Sp. Pl. 2: 683. 1753.
Subshrubs or herbs, annual or perennial, 0.2–1 m, rarely taller. Stems erect, minutely stellate-hairy, hairs to 0.5 mm. Leaves: stipules free from petiole, 1-veined, subulate, 3–6 mm, 1/2 as long as petiole; petiole 5–15 mm, usually 1/4–1/2 length of blade, sometimes shorter, minutely stellate-hairy, hairs to 0.5 mm, usually with small spinelike tubercle on stem just below its attachment; blade ovate, lanceolate, or narrowly oblong, 2–6 cm, smaller apically, 2–5 times longer than wide, base subcordate, margins crenate-serrate to base, apex usually acute, surfaces stellate-tomentulose abaxially, glabrate adaxially. Inflorescences axillary solitary or 2–4 clustered flowers. Pedicels 0.5–1 cm, subequal to calyx and subtending petiole. Flowers: calyx angulate, 5–7 mm, minutely tomentose, lobes triangular; petals yellow, rarely white, 5 mm; staminal column glabrous; style 5-branched. Schizocarps subconic, 4–5 mm diam., hairy; mericarps 5, 3–4 mm, somewhat rugose, apex spined, spines 1 mm, antrorsely hairy. 2n = 14, 28.
Phenology: Flowering year-round in warmer areas, summer elsewhere.
Habitat: Roadsides, pastures, disturbed ground
Elevation: 0–1500 m
Ont., Ala., Ariz., Ark., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Miss., Mo., Nebr., N.J., N.C., Ohio, Okla., Pa., S.C., Tenn., Tex., Va., W.Va., Mexico, West Indies, Central America, South America.
A small spur sometimes is present on the abaxial side of the petiole at the juncture with the stem, to which the specific epithet refers. It is not a spine and occasionally is absent.