- Name also: Bog Rhubarb, Devil’s Hat, Pestilence Wort (USA)
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Flowering stem develops in spring before leaves. Rhizomatous.
- Height: 15–40 cm (6–25 in.), in fruit up to 100 cm (40 in.). Stem at least in the beginning hairy at top, reddish brown–purple.
- Flower: Plant dioecious (male and female flowers on different plants), female plants very rare in Finland. Single flower-like capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets reddish, tubular. Stamens 5. Pistil of 2 fused carpels. Involucre 5–8 mm (0.2–0.32 in.) long, involucral bracts usually purple, with entire tips, only hairy at base. Capitula 15–50, sturdy, in a racemose cluster.
- Leaves: Alternate. Flowering stem leaves stalkless, scale-like, lanceolate, narrow-tipped, purple. Proper leaves long-stalked, blade kidney-shaped, up to 50–70 cm (20–28 in.) broad, cordate-based, angular, blunt-toothed, underside densely haired, top becoming glabrous, basal lobes round-tipped, basal gap often virtually closing.
- Fruit: Yellowish brown achene, tip with unbranched hairs. Fruits do not form in Finland.
- Habitat: Ditch banks, lake shores, field and road banks, woodland margins, yards, parks, wasteland. Ornamental, left over and escape from old gardens.
- Flowering time: (April–)May–June.
- Harmfulness: Harmful invasive species.
Common butterbur is originally native to southern Europe and western Asia. In Finland it is an old ornamental which also grows nowadays in the wild as a leftover from cultivation and as an escape. It was originally grown as a medicinal herb as its foul-smelling rootstock was believed to provide immunity from plague. It has also been used to treat a nervous stomach and menstrual problems, but it was discovered to contain poisonous compounds and fell out of use. Common butterbur began to be cultivated in Finland in the 17th century in Turku, which is nowadays in western Finland but at that time belonged to Sweden. Butterbur’s habitat was long restricted to the western side of the River Kymijoki, which marked Finland’s eastern border, and even there it only grew in the grounds of the most affluent houses. Nowadays common butterbur can be found here and there all over the country at least as far north as Lapland, both inside and outside gardens. In gardens its creeping rootstock helps it spread at the expense of other plants. It has escaped from the garden via pieces of root that are thrown away or by being deliberately planted.
Common butterbur does not produce seeds in Finland because it is dioecious and only staminate plants grow here. Early in spring, sometimes already in March–April, the plant produces its flowering stem. Its captivating, even extravagant-looking inflorescence attracts an abundance of insects when other sources of nutrition are hard to find. The first green leaves grow around the capitula when it has finished flowering. Later in the summer, when the inflorescence has passed, rhubarb-like leaves grow ever bigger, eventually covering the earth in a dense canopy. The earth remains bare here because there is not enough light for even the smallest plants to grow.
Japanese butterbur (P. japonicus) is a perennial which is slightly rarer and larger than common butterbur and which has also escaped into the wild in Finland. The leaves of this yellowish-flowered species have round margins and its basal lobes touch each other. In the north common butterbur can be confused with arctic butterbur (P. frigidus), which is native to Finland, but the latter is clearly smaller and its capitula have reddish disc florets and tongue-like ray-florets.