Annual Sunflower
Helianthus annuus
Aster family (Asteraceae)

Description: This is an annual wildflower with a large and stout central stem about 3-9' tall, although occasionally smaller. Toward the apex of the plant, there may be a few side stems, but it is tall and columnar overall. The central stem is light green to reddish green, terete, and covered with stiff spreading hairs. The large alternate leaves are up to 8" long and 6" across – they have a tendency to droop downward from the long petioles. They are cordate, ovate-cordate, or ovate with fine dentate margins, although some of the small upper leaves may have smooth margins and a lanceolate shape. The upper surface of the leaves is dull green and covered with short stiff hairs, providing it with a sandpapery feel. The petioles are light green to reddish green, and covered with short stiff hairs; the upper surface of each petiole is channeled.

The daisy-like flowerheads consist of numerous central disk florets (each about 1/8" across) that are yellow to brown; they are surrounded by approximately 20-40 ray florets. The petal-like extensions of the ray flowers are yellow. Each flowerhead is about 3-5" across. At the bottom of each flowerhead, there are large overlapping bracts in 2-3 series. These floral bracts are dull green, stiffly hairy, and ovate in shape, tapering abruptly to form long narrow tips. An average plant will bear from 1-12 of these flowerheads, and bloom from mid- to late summer for about 1 months. There is not much of a fragrance, although the florets have a musty smell that is peculiar to sunflowers. During the fall, the disk florets are replaced by large seeds that are ovoid and somewhat flattened in shapee; they are dispersed by gravity when the tall plants topple over during the winter. Like many other species in its genus, the Annual Sunflower exudes chemicals that kills off other kinds of vegetation. Thus, it has a tendency to form colonies that exclude other plants, particularly in disturbed areas.

Cultivation: The preference is full sun, moist to slightly dry conditions, and a fertile loamy soil. However, this wildflower often thrives in soil with a high clay or gravel content. There is a natural tendency for the lower leaves to shrivel and drop whenever there are extended spells of hot dry weather. Powdery mildew sometimes attacks the leaves during the fall, but this is usually after the plant has finished blooming and is forming seeds. It is easy to start new plants from seeds.

Range & Habitat: The native Annual Sunflower occurs throughout Illinois; it is especially common in the central and northern areas of the state (see Distribution Map). Habitats include disturbed areas of mesic to dry prairies, meadows in wooded areas, cultivated and abandoned fields, pastures, areas along railroads and roads, and urban waste areas. Annual Sunflower may occur sporadically as individual plants, or in small to large colonies that persist year after year. This rather weedy wildflower can be controlled by summer wildfires or periodic mowing.

Faunal Associations: Long-tongued bees are the most important pollinators of the flowers, including the honeybee, bumblebees, digger bees (Melissodes spp.), and leaf-cutting bees (Megachile spp.). Short-tongued bees that are important visitors of the flowers include Halictid bees, alkali bees, and some Andrenid bees. Some bees are specialist pollinators (oligoleges) of sunflowers; the oligolectic bees Andrena accepta, Andrena helianthi, Dufourea marginatus, Melissodes agilis, and Pseudopanurgus rugosus have been observed to visit the flowers of Annual Sunflower. Visitors of lesser importance include bee flies, butterflies, skippers, and the Goldenrod Soldier Beetle (Chauliognathus pennsylvanicus). These insects are searching for nectar and pollen. In addition to these floral visitors, many insects feed on the leaves and other parts of Annual Sunflower (see Insect Table). Because the seeds are abundant, large-sized, and nutritious, they are an attractive food source to many vertebrate animals, including upland gamebirds, songbirds, ground squirrels, tree squirrels, and mice. To some extent, the seeds are distributed by these animals to new locations. Mammalian herbivores, such as rabbits, ground hogs, and deer, often consume the foliage, particularly from young plants. When this sunflower grows near sources of water, muskrats and beavers sometimes eat its stems and other parts; beavers also use the stems in the construction of their lodges and dams.

Photographic Location: The above photographs were taken along a railroad near Champaign, Illinois.

Comments: The Annual Sunflower is thought to be adventive from western United States. However, it was cultivated as a source of food by native Americans, and was likely introduced to Illinois by them prior to European settlement. The cultivated sunflower of modern agriculture is a self-pollinating hybrid of this plant and another annual sunflower that occurs in the Great Plains. Because of its large heart-shaped leaves, it is easy to distinguish the Annual Sunflower from other Helianthus spp. (Sunflowers) that occur in the Midwest.