TOXIC: Giant Hogweed, Heracleum mantegazzianum : a dangerous plant! DO NOT TOUCH.

Heracleum mantegazzianum, commonly known as Giant Hogweed, Cartwheel-flower, Wild Parsnip, Wild Rhubarb, Giant Cow Parsnip, Giant Cow Parsley, Parsnip Tree, Hogsbane, is a plant in the large family Apiaceae.

In North America this species resembles Heracleum maximum (common cow-parsnip, American cow-parsnip), but is much larger. In Europe, the species resembles Heracleum sphondylium (common hogweed), but is larger, leaves are more serrated, and stem has red or purple patches. Giant hogweed’s relative Persian hogweed (H. persicum) is almost as large. Giant hogweed’s thick stem has red spots at the base while Persian hogweed is a uniform reddish brown. Persian hogweed’s leaf lobes are wider and have blunter teeth than giant hogweed. Superficially, it also resembles Heracleum sosnowskyi or garden angelica (Angelica archangelica).

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The sap of giant hogweed causes phytophotodermatitis in humans, resulting in blisters, long-lasting scars, and—if it comes in contact with eyes—blindness. These serious reactions are due to the furocoumarin derivatives in the rough hairs, leaves, roots, stems, flowers, and seeds of the plant. These phototoxic furanocoumarins combine with sunlight to cause skin burns that are difficult and slow to heal. Even giant hogweed’s particular smell is enough to induce allergenic symptoms in sensitive people, so protective clothing, masks and goggles should be worn when dealing with the plant.

Its edible relatives (carrot, parsnip, celery parsley, etc) also contain foumocoumarin, but in much lower concentrations, rarely causing skin reactions.
Growing form: Biennial or short-lived, usually once-flowering perennial herb (monocarp perennial).
Height: 150–320 cm (60–130 in.); 7-18 FEET (2–5.5 m) when in flower. Usually 1-stemmed, stem roughly haired, base reddish brown-spotted, up to 50–100 mm thick, hollow, joints with septa. With pungent fragrance.
Flower: Corolla regular (outer flowers slightly zygomorphic and larger than others), white, 12–25 mm (0.5–1 in.) wide; petals 5, deeply notched. Sepals 5. Stamens 5. Pistil of 2 fused carpels, styles 2. Inflorescence a compound umbel, secondary umbels 50–120. Primary umbel’s bracts quickly withering, secondary umbels’ 10–18 bracteoles enduring.
Leaves: Alternate, stalked, base sheath-like. Blade up to 1 metre long, usually max. as long as broad, underside densely haired–glabrous, top glabrous, pinnate, leaflets 3(–5). Leaflets large with long, narrow lobes, sharp-toothed, edges serrated.
Fruit: Elliptic, 2-sectioned, quite shallowly ridged, 9–11 mm (0.36–0.44 in.) long schizocarp, oil ducts clearly club-like.
The plant usually dies after flowering, but a large individual can produce up to 50,000 seeds. It is advisable to eradicate wild stands as quickly as possible, though eradication can be difficult.

Habitat: Roadsides, yards, parks, wasteland, forest margins and river banks. Also an ornamental.
Flowering: mid-May through July in warmer climates, August–September in cooler locations.

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