Petasites japonicus ssp. giganteus
- Name also: Sweet Coltsfoot, Fuki, Bog Rhubarb, Giant Butterbur
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Flowering stem develops in spring before leaves. Rhizomatous.
- Height: 10–50 cm (4–20 in.), (flowering stem). Stem yellow-brown, often brown-haired until base.
- Flower: Plant dioecious (staminate and pistillate flowers on different individuals), only male plants in Finland. Single flower-like capitula surrounded by involucral bracts. Capitulum lacks ray-florets; disk florets yellowish, tubular. Stamens 5. Gynoecium composed of 2 fused carpels Involucre 8–10 mm (0.3–0.4 in.) long, involucral bracts lime green or brown, usually entire tips, sparsely hairy. Capitula usually 20–35, dense, borne in a wide corymb.
- Leaves: Alternate. Flowering stem leaves stalkless, scale-like, ovate, with rounded tip, light green. Vegetative leaves long-stalked, blade kidney-shaped, up to 80 cm (32 in.) wide, irregularly toothed, underside sparsely hairy, upper surface almost glabrous, basal lobes with rounded tip, curving towards each other.
- Fruit: Achene with unbranched hairs on tip. Fruits do not form in Finland.
- Habitat: Rich coastal forests, lake shores, banks, parks. Ornamental, left over from old gardens and an escape.
- Flowering time: (April–)May.
- Harmfulness: Potentially or locally harmful alien species.
Japanese butterbur is a dioecious plant, meaning that the staminate and pistillate flowers are on different plants. In Finland the species has come from east Asia, but there are only male plants here so cypselas do not form: the plant can only spread through its rootstock. Japanese butterbur proliferates well however, and once it establishes a foothold it gradually takes over the whole area. It suffocates other plants with its umbrella-sized leaves, which are exceptionally big even for butterbur plants.
Like our native coltsfoot, butterbur flowers early in the spring and the leaves only reach their full size well after the plant has bloomed. The reasons for this strategy lie in pollination biology: there are plenty flowers to choose from in the middle of summer and part of the pollen is wasted on the wrong flowers. Additionally, butterburs form large clones of a single individual, so the pollinator has to travel long distances for the pollination to be successful. Japanese butterbur attempts to ensure this by blooming at a time when there are as few as possible other sources of nectar around. Butterburs are important nectar plants for e.g. the spring’s first butterflies and bees. Multiplying through vigorous growth is a part of the plant’s pioneering nature. Japanese butterbur thrives on river banks, but also e.g. volcanic ash fields, where it is one of the first species to take up residence. Apart from Japanese butterbur, common butterbur (P. hybridus) also grows in Finland as an escape. Its flowers have some red in them, its leaves are slightly angular and the basal lobes do not touch each other.