Giant Ragweed
Ambrosia trifida
Aster family (Asteraceae)

Description: This plant is a summer annual that becomes 3-12' tall, branching occasionally. The green stems are covered with white hairs. The opposite leaves are up to 12" long and 8" across. The larger leaves are divided into 3 or 5 lobes, usually serrated along the margins, and have long petioles that are sometimes winged. The smaller leaves near the base of an inflorescence are lanceolate and often hairy underneath. Many of the upper stems terminate in a cylindrical spike of flowers, about 3-6" long, with one or more smaller spikes near its base. The small flowers are are yellowish green and devoid of petals and sepals. They occur in small drooping clusters less than " across on short pedicels, and are densely arranged all around each spike. The fine pollen of the male flowers is easily carried aloft by the wind. This typically occurs during late summer or early fall. The seeds are large, tough-coated, and remain viable in the soil for several years. The root system is fibrous.

Cultivation: The preference is full sun to light shade, moist conditions, and fertile loamy soil. Under these circumstances, Giant Ragweed can develop into a huge plant. It also tolerates slightly drier conditions, but the large leaves have a tendency to wilt and wither away if there's a significant drought. Poor soil containing some clay or gravel is also tolerated – under these less than ideal conditions, this plant will be smaller. This common weed may introduce itself spontaneously to the wildflower garden or a native habitat without any official encouragement.

Range & Habitat: The native Giant Ragweed is a common plant that occurs in every county of Illinois (see Distribution Map). It can be found in disturbed areas of moist to mesic black soil prairies, especially along the margins near woodlands or fields. Other native habitats include disturbed areas of moist clay prairies, meadows in woodland areas or near rivers, thickets, and woodland borders. In more developed areas, it occurs in vacant lots, cropland, abandoned fields, poorly drained waste areas, areas along roadsides and railroads, and fence rows.

Faunal Associations: The flowers depend on wind pollination, therefore they attract few insects. However, Apis mellifera (Honey Bee) has been observed gathering the pollen. The caterpillars of several moths feed on the foliage and other parts of Giant Ragweed (see Moth Table). Because of their hard coat, the seeds of Giant Ragweed are less attractive to birds than the seeds of Common Ragweed. A few upland gamebirds eat the seeds to a limited extent, including the Greater Prairie Chicken. Cattle like to browse on the foliage of Giant Ragweed, and to a lesser extent it is also browsed by White-tailed Deer. Because of the poor digestibility of the seeds, it is possible that they are distributed by these birds and mammals.

Photographic Location: The photographs were taken at Meadowbrook Park in Urbana, Illinois.

Comments: This huge plant is probably an allergy sufferer's worst nightmare. It has some ecological value to various moths, but otherwise is less important than Ambrosia artemesiifolia (Common Ragweed). Giant Ragweed can be distinguished from other Ambrosia spp. (Ragweeds) by its palmately lobed leaves; other Ragweeds have leaves that are pinnatifid or bipinnatifid. The name of this genus of plants refers to ambrosia, "the food of the gods" in antiquity. This seems like a strange name for a group of unattractive plants, unless it refers to the value of the seeds of certain species from a bird's point of view.