Giant Hogweed is a member of the carrot family, Apiaceae, and is similar to the native Hogweed species, Cow Parsley, Heracleum sphondylium. Giant Hogweed is native to the Caucasus Mountains in south-west Asia. Giant Hogweed was introduced as an ornamental plant and has become invasive in Ireland. It grows in colder climates, such as Ireland, the UK, and Scandinavian countries. Seeds require cold temperatures to break dormancy. Giant Hogweed is commonly found along rivers and streams and its seeds are transported by water. The plant germinates early in the year in March or April. Early germination combined with fast growth to large sizes makes Giant Hogweed highly invasive and ability to outcompete native plants for light and space.
Classified as a high impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.
- Giant Hogweed is identified by its large size of up to 3-5m in height and umbels of white flowers from June to August.
- Giant Hogweed is generally biennial where a rosette of leaves occurs in the first year and a flower spike in the second.
- The stem is ribbed, purple-spotted, hollow and hairy bristles are often found.
- Leaves are serrated and long, 1-3m in length and up to 1.5m wide.
The sap of Giant Hogweed is phototoxic and can cause blistering on the skin when skin comes in contact with the sap and subsequently UV light. Reaction can take up to 24 hours to occur and blistering can be painful. The substance can stay in the skin for a number of years and if exposed to sunlight, again it will cause further burns. It is therefore essential that Giant Hogweed is prevented from making contact with the skin. If skin is exposed to sap, area should be covered immediately to prevent exposure to UV light, and washed with soap and water.
- Seed production is high, several thousand seeds can be produced by each flower head.
- Seeds can remain viable for up to 15 years and are spread by wind, water, and animals.
- Giant Hogweed can also regenerate from roots.
- Giant Hogweed grows early in the season and is tolerant of flooding, shade, and disturbed ground.
- They can outcompete other weeds as they grow to tall heights and have a high seed production.
- It can also be bad for flooding as hogweed growing on riverbanks and dies back during the winter leaving the riverbank exposed to erosion.
- Outcompete other plants due to its height and high seed production
- Cause blistering on skin because of phototoxic sap
Mechanical: JKI will undertake a programme of mechanical control in June. Young Giant Hogweed plants can be pulled when soil is moist. Any plant greater than 1.5m can be cut back and pulled out by the root. The central crown of the plant must be removed to prevent regeneration. Giant hogweed seed heads can be removed and should be bagged individually to prevent spread.
Chemical: Foliar spraying with a glyphosate-based herbicide in April / May for a period of at least 5 years. In protected sites or areas where sensitive vegetation occurs, the stem of the plant can be injected with glyphosate.
Biological: Grazing by cattle and sheep can be used as a form of biological control. In Denmark, sheep grazing over 6 years in a Giant Hogweed dominated field rendered the field Giant Hogweed free in six years. Soil samples taken from the field showed an absence of viable Giant Hogweed seeds (Anderson and Calov, 1996). Grazing should begin at the start of the growing season when plants are small. Using animals, such as sheep, with a darker pigmentation is likely to reduce inflammation the animal could be subject to from the Furanocoumarins in the sap. If an animal is showing blistering around the mouth, the animal should be removed. Other fodder should also be supplied to supplement diet (Nielsen et al., 2005).
(National Biodiversity Data Centre, Ireland, Giant Hogweed (Heracleum mantegazzianum), image, accessed 18 May 2022)