Giant hogweed (Heracleum mantegazanium) is a member of the carrot family. As its name suggests it is extremely large: in summer it produces white umbelliferous flowers which resemble cow parsley but on a substantially larger scale – the blooms may be over two feet in diameter. Capable of growing up to 20 feet in height (USA Forestry Images) it is unwelcome because of its capability for causing both environmental problems and damage to human health.
Native to the mountains of the Caucasus in central Asia, giant hogweed, like many other non-native terrestrial and aquatic plants, was brought to Europe by Victorian collectors who regarded it as a curiosity. It became popular among gardeners in Europe and elsewhere but soon escaped from gardens and became established in the countryside, where it favours disturbed ground. It is often found in urban areas, along paths, railway lines and waterways.
Giant Hogweed and the Law
Giant hogweed is considered such a serious problem that it is in many cases legally restricted. In the United Kingdom it is an offense to plant it or to allow it to grow in the wild, while any plant material (including the soil in which it is grown) is classified as controlled waste: its disposal requires documentation (UK Department of the Environment, Food and Rural Affairs). It is not, however, against the law to grow the plants in a garden.
In the USA, giant hogweed is classified as a Federal Noxious Weed, a designation which makes it illegal to bring the plant into the country or to move it across state lines. In addition, it is against the law to trade in the plants or their seeds, or to give them away (Michigan Department of Agriculture).
Environmental Problems of Giant Hogweed
In the USA and much of Europe, giant hogweed is considered to be an invasive non-native species. Because of its size, and the rapidity of its growth and spread, the number of seeds it produces (according to the New York State Department of Environmental Conservation, an average plant produces 20,000 seeds) it causes considerable environmental problems.
The plant crowds out other native species, denying them light (because of the area shaded by its leaves) and nutrients, through its deep taproots. When the plant dies back in winter, it exposes large areas of bare soil which are vulnerable to erosion, especially in one of the plant’s favoured habitats, along waterways (Applied Vegetation Dynamics Unit).
Medical Problems of Giant Hogweed
A detailed description of the impacts of giant hogweed on human health is produced by the New York State Department of Environmental Protection. The plant’s sap contains a range of photosensitising chemicals which react strongly to ultraviolet radiation. If skin with these chemicals on is exposed to sunlight, the effects can be severe, with blistering or burns occurring. These blisters may lead to scarring and if the sap gets into the eyes it can cause blindness. The scarring may last for several years.
Given the potential damage to human health, it is advisable not to touch the plants, and to avoid allowing the sap to come into contact with the skin. Anyone dealing with the plants should wear protective clothing and ensure that any sap on that clothing doesn’t come into contact with the skin at a later stage. If sap does touch the skin it should be washed off thoroughly with soap and cold water.
Getting Rid of Giant Hogweed
Giant hogweed grows rapidly, sets seed and then dies. The thousands of seeds which the plant produces are set in August and will grow rapidly the following year, or some years later. The deep taproot which is one of the plant’s characteristic must be removed if the plant is to be effectively controlled (NYDEP).
The AVDU notes that grazing by goats and sheep can be effective, as they seem to be unaffected by the plant’s chemical properties. In most cases, however, manual control is suitable for smaller infestations or for individual plants, although every precaution should be taken and protective clothing used.
Plants can either be regularly mowed, which will eventually kill them and prevent them from setting seeds, or dug out (with care being taken to remove the tap root). Covering the ground area with black plastic sheeting prevents re-growth. Mowing can also be effective for larger areas of giant hogweed, although like many other methods it must be regularly repeated.
Larger infestations will require chemical controls and are usually carried out by professional companies or by local authority contractors. Giant hogweed is susceptible to glyosphate, a systemic herbicide which is taken in through the plant’s leaves. This is a non-selective chemical, however, and will kill surrounding plants as well. An alternative is a weedkiller containing triclopyr, which kills only broadleaved plants.
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