T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Heracleum mantegazzianum (Giant hogweed)

Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Apiales
Family: Apiaceae
Genus: Heracleum
Species: H. mantegazzianum
Binomial name: Heracleum mantegazzianum
Common names: Giant hogweed, Cartwheel flower, Giant cow parsnip, Hogsbane, Giant cow parsley, Wild parsnip, Wild rhubarb

Heracleum mantegazzianum is a massive, erect, biennial or monocarpic perennial herb native to the Caucasus region and Central Asia. It was planted around the world as an ornamental plant. It typically grows to heights of 2–5.5 m. Superficially, it resembles common hogweed (Heracleum sphondylium) or garden angelica (Angelica archangelica). It is phototoxic and considered to be a noxious weed in many countries including New Zealand. This is because it is a prolific seed producer (>100,000) which are spread usually by water. 

Heracleum mantegazzianum forms dense colonies which suppress the growth of native plants and grasses. It will invade wastelands, road ditches and stream banks. The plants usually begin dying after they have set seed. Seeds may remain dormant in the soil for at least 5 years. The plant dies down in winter, leaving infested banks bare of vegetation and liable to erosion or to reinvasion by Heracleum mantegazzianum's seeds or other weed species.

Heracleum mantegazzianum grows from a forked or branched taproot. It has large 50-100 cm long leaves, made up of three bristled, deeply divided leaflets, forming a rosette at the base. The bright green stems are stout (5-10 cm diameter), grooved, bristled, are hollow and are frequently blotched and spotted with a dark red colour. The plant in its 2nd year it produces a tall flower stalk with leaves attached and large umbrella-like clusters (20-60 cm diameter) of white flowers (1.5 cm diameter). These are followed by dry, flattened, oval, light brown fruit (1 cm long).

Heracleum mantegazzianum sap is highly toxic and poisonous to humans if one is exposed either by touch or dust that is raised by machinery (e.g. weed eaters). Exposure will cause 'photodermatitis' or 'photosensitivity'. The skin may become very sensitive to sunlight and one may suffer from blistering, pigmentation and long-lasting scars. Contact with the eyes can lead to temporary or permanent blindness. Body areas which come into contact with this plant should be washed immediately and protected from direct sunlight, seek medical advice. Always wear gloves, clothes for skin protection and glasses for eye protection.
The serious reactions to this plant are due to the furocoumarin derivatives in the leaves, roots, stems, flowers, and seeds of the plant.

The red, hollow spotted stem with the poisonous sap starting to weep out.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/