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Giant Hogweed

Giant HogweedGiant Hogweed - credit RPS Group plc

Giant Hogweed is a huge umbellifer that invades lowland river banks.

Giant Hogweed was brought to the UK in the 1800s as an ornamental plant. Since then, it has spread widely. It grows vigorously and can hinder access to river banks. It should be treated with caution, as contact with sap can cause severe blistering.

Giant Hogweed is listed under Schedule 9 to the Wildlife and Countryside Act 1981 with respect to England, Wales and Scotland. It is an offence to plant or otherwise cause this species to grow in the wild. Under the Environmental Protection Act 1990, giant hogweed is also classified as controlled waste.

Form

Photograph by GBNNSSGiant Hogweed grows much taller than its native cousins, reaching heights of over 3 metres.

It is a biennial plant, growing in its first year and flowering in the second.  However it may survive longer than this if flowering is suppressed, for example due to grazing.

The upper stems are pale green, often with dark red spots. Lower stems can reach a thickness of 10cm and are red-purple in colour, especially towards the base. The stem is bristly and covered with white hairs.

In winter, just a brown skeleton of the plant may remain.

Leaves

Photograph courtesy of Victoria Freke - Avon Invasive Weed ForumLeaves at the bottom of the plant can also be huge: over 1 metre across.

They are bright green in colour, and sharply divided and serrated. They are bristly on the under side.

Flowers

Photograph courtesy of RPS Group plcGiant Hogweed flowers in its last year, from late Spring to mid Summer.

The flower heads (umbellifers) are huge and can be 80cm across. This is much larger than the native hogweed flower, which is around 15cm in diameter.

The flower heads consist of groups of small white, or occasionally pale pink, flowers.

Distribution

Giant Hogweed self-fertilises, but reproduces only once in its lifetime.

A single plant can produce more han 20,000 seeds, which are then spread by wind, water or animals.

The seeds over-winter in cold, damp soil and germinate the following year.  However some seeds can remain viable in the ground for 3 or more years.

Giant Hogweed thrives in moist, fertile soil and prefers partial shade. It is often found along river banks and in waste ground.

Impacts

The height of the plant, and the fact that it can grow in groups, means that native vegetation is out-competed.

When the plant dies back, it leaves bare ground leading to river bank erosion.

It can hinder access to the river bank by anglers and other river users.

The sap is photo-toxic, which means that it increases the skins sensitivity to ultra-violet light. This can lead to severe blistering, resulting in scars.

Treatment

Treatment at Earby BeckThe plant should not be cut down, as this will just delay flowing and will increase the chance of exposure to toxic sap.

Isolated or small groups of plants can be dug out. The stem should be cut through below ground level to prevent regrowth from the base.

Chemical treatment is effective, though this should be with chemical suitable for use near to water. This is best undertaken when the plant is actively growing in April-May.

The most effective treatment is by stem injection, but due to the toxicity of the plant it is more practical to spray.

As seed often spreads downstream, areas upstream should also be tackled to prevent recolonisation.

In all treatments, protective clothing must be worn.

Further Information

Further details and a fact sheets about this species can be found on the GB Non Native Species Secretariat website, provided by DEFRA.