Giant Hogweed (Heracleum mantegazzianum Sommier & Levier.) is a member of the Apiacaea family; also known as Cartwheel Flower because of the extremely large flower heads. It is not a native species to the UK, its natural distribution is the Caucasus mountains in North-Western Asia. As a non-native species it does not have any natural predators that would normally control its population and as a consequence it has naturalised along watercourses, roadsides, hedgerows, waste places and rough grassland. It can cause delays/additional costs on development sites where the plant must be removed as controlled waste in order to comply with legislation.
Giant Hogweed has been added to Schedule 9 by The Wildlife and Countryside Act 1981 (Variation of Schedule 9) (England and Wales) Order 2010: this means that it is illegal to plant or otherwise cause to grow Giant Hogweed in the wild.
Giant hogweed is highly invasive but more concerning are its ability to inflict serious injury and even blindness on people that come into contact with the plant. The sap of giant hogweed contains a toxic chemical which sensitises the skin and leads to severe blistering when exposed to sunlight. The burns can last for several months and even once they have died down the skin can remain sensitive to light for many years.
The leaves are large and deeply divided and can grow up to 1.5m across and 3m long. The flower-head is a flat topped umbel composed of many small white/pink florets; each cluster may be up to a diameter of 0.75 metres. Giant hogweed can grow up to 5 metres tall; with stems which are hollow and thick, growing to 10cm across with bristles.
Cultural – Cultural control should begin by preventing seed dispersal though seeds will continue to germinate for up to 7 years. Seedlings and young plants can be removed by hand but full protective clothing must be worn to prevent any contact with the plant, larger plants can be cut down to ground level or may be dug out though again full protective clothing must be worn to prevent any contact with the plant.
Chemical – Giant hogweed is susceptible to systemic herbicides. The benefit of spraying with a selective herbicide is that grasses that are present will not be affected and can happily recolonise the area occupied by the Giant Hogweed once it has died.
Long-lance sprayers can assist in accurate application to plants growing in inaccessible sites along river banks either by a spot treatment to individual plants, using hand-held equipment, or as an overall spray using machine-mounted spray booms. In the latter instance, total weed control of all vegetation will occur and it may be necessary to reseed the treated area with grass and other native plants. Establishing a good sward of grasses soon after treatment of the weed will help to reduce the rate of recolonisation of the area by seeds of Giant Hogweed. Application of any herbicide near water in the UK requires the specific approval of the Environment Agency.
Sites will need to be monitored annually for 5-10 years after treatment and re-treated as necessary to take account of newly emerged seedlings.
The ALS Contracts Team can be contacted on 01952 898518 or 01952 898519 for enquiries regarding Giant Hogweed Control or email firstname.lastname@example.org