Helianthus annuus head2
Introduced warm-season annual or biennial herb with a taproot; stems are 1–2 m tall and often branching towards apex. Leaves are ovate to ± cordate and 3–20 cm long; upper leaves are alternate, smaller and narrower. Heads are 1 or few per plant, solitary and nodding, usually 3–10 cm diam; involucral bracts are ovate to lanceolate, acuminate, scabrous and ciliate. Ray florets are bright yellow 2–3 cm long ligules. Disc florets are numerous, brownish and tubular. Achenes are flattened, 5–10 mm long, black with grey streaks and finely pubescent; pappus is absent.