Aconitum japonicum aka Japanese Monkshood

Aconitum japonicum is a perennial in the buttercup family, Ranunculaceae. Gardeners know it as Japanese monk's hood. Kew places its native range in central Honshu, Japan. Plants For A Future gives a wider East Asian range. That account includes Korea, northeastern China, and other Japanese islands. It records the plant in woods, woodland edges, and streamside sites.

This upright plant forms a clump and sheds its leaves. Garden plants reach about 1.6–3.3 feet [0.5–1 metre] tall. Their spread reaches about 1.6 feet [0.5 metres]. The glossy, dark green leaves have deep divisions. Each leaf can reach about 0.5 feet [0.16 metres] long. Deep blue flowers grow in branched clusters called panicles. Each bloom has a hood-like shape. The RHS places flowering in late summer. Plants take two to five years to reach full size.

Choose a sheltered spot with fertile soil that holds moisture. Partial shade suits this plant best, though it tolerates sun. The RHS lists both clay and loam as suitable soils. It also reports tolerance of most other soil types. Acid, neutral, and alkaline soils can all suit it. Keep the soil moist, but allow water to drain. The RHS gives it an H7 hardiness rating. This covers cold below minus 20 degrees Celsius under UK conditions. Its clumps suit flower beds, borders, and informal cottage gardens. The RHS also suggests it for wildlife gardens. Plants For A Future places it in woodland gardens with dappled shade.

Divide clumps in autumn or late winter to renew growth. Divided plants may take time to settle back in. Pruning needs are limited to removing dead stems. Wear gloves and protective gear when handling any plant parts. Aphids may attack the plant. Fungal stem rot, powdery mildew, and Verticillium wilt may also occur.

All parts are highly poisonous, with risks to people and pets. Eating the plant is dangerous; avoid skin contact as well. Plants For A Future reports numbness after contact in some people. It lists no known food uses. The same source records medicinal use of roots in China. These uses include treatments for pain and rheumatic complaints. Such records do not make this a safe home remedy. The source warns against internal use and calls for qualified supervision.