Annuals, 5–15(–50+) cm (usually strongly scented). Stems 1–5+ from bases, erect to ascending, branched ± throughout, ± puberulent or sparsely strigillose (hairs basifixed) and gland-dotted [glabrous]. Leaves mostly cauline; alternate; petiolate or sessile; blades ovate to obovate [linear], usually [1–]2(–3)-pinnately lobed, ultimate margins entire, faces ± puberulent or sparsely strigillose and gland-dotted [glabrous]. Heads discoid [radiate], borne singly or in corymbiform arrays (pedunculate). Involucres ± hemispheric or broader [cylindric], [3–]4–6[–8+] mm diam. Phyllaries persistent or tardily falling (± reflexed in fruit), [15–]22–35[–60+] in 3–4+ series, distinct, lance-triangular to ± ovate or elliptic (± navicular, carinate, sometimes each with central resin canal), unequal, margins and apices scarious. Receptacles [± flat] hemispheric, ± globose, or conic, epaleate (often muricate). Ray florets 0 [to 10, pistillate, fertile; corollas white, laminae ± obovate]. Disc florets [20–]100–250+, bisexual, fertile; corollas yellow, tubes ± cylindric (often basally invaginated and ± decurrent onto ovaries; tubes and proximal portions of throats externally continuous and ± dilated with relatively large, quadrate cells, often oblique, fragile when dry, distally constricted and texturally distinct from ± campanulate distal portions of throats), lobes 4, triangular. Cypselae columnar to prismatic (proximally tapered), ribs 4, faces gland-dotted between ribs, especially adaxially (pericarps without myxogenic cells or resin sacs; embryo sac development monosporic); pappi coroniform (shorter adaxially), dentate or entire. x = 6, 8.
Introduced; s Africa, also introduced in Australia.
Species 8 (1 in the flora).
Oncosiphon suffruticosum (Linnaeus) Källersjö [Matricaria suffruticosa (Linnaeus) Druce] was collected on ballast at Portland, Oregon, in 1902. It differs from O. piluliferum by smaller (3–5 mm diam.) heads clustered in dense, ± flat-topped, corymbiform arrays. Both O. piluliferum and O. suffruticosum are common weeds in their native southern Africa and in Australia. Both have the potential to become serious pests in western North America. They are unpalatable to livestock and tend to dominate infested pasturelands. When consumed by animals, they taint the flavor of milk and meat.