- Name also: Cartwheel-flower, Wild Parsnip, Wild Rhubarb, Giant Cow Parsnip, Giant Cow Parsley, Parsnip Tree
- Family: Carrot Family – Apiaceae (Umbelliferae)
- Growing form: Biennial or short-lived, usually once-flowering perennial herb.
- Height: 150–320 cm (60–130 in.). 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.
- Fruit: Elliptic, 2-sectioned, quite shallowly ridged, 9–11 mm (0.36–0.44 in.) long schizocarp, oil ducts clearly club-like.
- Habitat: Roadsides, yards, parks, wasteland, forest margins and river banks. Also an ornamental.
- Flowering time: August–September.
- Harmfulness: Extremely harmful invasive species.
Giant hogweed is a truly arresting sight with stems up to four metres tall and metre-long leaves. This impressive species was brought to Finland as an ornamental, but unfortunately is has been problematic from the outset as the furanocoumarins in its rough hairs combine with sunlight to cause skin burns that are difficult and slow to heal. Doctors call this kind of chemically induced skin irritation that requires light phototoxicity. 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.
Giant hogweed only grows ferally high up in the mountains of the western Caucasus. Nowadays it has spread into the wild in many parts of the south of Finland. A large individual can produce up to 50,000 seeds, so if even only a small proportion of these germinate hundreds of millions of seeds are produced within the space of a couple of years. The plant usually dies after flowering so it would die out if it was unable to seed. It is advisable to eradicate wild stands as quickly as possible, although a seed-bank will survive in the soil for years, so getting rid of established populations is often a difficult and very time-consuming job. Giant hogweed has made such a nuisance of itself in many places that serious consideration is being given to banning the sale of their seeds and shoots.
Giant hogweed’s relative Persian hogweed (H. persicum) is almost as large, but this perennial is often also many-branched. 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.
The third hogweed that grows in Finland is Sosnowsky’s hogweed which can grow up to 5 metres high. The species was originally developed as animal fodder and has escaped from arable land.