Herbs, perennial, 1–15 dm diam.; from branched, woody caudex or stoloniferous. Stems 1–20, creeping and rooting or suberect, basally woody, clad in old leaf bases, glabrous or sparsely pilose. Leaves persistent, basal and/or cauline; stipules absent or persistent, adnate to petiole, free portion ovate to obovate, deeply 2–5-fid, or linear, margins entire; petiole present; blade spatulate, [1–]2–15[–18] cm, herbaceous, leaflets 4–10 per side, opposite, ovate, oblong, or elliptic, margins flat or revolute, incised or pinnatisect, dentate or entire, surfaces hairy or glabrous. Inflorescences terminal or axillary, [4–]15–100-flowered, spikes or globose heads; peduncles present or absent; bracts absent; bracteoles present. Pedicels present. Flowers 3.5–6 mm diam.; hypanthium obtriangular, ovoid, or obconic, 3.5–6 mm diam., spiny [or winged or neither], tomentose or glabrous; sepals 4–5[–6], spreading, elliptic or triangular; petals 0; stamens 2–4[–6], longer than sepals; carpels 1[or 2], styles white or red, fimbriate. Fruits achenes, 1[–2], spindle-shaped; hypanthium persistent, enclosing achenes, gray- to red-brown, ovoid, oblong, globose, or obconic, 3–6 mm diam., hardened, sometimes winged, distinctly or obscurely angled with a single spine at apex of each of 4 angles, or covered with 10–20[–50] spines [or without spines], spines usually bearing retrorse hairs or barbs at apices, pubescent or glabrous, eglandular or glandular; sepals persistent, spreading. x = 21.
w United States, Mexico, South America (Argentina, Chile), s Africa (Cape Province), Atlantic Islands (Gough Island, South Georgia, Tristan da Cunha), Indian Ocean Islands (Kerguelen, Prince Edward Island), Pacific Islands (Hawaii, New Guinea, New Zealand), Australia, introduced in Europe (England, Ireland).
Species ca. 45 (3 in the flora).
|1||Inflorescences interrupted spikes; fruiting hypanthia bearing 10–20 spines 1–3 mm; plants decumbent to suberect; leaflets pinnatisect.||Acaena pinnatifida|
|1||Inflorescences globose heads; fruiting hypanthia 4-angled, each angle with a spine at apex (arising distally) 7–15 mm; plants creeping or stoloniferous; leaflets dentate||> 2|
|2||Fruiting hypanthia: spines 7–12 mm, without subsidiary spines on angles; stems 1.5–2 mm diam.; leaflet adaxial surfaces bright shining green, smooth, abaxial surfaces glaucescent.||Acaena novae-zelandiae|
|2||Fruiting hypanthia: spines 9–15 mm, with subsidiary spines on angles; stems 2–3 mm diam.; leaflet adaxial surfaces glossy green, rugulose, abaxial surfaces whitish with uneven wax layer.||Acaena pallida|