Flower:

Single daisy-like flower at the end of a branching stem. Flowers are 3 to 6 inches across with 17 to 40 yellow rays (petals). The center disk is larger than most sunflowers, rarely less than 1¼ inches across, the disk flowers dark reddish brown to yellow, with yellow styles.

The bracts behind the flower are in 2 or 3 layers, generally oval with a long abruptly tapered tip, and short stiff hairs around the edges. A plant typically has 1 to 12 flowers

Leaves and stems:

Leaves are 4 to 16 inches long, 2 to 8 inches wide, egg to heart-shaped to triangular with rounded angles at the base, rough-textured on both surfaces, usually toothed edges that may be a bit wavy, 3 primary veins radiating from the base, and a stalk ¾ to 8 inches long. Attachment is mostly alternate but sometimes opposite in the lowest leaves. The main stem is quite stout and covered to varying degrees in stiff hairs.

Notes:

Common Sunflower is present in all 50 states, though only native to the lower 48, and is the plant from which many cultivated sunflowers were derived. I have mostly seen it in full sun along roadsides and construction sites, where the soil is heavily disturbed and usually dry. Common Sunflower has been labeled a noxious weedy species in some agricultural areas but modern herbicides have taken care of that. Overall it resembles Prairie Sunflower (Helianthus petiolaris) which has leaves and a center disk that are proportionately much smaller.