Giant Hogweed (Heracleum mantegazzianum): a dangerous neophyte

Riesenbärenklau: Ein gefährlicher Neophyt
Giant Hogweed (Heracleum mantegazzianum)

Origin

The giant hogweed (Heracleum mantegazzianum), also called Hercules, is one of the invasive neophytes. It is a so-called newcomer, which was only introduced to Central Europe from the Caucasus as a garden plant around 1900, and then to the United States. It likes it here so much that it has been spreading rapidly in the wild ever since, displacing native plants. The reasons for this are obvious: the perennial shows enormous growth and can reach a height of up to four metres under good conditions. Added to this are its large white umbel flowers, which are also very popular among beekeepers as bee food.

Appearance and growth

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.

Toxic plant sap

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.

Spread

For native plants, the giant hogweed is an overwhelming competitor because of its strong growth in most locations: although it is not long-lived, its enormously high seed production ensures that it can spread and the large leaves cast so much shadow that weaker species are reliably suppressed. After sowing, the mother plant dies in most cases, but several new Hercules perennials soon emerge in its vicinity.

Fight Giant Hogweed

If the giant hogweed is spreading in the garden, it should definitely be removed before it can seed itself. Protect all bare skin with waterproof clothing and put on rubber gloves to prevent contact with the plant sap. It is not enough to simply knock the plants down to ground level, as they will then usually sprout again from the tap root. You should therefore remove them with a spade or weeding knife afterwards. By the way: many local environmental associations also organise occasional action days to drive back the giant hogweed in the wild. Volunteers are also always welcome here.

Profile

Growth type
  • Perennial plant
  • biennial or short-lived
Growth height
from 300.00cm to 350.00cm
Growth width
from 150.00cm to 250.00cm
Flowering time (month)
  • June to July
Sheet shape
  • fiddly
  • feathery
Light
  • sunny
  • semi-shade
Type of soil
  • sandy to loamy
Soil Moisture
  • moderately moist to moist
pH-value
  • neutral to slightly acidic
Lime tolerance
  • lime-tolerant
Nutrient requirements
  • nutrient-rich
Decorative or utility value
  • Flower decoration
  • Leaf decoration
  • picturesque growth
  • Nectar or pollen plant
Use
  • Underplanting