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Giant Hogweed – Heracleum mantegazzianum

Posted on by phlorum

Giant Hogweed is a non-native plant species that grows abundantly along the banks of rivers and streams. It is a member of the family Apiaceae (carrots/umbellifers), native to the Caucasus Region and Central Asia. Giant Hogweed is a perennial with tuberous rootstalks which form perennating buds each year. It flowers from late spring to mid summer, with numerous white flowers clustered in an umbrella-shaped head that is up to 80 cm (2.5 ft) in diameter across its flat top.

Hogweed is a cause for concern in that it is both a human health hazard, and exerts a negative ecological impact on infested river corridors. Giant Hogweed contains a substance within its sap that makes the skin sensitive to ultra violet light.  This can result in severe burns to the affected areas, producing swelling and severe, painful blistering. Giant Hogweed is an aggressive competitor, which is able to out-compete native plant species, reducing the amount of suitable habitat available for insects, birds and mammals.  Giant Hogweed produces approximately 1500 seeds per flower head in late summer. These seeds can remain inactive in the soil for several years. The movement of soil polluted with Giant Hogweed seeds must be carefully controlled to prevent the spread of the plant.

In the UK the Wildlife and Countryside Act 1982 makes it an offence to plant or cause Giant Hogweed to spread in the wild. Any Giant Hogweed polluted soil or plant material that you discard, intend to discard or are required to discard is classed as controlled waste and should be accompanied by appropriate Waste Transfer documentation.

  • It is characterized by its size and may grow over 2-5m tall, It is further distinguished by a stout, dark reddish-purple stem and spotted leaf stalks that are hollow and produce sturdy bristles.
  • Stems vary from 3-8 cm in diameter, occasionally up to 10 cm. The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk.
  • The plant has deeply incised compound leaves which grow up to 1-1.7 m in width. The plant produces flattened, 1cm long, oval dry seeds that have a broadly rounded base, and broad marginal ridges.
  • The Giant Hogweed flowers from late spring to mid summer, and then produces numerous, large flattened elliptic dry seeds (between 1,500-100,000). Shoots die down in the autumn. Tall stems mark its locations during winter.

Control

Spraying with an appropriate herbicide such as glyphosate is the most effective treatment option available, although it can take several years to eradicate these species. The soil beneath an established stand of Giant Hogweed will contain thousands of seeds that will continue to produce new plants.  Although the eradication of this species before or during construction is unlikely, herbicides can be used to achieve short-term control of the plant, allowing construction works to continue and reducing the risk to the workforce.

A catchment approach to spraying with glyphosate is required, starting at the furthest upstream site for the plant. Spraying should commence in March/early April when leaf growth has occurred and the height is >15cm. Experience has shown that seedlings are less susceptible to glyphosate treatment. A dose rate of 5 l/ha -1 is sufficient to kill treated vegetation.

Sections treated in March/April should be re-treated in May. The catchment area should be surveyed again in July and any plants that have flowered or are likely to flower must be deheaded before seeds are produced. The cut umbels must be removed from the area and destroyed. The plant should be sprayed again with glyphosate or it will attempt to flower again and set seed. A further spraying of glyphosate throughout the catchment area in September will kill or suppress the growth of autumn flowering or late-developing plants.

A quicker method of removing Giant Hogweed involves the clearing of above ground leaf/stem material and the removal of ground material polluted with roots and seeds.  Due to the risk of contact with sap from this plant, removal by hand should be restricted and not considered once the plant has grown above one metre in height.

The majority of seeds that are produced fall to ground within four metres of the parent plant as they are heavy and rely on water and the wind to transport them.  This four-metre radius around Giant Hogweed plants should be considered as polluted.  Even with great care, a certain number of seeds may remain.  A watching brief is advised after polluted ground material has been removed.  Any regrowth should be treated with an appropriate herbicide as discussed above.