Common Sowthistle is an annual herb to 1.2–2 m high. Stems hollow. Leaves variable; basal leaves 5–25 cm long, lanceolate, base not stem clasping; stem leaves 6–35 cm long, lanceolate, usually lobed and with pointed stem clasping basal lobes. Flowerhead made up of heads in flat-topped panicles. Seeds brown.
Distinguished by non-rhizomatous habit; milky sap; leaves thin and soft, margins of leaves lobed to toothed; flower stalks and bracts around heads with or without glandular hairs, bracts around heads without dense hairs when young; bracts around heads in 3 rows, the inner row longest; all florets yellow, bisexual, fertile and with ligules 4–6 mm long; area where seeds attach to the head (receptacle) pitted, hairless; seeds 2.5–4 mm long, compressed, wrinkled, margins narrow, without a beak and topped by hairs 5–8 mm long.