Giant Hogweed (Heracleum mantegazzianum): a dangerous neophyte

The giant hogweed is a short-lived perennial and rarely grows older than two years. After germination of the seeds, it remains relatively compact in the first year and only produces leaves. Only in the second season does it grow rapidly upwards, reaching a height of over two metres within a few weeks. Its hollow, tubular stem is finely hairy and covered with irregular purple dots. Depending on the size of the plant, the stem can become lignified near the ground, very stable and up to ten centimetres in diameter.

Starting from the stem, the giant hogweed usually forms jagged, multi-fingered leaves up to one metre long. Depending on the size of the plant, the individual leaf blades are three- to nine-lobed and fiddle-cut. One of the reasons why the plant was imported into our regions is its striking white inflorescences. Depending on the size of the plant, the double umbels reach a diameter of 30 to 50 centimetres and consist of 30 to 150 flower stems. It is noticeable that the flowers are larger at the edge of the umbels than inside the umbels. Despite its bad reputation, the perennial plant is interesting for beekeepers because it forms around 80,000 individual flowers per plant and thus provides the bees with a rich supply of food. When the flowering season is over between June and July, a single plant produces on average around 20,000 seeds.

What makes the giant hogweed particularly dangerous is its plant sap. The liquid contains phototoxic substances that cause severe burns on the skin when exposed to sunlight. These burns are also called meadow grass dermatitis, are very painful and often leave pigment changes after healing. Because of its potential danger and the rapid spread of the plant, the giant hogweed was awarded the dubious “Poisonous Plant of the Year” award in 2008.

Especially for children and animals the contact with the giant hogweed can be a painful experience. Burnt legs, arms and hands in children and burnt noses in dogs are the most common injuries. The contact is favoured by the locations that the giant hogweed prefers: the perennial plant loves nitrogenous, moist soil and therefore often grows on water banks and forest edges in clearings and along paths. Especially the last location is of course predestined to come into contact with Giant Hogweed. If you have come into contact with the plant sap, you should immediately protect your skin from sunlight at this point and then wash the sap off thoroughly with warm water and soap.