Virginia Tech Weed Identification Guide

Giant Ragweed: Ambrosia trifida

Weed Description: Erect summer annual that may reach 16 feet in height.  Leaves are large and distinctively 3-lobed, or less often 5-lobed.  Primarily a weed of agronomic crops that thrives in fertile soils.  Found throughout the U.S. except the Pacific Coast, areas of the Southwest, and portions of Florida and Maine.
Seedling:  Cotyledons round to oblong, thick, and 3-4 times larger than those of common ragweed.   The stem below the cotyledon (hypocotyl) is often purple.  The first pair of true leaves is unlobed and lanceolate in shape, with toothed margins.

Roots:  Taproot.

Stems:  Erect, freely branched, hairy.

Leaves:  All leaves subsequent to the first pair of true leaves are 3-lobed, or less often 5-lobed.   Lobes arise from the same point (palmately lobed), and each lobe is lanceolate in shape with toothed margins.  Leaves are opposite, hairy, occur on long petioles and are large (4-8 inches wide by 6 inches long).
Flowers:  Occur in long slender racemes at the ends of branches (male) or in the leaf axils of the upper leaves (female).   Individual flowers are small and greenish.

Fruit:  A large (6-12 mm long) black crown-shaped achene.

Identifying Characteristics:  Large, 3-lobed leaves and crown-shaped achene.  The first true unlobed leaves of giant ragweed may lead to the confusion of this weed with Common Cocklebur (Xanthium strumarium).  However, the leaves of cocklebur are alternate.