Petasites japonicus aka Japanese Butterbur

Petasites japonicus is a perennial known as Japanese butterbur. It belongs to the daisy family, Asteraceae. Its native range spans parts of temperate eastern Asia. These include China, Korea, Japan, and Sakhalin. In the wild, it grows in damp woods and thickets. It is not native to Britain or Ireland. Underground rhizomes let it spread and form broad mats.

Garden plants reach about 3–5 feet [0.9–1.5 metres] tall. The RHS gives a spread of 13–26 feet [4–8 metres]. Its large, nearly round leaves define its summer form. Each leaf can reach 3 feet [0.9 metres] across. The plant sheds its leaves as the season ends. Its wide spread needs more room than most small borders offer. The leaves make it useful as ground cover in damp shade.

Flowers appear in late winter or early spring. The RHS reports February blooms; Chicago records March to April. Their colour ranges from cream or white to yellowish white. The blooms have scent and stand on stout, upright stems. These stems reach about 1 foot [0.3 metres] tall. Plants sold for gardens are most often male. The flowers add interest well before the large summer leaves develop.

Grow this plant in partial shade or full shade. It needs deep, fertile soil rich in humus. Keep the soil moist, but avoid stagnant conditions. The RHS lists clay and loam as suitable soils. It accepts acid, neutral, and alkaline soil. Chicago lists USDA zones 5–9; the RHS gives H6. Suitable sites include pond banks, stream edges, and bog gardens. It also suits moist woodland borders and helps cover erosion-prone ground. Divide plants in spring to make new plants. Remove old leaves in late winter, before fresh growth starts.

Rhizomes spread strongly unless containers or barriers hold them back. This can make the plant a nuisance in garden beds. Most insects and diseases cause few problems, though slugs need watching. PFAF records cooked leaf stalks, flower buds, and young flowering stems as foods. The stalks may also be pickled or kept in miso. Flower buds serve as a bitter flavouring in Japanese dishes. PFAF lists no known hazards, but gives no firm safety assessment. It also records traditional medicinal use for coughs and breathing problems. These reported uses do not establish treatment safety or benefit.