Giant Hogweed - Invasive Species Information

What Is Giant Hogweed - (Heracleum mantegazzianum)?

Habitat: commonly found along streams, ditches, roadsides and wet meadows.
Distribution in Ireland: Widespread

Status: Established

Family name: Apiaceae

Reproduction: Giant hogweed produces 1,500 to 100,000 seeds per plant. These are coin sized and paper thin. Some can be spread by the surface run-off of rain or in the wind. A small number of seeds can even be transported unintentionally by humans or livestock if trampled upon. Giant hogweed seeds can remain viable in the soil for up to 15 years.

Giant hogweed toxic spines

Giant hogweed leaf

Giant hogweed is an invasive biennial or perennial plant species which is native to the Caucasus Mountains at the border of Europe and Asia. It was introduced to Ireland in the 19th Century as an ornamental garden plant. Since then, it has escaped into the wild and is scattered throughout the country. Giant hogweed prefers in rich, moist soil.

Giant hogweed Heracleum mantegazzianum flower

Giant hogweed in flower

How To Identify Giant Hogweed?

Leaf: Leaf edges are very coarse and serrated

Flowers: Look like very large Cow parsley

Stem: Fine (toxic) hairs grow from red spots

Size: Can grow up to between 3-5 metres high

Giant Hogweed - Heracleum mantegazzianum ID Guide V8

Giant hogweed - Heracleum mantegazzianum seeds

Giant hogweed - Heracleum mantegazzianum in winter

Hollow Giant - Heracleum mantegazzianum hogweed stem

Giant hogweed seeds

Giant hogweed - Heracleum mantegazzianum ID Guide

Giant hogweed in winter

Giant hogweed stem

Why Is Giant Hogweed A Problem?

Giant hogweed is considered to be a significant danger to public health. The stems, edges and undersides of the leaves are coated with fine hairs containing phototoxic sap*. The slightest contact with human skin can cause severe dermatitis in the presence of sunlight. Effects may include welts, rashes, and blistering.

The sap* can also cause temporary or permanent blindness following contact with eyes. If skin comes into contact with Giant hogweed the affected area should be covered immediately from sunlight.

The skin should then be washed with cold water and medical advice should be sought as soon as possible. Giant hogweed can invade farm and forestry land, reducing available agricultural area and biodiversity.

Animals grazing Giant hogweed may be susceptible to poisoning or digestive disorders. Ingestion of Giant hogweed is known to interfere with the fertility of female livestock.

Due to the serious health risks associated with Giant hogweed, it is recommended that all methods of control are carried out by trained and experienced personnel.

Giant hogweed - Heracleum mantegazzianum UV burns and blisters

Giant knotweed - Heracleum mantegazzianum in flower

Giant hogweed is an invasive species which is extremely difficult to eliminate from a site. The control of Giant hogweed should aim to eradicate the plant entirely or at a minimum, prevent the plant from producing seed. A number control methods may be employed to exhaust the plant’s seed bank.

As the seeds remain viable for several years, control requires continued input and follow-up over a number of years.

Giant hogweed burns

Giant hogweed in flower

*Sap/Juices contain Furanocoumarin.

Furanocoumarins, or furocoumarins, are a class of organic chemical compounds produced by a variety of plants. They are biosynthesized partly through the phenylpropanoid pathway and the mevalonate pathway, which is biosynthesized by a coupling of dimethylallyl pyrophosphate (DMAPP) and 7-hydroxycoumarin (umbelliferone).

Many furanocoumarins are toxic and are produced by plants as a defense mechanism against predators such as insects and mammals. This class of phytochemical is responsible for the phytophotodermatitis seen in exposure to the juices of the wild parsnip and the giant hogweed..

Medication interactions:-

Furanocoumarins have other biological effects. For example, in humans, bergamottin and 6',7'-dihydroxybergamottin are responsible for the "grapefruit juice effect", in which furanocoumarins affect certain P450 liver and gut enzymes, such as the inhibition of CYP3A4 which either activates or deactivates many drugs, leading to higher or lower levels in the bloodstream.

Furanocoumarins have various effects which can specifically increase or decrease (depending on the drug) the blood levels of many pharmaceuticals in ways that can be life-threatening. Approved drugs will include warnings for grapefruit.