Sp. Pl. 2: 884. 1753.
Gen. Pl. ed. 5, 376. 1754.
Perennials, 5–100 cm (rhizomes relatively long and thin; caudices woody, relatively short and thick). Stems erect, simple or branched. Leaves basal (sterile basal rosettes often present) and/or cauline; mostly opposite (usually 1–10 pairs, distalmost sometimes alternate and usually smaller); petiolate or sessile; blades mostly cordate, deltate, elliptic, lanceolate, linear, oblanceolate, oblong, obovate, ovate, or spatulate, margins entire or toothed (usually dentate, denticulate, or serrate, sometimes crenate or slightly lobed), faces glabrous, hirsute, hispidulous, pilose, puberulent, scabrous, tomentose, villous, or woolly, often stipitate-glandular as well. Heads radiate or discoid, borne singly or in cymiform or corymbiform arrays. Involucres campanulate, hemispheric, or turbinate, mostly 6–20+ mm diam. Phyllaries persistent, 5–23 in (1–)2 series. Receptacles convex, smooth or pitted, epaleate. Ray florets 0, or 5–22, pistillate, fertile; corollas yellow to orange. Disc florets 10–120, usually bisexual and fertile (functionally staminate in A. dealbata); corollas usually yellow, rarely cream, tubes shorter than funnelform throats, lobes 5, ± deltate (anthers usually yellow, purple in A. lessingii and A. unalaschcensis). Cypselae (gray or brown to black) ± conic, fusiform, or obovoid-cylindric, nerves 5–10(–20), faces hairy, glandular, or glabrous; pappi usually persistent, of 10–50 white or stramineous to tawny, fine, barbellate or subplumose to plumose bristles (0 in A. dealbata). x = 19.
North America, Mexico, Europe, Asia (Japan, Russia).
Species 29 (26 in the flora).
Arnica is circumboreal, predominantly montane, and exhibits maximum species diversity in western North America. It includes common and very widespread species as well as relatively uncommon, narrow endemics. Polyploidy and apomixis are common in the genus, resulting in considerable morphologic variability. Arnica montana from Europe has been used medicinally for centuries, and unsubstantiated claims have been made regarding the medicinal properties of some North American species.
Here, in key leads and descriptions, numbers of pairs of leaves refer to flowering shoots.
|1||Pappi 0 (ray corollas marcescent; disc florets functionally staminate)||Arnica dealbata|
|1||Pappi of 10–50, fine bristles||> 2|
|2||Anthers purple||> 3|
|2||Anthers yellow||> 4|
|3||Leaves mostly cauline; heads erect; pappus bristles subplumose||Arnica unalaschcensis|
|3||Leaves mostly basal; heads nodding; pappus bristles usually barbellate||Arnica lessingii|
|4||Ray florets 0 (disc corollas sometimes dilated in A. parryi, resembling rays)||> 5|
|4||Ray florets 5–22||> 9|
|5||Cauline leaves usually sessile, rarely broadly petiolate||> 6|
|5||Cauline leaves petiolate (petioles sometimes relatively broad)||> 7|
|6||Leaves: (reticulate venation inconspicuous) margins ± entire, abaxial and adaxial faces sparsely to densely pilose and stipitate-glandular; disc corollascream||Arnica viscosa|
|6||Leaves: (reticulate venation conspicuous) margins irregularly and coarsely serrate, abaxial faces pilose and stipitate-glandular, adaxial glabrate tostipitate-glandular; disc corollas yellow||Arnica venosa|
|7||Heads 1–14 (nodding in bud); phyllaries linear to narrowly lanceolate; pappi usually stramineous, rarely tawny, bristles barbellate to ± subplumose||Arnica parryi|
|7||Heads 1–30 (erect); phyllaries lanceolate, ovate, or ovate-lanceolate; pappi white, bristles usually barbellate, sometimes subplumose||> 8|
|8||Leaves lanceolate, ovate, or subcordate (petioles usually narrow or scarcely winged); cypselae stipitate-glandular and hirsute (hairs duplex)||Arnica discoidea|
|8||Leaves elliptic-ovate to spatulate (petioles relatively broad); cypselae sparsely stipitate-glandular (lacking duplex hairs)||Arnica spathulata|
