre. Hardly giganteum!! I also note, however, that another invasive hogweed, Heracleum persicum, which is observed growing to 5 metres, only achieves 1.5 metres in the pages of the same dictionary.

The descriptive text about Heracleum stevenii is interesting. Parts are hairy; parts are rough and hairy. "villosum" means shaggily hairy.

Let us roll back the clock to 1849, when seeds of "Heracleum giganteum, one of the most magnificent plants in the world" were being offered by Hardy and Sons of Maldon. I suspect Miss Jekyll was right; this was a different name for a different plant.

If the history of nomenclature is correct, it may have been Heracleum stevenu, which has another modern synonym, which I will tell you about later.

Let us roll back a few more years, to 1836, when the gardener and architect and "conductor" of the "Gardeners' Magazine", John Loudon, was praising the "Giant Siberian Cow Parsnip, Heracleum asperum" and noting how it grew to over 12 feet tall in 5 months. What was this plant? Was it mantegazzianum or the former giganteum, which may be may be the modern Stevenii, or might it be something else? The description "asperum" means rough and bristly, which reasonably corresponds with the rough and hairy of stevenii.

Another interesting diversion now blows up. Loudon was so convinced that this plant should be planted in woodland glades or quiet churchyards, that he gave seeds to friends with instructions to plant in suitable places. One such friend was about to tour Norway. Norway has a great deal of giant hogweed, but it is known as "The Tromsø Palm" or Heracleum laciniatum. This plant is now known to be Heracleum persicum. The other accepted synonym of Heracleum laciniatum, which I promised to tell you about, is again Heracleum stevenii! The situation is far from clear.

Miss Jekyll's Heracleum giganteum might well have been true Heracleum persicum, which is slighter than the Heracleum mantegazzianum she came to prefer, and has far smaller flowerheads. Perhaps the seed, offered in 1849 as Heracleum giganteum, was also Heracleum persicum, and perhaps also Loudon's well-meaning friend, scattering the seeds of Heracleum asperum, was planting Heracleum persicum, and started the career of the Tromsø Palm in Norway. The introduction date there and recorded English origin of seeds certainly suggests so.

I am inclined to think that some plantings which are now assumed to have been Heracleum mantegazzianum in the UK in the 19th century, may well have been Heracleum persicum. (Heracleum persicum has a far more restricted and eclectic introduced range than Heracleum mantegazzianum, and has not become established here.) Heracleum mantegazzianum, if Jekyll did not know it until circa 1900, might have been planted rather less frequently than had been supposed, owing to taxonomical misinterpretations. Planted, however, it was; to spread with great vigour, becoming exclusively the problem hogweed here.

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 lost 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.

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.

Once again, the dramatic alien, introduced by gardeners at multiple sites, has grasped the agenda and used its natural advantages to invade. It is time for it to go.

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. Bracteoles in an involucel and are linear. Petals are white to dirty pink, measuring 12mm. Fruits are glabrous, sometimes villous, 9-11mm in diameter and bearing brown swollen resin canals.

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.

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.

Control Methods
Protection of the operators affecting the control of this plant is the first consideration when instituting a control programme. Long gauntlet gloves, waterproof overalls and boots and eye protection must be worn. Where powered cutters employing high-speed blades are employed, suitable 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 surprisingly, 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 muzzle, 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 several 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.

Removal of whole larger plants or umbel cutting can prevent seeding and with repetition, will cause the death of the plant. Timing should be early in full flower, which will reduce the vigour of re-growth. Seed maturation should not have begun as seeds can continue to ripen on cut plants. It will probably be necessary to collect and burn flowering material.

Because most seed is in the top 5cm of soil, ploughing to a depth of 24cm can effectively bury it and prevent further germination. Ploughing is most effective as a follow up to other methods of control.

Herbicidal treatments conducted when plants are between 20 and 50cm in height are highly effective, using Knapsack or sometimes weed-wipe methods.

The chemicals often used are 2,4-D and Triclopyr, both of which spare grasses Triclopyr, however, may not be used near water. Alternatively, the total herbicide Glyphosate may be used, which has the advantage of not inducing delay in revegetation because of soil persistence.

Follow up spraying before the end or May is advisable to catch late germinators, not present during the first procedure.

There may be some benefit in experimenting with Ammonium Sulphamate, which might assist re-vegetation by nitrifying the soil when it breaks down. This chemical is effective against native hogweed. (Heracleum sphondylium)

At this time, no viable biological controls have been identified.

Part of the control programme should include consideration of preventing re-infestation and suitable revegetation of the site.

In some instances, the quickest way to a competitive sward is to kill all remaining vegetation with glyphosate followed by deep ploughing and sowing at high densities with suitable grasses. Frequent mowing is then advised to produce a dense sward discouraging hogweed germination.

Combinations of sowing with grasses and mowing without prior herbicide use, in sensitive areas, for example by watercourses, can gradually eliminate hogweed and establish the dense sward, resistant to further infestation.

Grasses used in post-hogweed revegetation:
• Lolium Perenne
• Dactylis Glomerata
• Festuca Rubra
• Poa pratensis
• Festuca arundinacea

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