Giant Hogweed

Introduction

Heracleum mantegazzianum Sommier & Levier.

Giant Hogweed is a member of the Apiacaea family; also known as Cartwheel Flower because of the extremely large flower heads.

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. 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 is highly invasive but more concerning is 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.

• Contact with the cut material in sunlight produces a reaction in almost everyone. The degree of symptoms will vary between individuals, but children are known to be particularly sensitive.
• The cut material remains active for several hours after cutting
• Blistering symptoms occur after 24-48 hours post exposure, and dense post-inflammatory hyper-pigmentation is visible after 3-5 days and may persist for at least 6 years.
• The threshold concentration of furanocoumarins (10-100g/ml) for a light sensitive skin reaction is far exceeded during the growing season. The highest concentration is in the leaves, the lowest in the stems (often the cause of mouth blisters in young children) and petioles, and the root intermediate.
• The mode of action in the photosensitive reaction causes damage to DNA, thus inducing cellular damage, especially in melanocytic cells. The mechanism is thought to be due to photoinduced gene suppression leading to increased melanocyte production. These mechanisms are also involved in sunlight induced skin cancer, or melanoma, but evidence suggests that furanocoumarins are unlikely to be a genetic or carcinogenic hazard to humans.
• High light and nitrate habitats (e.g. streamsides, roadsides and waste ground) tend to produce higher quantities of furanocoumarins.
• The furanocoumarins are produced by the plant as a defence against insect herbivory. There may be loss of insect biodiversity in dense stands of this species, due to the presence of only specialist herbivore insects.
• Furanocoumarins are known to be antifungal, possibly explaining their presence in the roots. This may lead to suppression of soil fungi, essential for soil fertility.

Identification

The leaves are large and deeply divided and can grow up to 1m or more in size. 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 Methods Of Control

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. The taproot should be cut 8-12 cm below ground level. Cutting plants at or above ground level encourages vigorous regrowth. Plants should be cut every 2 weeks in spring. It is not advisable to attempt cutting plants that are taller than 1.5 m. Bare areas of soil left after removal of the weed should be sown or planted with native vegetation to help prevent re-colonisation by the giant hogweed.

Cattle, sheep, pigs and goats can graze the plant with no apparent ill effects. Grazing should begin in mid-March and continue through the growing season. Growth is suppressed but rotational grazing does not kill the plants. Pig foraging should eradicate the plant through root damage.

Biological Methods Of Control

The toxic sap is thought to act as a feeding deterrent for many insects, although there are some specialist butterflies whose caterpillars may offer some degree of control though there are no plans currently to instigate a trial.

Chemical Methods Of Control

Professional Pesticides

The only herbicide which is known to control Giant Hogweed and which is approved for use in or near water is glyphosate, the active ingredient in Roundup ProVantage and Roundup ProActive. The plants can be sprayed with glyphosate when the plants are growing actively but still less than about 1 m high. This is usually in April and May. Long-lance sprayers may assist in accurate application of glyphosate to plants growing in inaccessible sites along riverbanks. Glyphosate can be applied as 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.

Application should be carried out wearing face shield, gloves, overalls and wellingtons and contact with the plant sap avoided. Try not to spray in direct sunlight. Monitor sites annually for 5-10 years and re-treat as necessary to take account of newly emerged seedlings.

Giant Hogweed is susceptible to systemic herbicides, such as glyphosate and triclopyr. Triclopyr is the active ingredient in Grazon Pro and Icade. These are selective herbicides that are particularly effective at controlling woody weeds such as Brambles, Buddleia, Rhododendron, Sycamore and Ash trees as well as difficult-to-control weeds such as Giant Hogweed and Japanese Knotweed. 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.

Domestic Weed Killers

As with the professional pesticides, two active ingredients are available: glyphosate is the active ingredient in Barclay Gallup Home & Garden. Barclay Gallup Home & Garden is a very effective herbicide that starts to degrade almost as soon as it is applied however it is not selective and will kill any plant it comes into contact with.

Large areas - Rates, Timings and Water Volumes

Triclopyr is the active ingredient in SBK Brushwood Killer a selective brushwood killer that will control broadleaved weeds without affecting grass. SBK is particularly effective at controlling woody weeds.