Arnica is a genus of perennial, herbaceous flowering plants in the family Asteraceae, comprising approximately 31–32 accepted species. Plants grow from 5 to 100 cm tall, typically arising from long, thin rhizomes or woody caudices. Stems are erect and may be simple or branched, bearing mostly opposite leaves that range widely in shape — from cordate and ovate to lanceolate and spatulate. The foliage is downy and soft, and the entire plant emits a distinctive pine-sage scent when the leaves are bruised or rubbed.
Flowers are the genus's most striking feature: large, daisy-like heads 6–8 cm across with showy yellow or orange ray florets. The inflorescence structure includes 5–23 persistent phyllaries arranged in one or two series, and the seed-like fruits (cypselae) bear persistent pappi of barbellate to plumose bristles. Occasionally, discoid (rayless) forms occur. Polyploidy and apomixis are common throughout the genus, contributing to considerable morphological variability and taxonomic complexity.
The genus has a circumboreal, predominantly montane distribution, with the greatest species diversity in western North America. Species occupy a range of mountain habitats — from open pastures and woodland on neutral to calcareous soils to bare rocky alpine slopes and summits.
Arnica is best known for Arnica montana, a European species with a long history of use as a topical herbal remedy for bruising, sprains, and inflammation. The whole plant contains helenalin, a sesquiterpene lactone that is toxic if ingested; medicinal preparations are therefore restricted to external use on unbroken skin. The genus name is thought to derive from the Greek arni, meaning "lamb," in reference to the plants' characteristically soft, woolly leaves.
Etymology
The genus name Arnica is thought to derive from the Greek word arni, meaning "lamb," an allusion to the plants' characteristically soft and downy foliage. The name was formally established by Linnaeus in his Species Plantarum (1753), where the genus was described on page 884. Several later synonyms — including Arnicula Kuntze, Aliseta Raf., Mallotopus Franch. & Sav., Peritris Raf., and Whitneya A.Gray — have since been reduced to synonymy under Arnica L.
Distribution
Arnica has a circumboreal, predominantly montane distribution centered on the Northern Hemisphere. The greatest concentration of species diversity occurs in western North America, from Alaska and British Columbia south through the Rocky Mountain states to California and Arizona. The genus also occurs across northern Europe (including Austria, Belgium, Bulgaria, Denmark, Finland, and the Baltic states) and into central and northern Asia (including the Altay, Buryatiya, and Aleutian Islands regions). In Switzerland, two species are recorded: Arnica chamissonis and Arnica montana. The genus has been introduced into the western Himalaya region. Individual species range from common and widespread to narrow endemics restricted to specific mountain environments.
Taxonomy
Arnica belongs to order Asterales, family Asteraceae. Its current tribal placement is tribe Madieae, subtribe Arnicinae — a reclassification from the earlier assignment to tribe Senecioneae, which was based on the fine-bristled pappus structure shared with that group. POWO recognizes 31 accepted species; SEINet catalogs over 50 names including synonyms, varieties, and infraspecific taxa. GBIF records 84 descendant entries under the backbone taxon.
A notable feature of the genus is a base chromosome number of x = 19. Polyploidy and apomixis are widespread, producing considerable morphological variability and making species delimitation challenging. Five heterotypic synonyms at genus rank are currently recognized by POWO: Aliseta Raf., Arnicula Kuntze, Mallotopus Franch. & Sav., Peritris Raf., and Whitneya A.Gray.
Ecology
Arnica species are characteristic plants of montane and subalpine zones. They occupy a variety of open habitats including pastures, open woodland on neutral to calcareous soils, and bare rocky alpine slopes and summits. The genus is adapted to cool, moist conditions and tends to favor well-drained, humus-rich substrates. Species diversity within the genus reflects adaptation to specific mountain environments, with some taxa being widespread generalists and others highly restricted endemics. Arnica species also serve as larval food plants for certain Lepidoptera, including Bucculatrix arnicella.
Cultivation
Arnica is cultivated primarily as an ornamental and medicinal herb. Plants are hardy in USDA zones 3–7 and require moist, well-drained, humus-rich soil; most species prefer lime-free conditions, though some are more lime-tolerant. They grow in semi-shade to full sun and tolerate a range of soil textures from light to heavy. In garden settings, Arnica benefits from cool temperatures and good drainage that mimics its native montane habitat.
Propagation
Arnica can be propagated from seed or by division. Seeds should be sown in early spring in a cold frame, covered lightly with soil, and kept cool until germination. Division of established clumps is carried out in spring. The genus's tendency toward apomixis means that some seed-raised plants may be genetically identical to the parent.
Uses & Folklore
Arnica montana has been used in European folk and herbal medicine for centuries, primarily as a topical treatment for bruising, sprains, muscle pain, and inflammation. The plant contains helenalin, a sesquiterpene lactone with anti-inflammatory and vulnerary properties that promotes blood circulation and accelerates tissue healing. Because helenalin is toxic if ingested — causing headache, drowsiness, and in severe cases collapse — medicinal use is restricted to external application on unbroken skin. Internal preparations require professional medical supervision.
Arnica is also widely used in homeopathic preparations marketed for traumatic injury and shock relief; however, clinical evidence for the effectiveness of homeopathic Arnica is not established. Historically, the dried leaves of some species were smoked as a tobacco substitute, and preparations have also been used as a hair conditioner. Claims regarding medicinal properties of North American Arnica species remain largely unsubstantiated compared to the European A. montana.
