Introduction

Introduced originally as an ornamental plant in the early 19th century, Giant Hogweed is an invasive plant which can be found in a variety of different places, particularly in areas of wasteland and riverbanks.

Giant Hogweed is named under The Wildlife and Countryside Act 1981, Schedule 9, which makes it illegal to cause the spread, either deliberately or accidentally, of Giant Hogweed and places a duty of care on anybody dealing with it

Description

Giant Hogweed is an invasive biennial to short-lived perennial plant. It takes up to 4 years to mature and flower. It can grow up to 5m high, which distinguishes it from the much smaller native Hogweed.

Stems can reach 12 cm in diameter and are up to 5m in height. The leaves can measure up to 3 metres.

Its white flowers appear in June or July and can reach up to 50 cm in diameter.

Impacts

Its height can become a considerable problem since it prevents other species from growing through light deprivation, drastically reducing species diversity. Each flower can produce up to 50,000 seeds that are easily dispersed by water. This coupled with the fact that seeds can remain viable in the soil for up to 15 years allows Giant Hogweed to rapidly colonise areas especially along riverbanks, road and rail corridors.

Furthermore, Giant Hogweed can be hazardous to human health – the leaves and stems contain a photosynthesising compound, which when exposed to light can cause severe third degree burns which can persist for up to 6 years. Any person dealing with Giant Hogweed should have all skin covered by a strong thick, waterproof material and should handle it with great care.

Control Methods

The most effective method of control is a herbicide treatment programme. Spraying can start as soon as the plant is about 1m high, usually in March and continue throughout the summer.  More than one application is often necessary and follow-up spraying will be required to kill seedlings in subsequent years. The herbicides often used are 2-4D and Glyphosate.

There are also mechanical methods. Cutting down stems with a sharp scythe before flowering and excavating to a depth of 10 cm below ground level will help control this plant.

When clearing a site for development, it is necessary to remove not only the above ground plant material but also the seed base in the soil and the plants root system to ensure Giant Hogweed isn't spread further across the site.

 

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