common sunflower
Helianthus annuus                                      

Identifying Characteristics:

Seed:  white,grayish, or brown with black mottling, ovate to wedge-shaped, flattened, enclosed in nutlet

Seedling:  hypocotyl stout, green, stiff white trichomes; cotyledons glabrous, prominent midvein; lower true leaves opposite

Mature Plant: fibrous roots from taproot; erect stem, tall, green, stout and course; mostly alternate leaves, ovate to obtuse, simple, rough on both surfaces; showy inflorescences, yellow to orange-yellow, numerous, red to dark reddish brown; disk florets perfect

Method of Propagation: seed

Life Cycle: erect annual

References: 
Bryson, Charles T. & Michael S DeFelice. "Weeds of the Midwestern United States & Central Canada."