- Name also: Woolly Butterbur
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Flowering stem develops in spring before leaves. Rhizomatous.
- Height: 20–45 cm (8–18 in.), taller in fruit (flowering stem). Stem very hairy at top.
- Flower: Plant dioecious (staminate and pistillate flowers on different plants), only male plants in Finland. Single flower-like capitula surrounded by involucral bracts. Capitulum flowers pale yellow, fading, ray-florets tongue-like; disk florets tubular. Stamens 5. Gynoecium composed of 2 fused carpels. Involucre 5.5–8 mm (0.2–0.3 in.), involucral bracts dark olive green–brownish, shortly haired only at tip. Capitula 40–70, borne in a corymbose cluster.
- Leaves: Alternate. Flowering stem leaves stalkless, lower sheath-like, upper scale-like, narrowly elliptic, lime green or sometimes at least outer leaves becoming red. Vegetative leaves long-stalked, blade broadly triangular, approx. 20 cm (8 in) wide, 2–4-lobed on both sides as base, becoming glabrous on top, densely haired underneath, basal lobes straggly.
- Fruit: Achene with unbranched hairs on tip.
- Habitat: Waterside sandpits.
- Flowering time: May.
- Endangerment: Critically endangered.
Wooly butterbur has established itself in Finland on only one island in eastern Helsinki. The stand is around 2 acres big, but seems to be growing. It was found in 1974, but one can only guess when and where it arrived. It has perhaps spread from drifting rootstock; on the other hand a whole group of other special plants have escaped into the Finnish wild from villa gardens. Like its relatives, wooly butterbur inflorescence and plant shoots develop at different times. Flowering stems break through the ground early in the spring, while long-stalked leaves do not appear until the beginning of summer. Many wooly butterbur plants are at least partly dioecious; its female flowers’ stamens’ anthers are rudimentary and in the male flowers the pistil’s ovary is likewise undeveloped. In Finland the plant has only been observed with a male inflorescence, so the whole stand is probable one and the same plant, like clones. Seeds cannot be produced in this place, so the plant cannot easily spread to new habitats. Wooly butterbur spreads efficiently however via its rootstock.
Wooly butterbur was also observed in Finland in Eckerö on the Åland Islands in the 1930s, but this temporary stand has now vanished. The plant grows along many coastal areas of the Baltic Sea, in Denmark and Sweden too, as far north as Gotland. It also grows in Estonia, albeit rarely, and in principle it could spread from there to the Baltic archipelago and sandy southern coastal areas of Finland. Wooly butterbur is easy to differentiate from common butterbur (P. hybridus), which is an ornamental and occasional escape in Finland, based on the difference in the inflorescence of their capitula; wooly butterbur’s is bright yellow while common butterbur’s is red.