Common Sunflower

Helianthus annuus

Common sunflower
Common sunflower sunflower. Photo courtesy of Forest and Kim Starr Licensed under CC BY 2.0.
The common sunflower is an annual herb that grows to a height of between 1.5 and 8 feet. It is easily identified by it's bright yellow flower head and course, hairy exterior.
Seedlings germinate in dry to somewhat moist soils, initially growing from a taproot. Cotyledons are oblong and smooth. The first two leaves are arranged opposite, a dull green color, and are covered in short, bristly hairs. Edges of these leaves are weakly-toothed, with each tooth being rounded. Subsequent leaves are arranged alternate and maintain the bristly exterior.
Common sunflower seedling
Common sunflower seedling. Photo courtesy of Phil Westra. Licensed under CC BY 3.0.
The lower leaves of the common sunflower are bigger than those at the top, and may be opposite. Upper leaves are alternate. The stem is generally highly branched and rough to the touch. The leaves are heart-shaped or oval and have coarsely serrated edges. Each leaf is connected to the stem by a long petiole.
Common Sunflower Foliage
Common sunflower foliage. Photo courtesy of Howard F. Schwartz. Licensed under CC BY 3.0.
Each flower head (often erroneously referred to as the flower itself) sits on top of a single stem. It is large and showy, surrounded by a ray of bright-yellow petals. The central disk of the flower head is maroon and filled with tiny maroon flowers that eventually become black sunflower seeds. The common sunflower often has a difficult time propagating due to its attractiveness as a food source for birds and other animals.
Common Sunflower Seedhead
Common sunflower seedhead. Photo courtesy of Zion National Park. Licensed under CC BY 2.0.
The common sunflower is listed as a noxious weed throughout many parts of the United States. It competes heavily for nutrients and water, often difficult to control once spreading of seeds has begun.
Always be sure to buy clean seed, drive around (rather than through) weedy areas, and clean equipment that has been used in a weedy area in order to prevent infestation. In the case of an established crop, hoeing and cultivation can be used to effectively control common sunflowers. If seeds continue to germinate, harrowing may need to be done multiple times.
Timing is very important in dealing with common sunflowers. Young plants are much more susceptible to herbicide treatment, as mature plants have established a strong root system. Herbicides applied to a stressed plant will have no effect. The best time to apply any sort of chemical herbicide to common sunflowers is after a rain, since this is the time when they will be more likely to take in a higher volume through the roots. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.