|9||Leaves mostly basal (blades usually with 3, 5, or 7 prominent, subparallel veins; cauline leaves 0 or 1–2 and much reduced)||> 10|
|9||Leaves mostly cauline, sometimes also basal (blades lacking prominent, subparallel veins)||> 12|
|10||Leaves broadly elliptic, ovate, or rhombic; involucres campanulate; raysyellow; e United States||Arnica acaulis|
|10||Leaves oblanceolate, oblong, oval, or spatulate; involucres hemispheric; rays yellow-orange; Canada, w United States||> 11|
|11||Leaf blades 4.5–20 × 0.5–2.5 cm (axils with tufts of brown wool)||Arnica fulgens|
|11||Leaf blades 3.5–14.5 × 0.6–2.4 cm (axils lacking tufts of brown wool)||Arnica sororia|
|12||Cauline leaves (at least at mid stems and exclusive of distal reduced leaves) sessile||> 13|
|12||Cauline leaves (at least mid-stem pair) petiolate (petioles sometimes broad)||> 18|
|13||Pappi white, bristles barbellate||> 14|
|13||Pappi stramineous to tawny, bristles usually subplumose to plumose sometimes barbellate||> 15|
|14||Leaf blades ovate to elliptic-lanceolate, 2–10 × 1–6 cm, margins serrate to dentate||Arnica latifolia|
|14||Leaf blades oblanceolate to spatulate, 2–7 × 0.5–2.5 cm, margins usually entire, sometimes denticulate||Arnica rydbergii|
|15||Cauline leaves (2–)3(–4) pairs (pappus bristles with deep, amberlikedeposits)||Arnica mollis|
|15||Cauline leaves usually 4–10 pairs (pappus bristles lacking deep, amberlike deposits)||> 16|
|16||Phyllaries 8–23, lanceolate to linear (apices with conspicuoustufts of white hairs)||Arnica chamissonis|
|16||Phyllaries 8–20, narrowly to broadly lanceolate (apices lacking conspicuous tufts of white hairs)||> 17|
|17||Leaves lance-elliptic, obovate, or ovate, margins subentire todentate-serrate (basal leaves present at flowering)||Arnica lanceolata|
|17||Leaves lanceolate to lance-elliptic, margins usually entire(basal leaves withered by flowering)||Arnica longifolia|
|18||Cauline leaf blades elliptic, lanceolate, linear, oblanceolate, ovate, or spatulate (lengths 2.5–10 times widths; basal leaves withered by flowering)||> 19|
|18||Cauline leaf blades cordate, deltate, elliptic, ovate, ovate-lanceolate, or subcordate (lengths 1–3 times widths; basal leaves present at flowering)||> 22|
|19||Heads 1–8 (erect)||> 20|
|19||Heads 1–3 (usually nodding)||> 21|
|20||Leaves 3–7 pairs (petioles relatively narrow, lengths ± equaling blades)||Arnica lonchophylla|
|20||Leaves 1–5 pairs (petioles relatively broadly to narrowly winged, lengths shorterthan blades)||Arnica angustifolia|
|21||Leaves: margins usually entire, rarely denticulate or slightly undulate, faces ± densely stipitate-glandular; cypselae ± glandular; Alberta||Arnica louiseana|
|21||Leaves: margins usually denticulate to dentate, rarely entire, faces rarely stipitate-glandular; cypselae rarely glandular; e Canada or Alaska, nw Canada (not Alberta)||Arnica griscomii|
|22||Pappi stramineous to tawny, bristles subplumose (sterile basal leaf rosetteslacking)||Arnica ovata|
|22||Pappi white, bristles usually barbellate, rarely subplumose (sterile basal leaf rosettes present)||> 23|
|23||Leaves (relatively thick, often succulent and reddish) glabrous or scabrous; heads 1(–3; often nodding in bud; serpentine in nw California, sw Oregon)||Arnica cernua|
|23||Leaves (relatively thin, not succulent nor reddish) hairy; heads 1 or 3–15 (erect in bud; plants not of serpentine)||> 24|
|24||Stems usually much branched distally (often in dense clumps); heads (1–)5–15; cypselae black||Arnica gracilis|
|24||Stems usually simple (not in dense clumps); heads 1 or 3–5(–10); cypselae gray||> 25|
|25||Leaves cordate, ovate, or subcordate, margins dentate; involucres broadlycampanulate||Arnica cordifolia|
|25||Leaves elliptic to ovate, margins entire or denticulate; involucres campanulate-turbinate||Arnica nevadensis|