Small annual herbs. Stems decumbent, well branched, branches ascending, pilose. Leaves alternate, tripinnatifid to tripinnate, 1.5-5 cm long, pilose on both surfaces, petiole widened beneath flower heads. Heads solitary in leaf axils, sessile, ca. 3 mm long, 5 mm across, scattered along stem. Involucre hemispheric, bracts ca. 2 series, subequal, herbaceous, greenish, oblong to lanceolate, pilose on adaxial surface, entire, margins ciliolate, 4-4.5×1.5-2 mm. Outer florets 13-15, in several rows, style bifurcate at tip, 1 mm long. Central florets 5-6; corolla greenish, tubular, 1.6-1.8 mm long, 4-lobed at apex; style capitate, not branched at tip; stamens 4. Achenes ca. 2×3 mm, minutely pubescent, with thin planar wings, wings indented below middle resulting in a large upper lobe and a smaller basal lobe; styles becoming hard at maturity of achene, persistent, spinose, often with an additional incurved spine on shoulder of each upper lobe.

TAIPEI: Natl. Taiwan Univ., Peng 14818; Yangmingshan, Peng 13985. TAOYUAN: Tayuan, Chiang-kaishek International Airport, Peng 17111.

A native of South America, S. pterosperma has become fully naturalized in lawns of university campuses and city parks in northern Taiwan within the past decade. It is a particularly noxious weed in lawns, because of the abundant prickly infructescences.