A giant perennial herb, usually branched, sometimes occurring in dense colonies of clumping stems. Lower stems often hairless, reddish, sometimes with a white waxy coating. Flowerheads all yellow, to 3½ inches across, with 10–25 fairly wide ray florets. Blooms July–October. Leaves sessile or with very short petioles, the petioles often with small wings. Leaves about 10 inches long and 2½ inches wide, coarsely toothed, lanceolate; upper leaves closely spaced, usually alternate.
Similar species: Sunflowers readily hybridize with each other, which can make identification difficult. Not counting hybrids, there are 16 species of Helianthus recorded for Missouri. This species is perhaps best identified by its leaves, which are mostly alternate, very narrow, folded lengthwise along the midvein, with flat (uncurled) leaf margins, and yellow disk florets.