Giant ragweed Ambrosia trifida L.
Family: Asteraceae (Composite family)
Life cycle: Annual
Native status: Native to North America
Habitat: Crop fields, flood plains, field edges
General description: Erect plant up to 12 ft tall. Stems and leaves rough, leaves opposite with 3 to 5 deep lobes. The first leaves occasionally have entire leaf margins. Staminate (male) flowers occur in terminal racemes, pistillate (female) flowers in axillary clusters. Fruit is a large (1/3 in) crown-shaped achene.
Key ID traits: Deeply lobed opposite leaves, rough stems and leaves.
Similar species: Occasionally first leaves of giant ragweed may be unlobed, in which it may appear similar to cocklebur or sunflower.
Miscellaneous: Native to North America, a major source of pollen causing hay fever. One of the earliest emerging summer annuals, which allows it to dominate plant communities in disturbed habitats. Biotypes with extended emergence have evolved which favors survival in crop fields. Commonly called horseweed because of horses’ fondness of giant ragweed foliage. Giant ragweed biotypes have been identified resistant to Group 2 (ALS) and Group 9 (glyphosate) herbicides.