An annual sunflower, extremely variable in height and appearance, with hairy stems. Flowerheads of wild form are many per plant, large, with brown disks, and frequently with a double set of yellow ray florets. Disk usually a couple inches in diameter, not including the rays. Blooms July–November. Cultivated forms often have only one, often huge, flowerhead per plant. Leaves are large, ovate to broad, with irregular, large teeth, mostly alternate except the uppermost ones. Lower leaves are usually heart-shaped. All leaves are rough and hairy.
Similar species: There are 16 species of Helianthus in Missouri. Common sunflower’s leaves have long petioles, lacking wings, and are rather broad, with the larger leaves ovate to heart-shaped with irregular, large teeth, and are mostly alternate except those at the very top. Also, its disks are reddish brown to dark purple, while several other sunflowers have yellow disks.