Giant hogweed Heracleum mantegazzianum
Characteristically large, the foreign giant hogweed can grow up to 5 metres tall and greatly hinders the growth of native plant species. It is also a toxic plant with poisonous sap in its stems and leaves.
Conservation status in Norfolk
Giant hogweed is a non-native plant which out-competes native plant species, preventing them from growing. Under the Wildlife & Countryside Act 1981 it is an offence to “plant or otherwise cause Giant hogweed to grow” in the wild.
How to help
As part of its Natural Connections project, Norfolk Wildlife Trust is working in partnership with the Norfolk County Council (Norfolk Biodiversity Partnership) to map the distribution of Himalayan balsam in Norfolk. The aim is to engage the general public in helping to find out where these species are found in Norfolk and help discover what impact they are having on Norfolk’s countryside. The survey will run from end February through to end October. For further information or a copy of the Freepost survey form telephone 01603 598333, or visit our website at
www.norfolkwildlifetrust.org.uk/naturalconnections/surveys where copies of the survey form can be downloaded.
Giant Hogweed: Elizabeth Dack
Giant Hogweed Seedhead: David North
Information on the Giant hogweed
How to recognise
Characterised by its impressive size, Giant hogweed can stand up to 5 metres tall. This plant is not native to the UK, and was introduced from south-west Asia in 1893 as an ornamental plant. The hollow stems are green with reddish-purple blotches, and up to 100 mm across. The leaves are dark green and jagged, ending in a spike, and arranged in rosettes around the stem. The many small white flowers form an umbrella-shaped head (umbel) up to 500 mm across.
Where to see
Giant hogweed grows well where the soil has been disturbed, such as waste ground, roadsides and riverbanks. The seeds are easily dispersed by water, so it is often seen spreading along watercourses. It is widespread in Norfolk with ‘hot-spots’ to the south and east of Norwich.
When to see
Giant hogweed is a perennial and starts to grow from a large fleshy taproot in March-April. It flowers in June and July. It can take up to four years for a Giant hogweed to flower, but each plant can produce 50 – 80,000 seeds. After flowering, the whole plant dies off. This can cause problems with soil erosion if large areas of riverbanks are left bare over the winter.
Did you know?
Giant hogweed is a toxic plant and a public health hazard. The stems, edges and undersides of the leaves have small hairs which contain poisonous sap. The slightest touch causes the skin to become photo-sensitive, so that exposure to sunlight causes severe burns and blistering. For this reason, protective clothing must be worn when dealing with this species.
Related questions
Giant Hogweed How do I recognise it?
Giant hogweed is an impressive plant and can reach heights of up to 5-6 metres. It is therefore much taller and has altogether larger dimensions than any related native plant. The plant is not native to the UK, and was introduced from south-west Asia in 1893 as an ornamental plant. The hollow stems are green with reddish-purple blotches, and up to 100 mm across. The leaves are dark green and jagged, ending in a spike, and arranged in rosettes around the stem. The many small white flowers appear in June or July and form an umbrella-shaped head (umbel) up to 500 mm across. It can take up to four years for a Giant hogweed to flower, but each plant can produce 50 – 80,000 seeds
Giant hogweed grows well where the soil has been disturbed, such as wasteground, roadsides and riverbanks. Here they will shade out native vegetation. The seeds are easily dispersed by water, so it is often seen spreading along watercourses. It is widespread in Norfolk with ‘hot-spots’ to the south and east of Norwich. After flowering, the whole plant dies off. This can cause problems with soil erosion if large areas of riverbanks are left bare over the winter.
NOTE: Giant hogweed is a toxic plant and a public health hazard. The stems, edges and undersides of the leaves have small hairs which contain poisonous sap. The slightest touch causes the skin to become photo-sensitive, so that exposure to sunlight causes severe burns and blistering. For this reason, protective clothing must be worn when dealing with this species.