Plants annual, 20–50(–90) cm, essentially glabrous. Leaves petiolate, petiole 1–5 cm; blade triangular-ovate in outline, 2–8 × 2–6 cm, 2–3-pinnate/pinnatisect; ultimate segments linear–filiform to 15 × 0.2–0.5 mm. Umbels 2.5–5 cm across; bracts 3–8, linear-subulate, 5–7 mm; rays 6–20, 1–3 cm; bracteoles 5–10, linear, 2–3 mm; umbellules ca. 1 cm across, ca. 20-flowered; pedicels 0.5–4 mm, unequal. Calyx teeth conspicuous, minute, ovate or obsolete. Petals ca. 1.3 × 1.3 mm. Fruit 1.2–2 × 1.2–1.8 mm, densely covered in whitish minute papillae. Fl. & fr. May–Aug.
Annual herb, ± 25–65 cm. tall, much branched from near the base upwards except in depauperate plants, with long divergent or more divaricate branches; stem and branches glabrous, slender, striate and terete or the stem somewhat sulcate.
Petals white or tinged with pink, obcordate with broad lobes, 1–1.25 mm., shortly white-hairy; anthers dark purplish black.
Fruit ovate, ± 2–2.5 × 1.75–2 mm., covered with white papillae or at the extreme apex frequently glabrous or almost so and deeply grooved, ribs prominent; stylopodia bluntly conical; styles slender, deflexed or flexuose, subequalling to twice as long as the stylopodia.
Basal leaves withered at time of flowering; lowest stem leaves ± 5–14 × 2–6 cm., lamina broadly oblong or ovate, ± bi- or subtripinnate with 3–5 pairs of pinnae, basal pair sessile or with petiolules up to ± 5 mm.; pinnae rather irregularly divided into narrowly linear, glabrous, rather bluntly mucronate segments up to ± 1.4 cm. long and ± 0.5 mm. wide, with ± revolute margins and not dentate; petioles ± 2.5–8 cm. including the tapering, 8–12 mm., ribbed sheath; upper stem leaves reduced and more shortly petiolate and finally sessile but similar; uppermost leaves also much divided, but small.
Umbels numerous, with 6–20 usually obviously unequal rays 5–21 mm. long, glabrous to minutely scabrid; involucre conspicuous, of 2–4 linear or sometimes trisect or ± pinnate, scabrid-margined, often unequal bracts 3–20 mm. long; partial umbels ± 8–24-flowered, pedicels 1–4 mm., minutely scabrid to subglabrous; involucel of ± 6–8 subulate bracteoles 1–5 mm. long, with scabrid margins; peduncles ± 2–8.5 cm., glabrous.