Today's “Plant of the Day” is one the most notorious plants at all. I'm speaking of Heracleum mantegazzianum Somm. Et Lev. from the Apicaceae family. In Germany, this species is known as “Bärenklau” or “Herkulesstaude”, while English names are “giant hogweed”, “wild parsnip” or “giant cow parsley”. In older literature, this plant is still known as Heracleum giganteum.
H. mantegazzianum - habitus
It's a large perennial plant, which can reach heights between 2 and 3,5 metres (6.6 to 11.5 feet). The hollow stalk grows very fast and H. mantegazzium can reach these heights within a few weeks after sprouting. The stalk is very strong with up to 10 centimetres in diameter at the base. It's very hairy and also speckled purple.
H. mantegazzianum - stalk
The leaves are pinnate with 3 to 5 digitate “leaflets”. As the rest of this plant, they have an impressive size with up to 1 metre. Their sheaths are also very conspicuous and remain as a leathery structure at the leaf base.
H. mantegazzianum - leaflet
The inflorescences are umbels. These umbels can have up to 50 centimetres in diameter and can consists of 30 to 150 small flowers (you see, it's a very imposing plant). The flowers in the periphery of an umbel are normally larger than the flowers in the centre. Each flower has five white petals, which are indented a little bit. Flowering time is between June and September. The ripe fruits are shizocarp, what is typical for the Apiaceae. They are disk-shaped and their edge rips are bristly.
H. mantegazzianum - umbel
H. mantegazzianum is native to the Caucasian, but was brought as ornamental plant to Middle Europe in 1900. Later, it starts to grow wild and spread out along roads and rivers. So today, you can found this species e. g. on ruderal wastelands, at roadsides or in river valleys. It's very undemanding towards the soil but likes nutrient-rich, not so acid places.
H. mantegazzianum - sheaths
As fast growing neophyte, H. mantegazzianum could be a great threat for native biodiversity, because it has the ability to overgrow places and to displace other species quickly. However, H. mantegazzianum is not such a large problem like e. g. Fallopia japonica (Japanese knotweed) or Robinia pseudoacacia (Black Locust). Regular mowing is a good way to fight large stocks of this species.
H. mantegazzianum - stalk with hairs
However, in popular culture, H. mantegazzianum is one of the most feared plants. The reason for its bad reputation are the ingredients of the plant.
All parts of H. mantegazzianum contain a phototoxic liquid, which is dispensed by the hairs. This liquid reacts with UV radiation, what triggers a chemical reaction, which can caused serious burns. So, it's not a good idea to touch this plant with bare hands. To avoid contact with the skin, people should always wear protective clothing, if they want to remove plants.