Giant hogweed: the facts

You may have heard about the dangers of giant hogweed – a non-native invasive plant which can cause burns. So we’ve put together the facts for you, from identification to staying safe.

What is giant hogweed?

Giant hogweed (Heracleum mantegazzianum) is a plant in the Apiaceae family (previously known as the Umbelliferae). This family includes some well-known plants such as parsley, carrot, parsnip, cumin and coriander.

Where does giant hogweed grow?

Giant hogweed is not native to the UK. It originates from the Caucasus Mountains and Central Asia. It was first introduced to the UK as an ornamental in the 19th century when it escaped and naturalised in the wild. It is now classed as an invasive species and can be found throughout much of the UK, especially colonising river banks where its seeds are transported by the water.

What does giant hogweed look like?

Giant hogweed looks like an enormous cow parsley plant. When it's fully grown, it can reach towering heights of between 1.5m to 5m and have a spread of between 1 and 2m. It forms a rosette of jagged, lobed leaves in the first year before sending up a flower spike in the second year and then setting seed.

Stems

Green and often with purple blotches and stiff, white, britly hairs. Stems are hollow with ridges and have a thick circle of hairs at base of each leaf stalk.

Leaves

Huge, up to 1.5m wide and 3m long and is deeply divided into smaller leaflets. It looks a bit like a rhubarb leaf, with irregular and very sharp or jagged edges - which has given rise to one of its other common names - wild rhubarb.The underside of the leaf is hairy.

Flowers

Appear in June and July. They are small and white (or slightly pink) and are clustered on umbrella-like heads known as umbels that can reach a diameter of 60cm. All the flowers on the umbel face upwards.

Seeds

Dry, flattened, and oval. Almost 1cm long with tan with brown lines extending 3/4 of the seed length.

Is giant hogweed dangerous?

In short - the sap of giant hogweed can cause burns. It contains furocoumarin, which makes skin extremely sensitive to sunlight (phytophotodermatitis).

If the sap gets onto your skin and it's then exposed to the sun, your skin can blister badly. Blistering can then recur over months and even years. This is known as phytotoxicity.

The best way to avoid injury is to familiarise yourself with the plant and avoid contact with your skin. Brushing through patches of giant hogweed and exposing yourself to plants that have been cut might cause you to get sap on your skin. 

What happens if you touch giant hogweed?

If you do get giant hogweed sap on your skin, be sure to wash the area thoroughly and immediately. Seek medical advice and do not expose the area to sunlight for a few days.

How to tell giant hogweed apart from similar plants

Here are some common plants that could be confused with giant hogweed with images and descriptions to help you with identification.

Common hogweed is very similar-looking to giant hogweed but is much smaller. Its stems aren’t blotchy like those of giant hogweed (their colour graduates smoothly from green to purple) and are ridged, hollow and hairy.

Common hogweed only reaches a maximum height of 2 metres (6ft 7) and the symmetrical flower heads only reach 20cm across. Its seeds are much smaller and lighter than those of giant hogweed, and the leaves are less jagged and more rounded at the edges than giant hogweed.

Cw parsley is a native plant common along hedgerows and roadsides. It grows to 60–170 cm and also has clusters of white flowers in umbels. The leaves are very different to giant hogweed - they are tripinnate (the leaflets are themselves divided and then these divisions are divided again). Confusingly, some people refer to giant hogweed as giant cow parsley.

Ground elder is a non-native plant (introduced by the Roman possibly as an edible) which has made its way into the countryside and people's gardens. It is not related to elder trees (Sambucus nigra) but gets its name from having elder-like leaves. Its leaves are very different to giant hogweed, although it has similar flat-topped clusters of small white flowers which can grow to 20cm across.