Giant ragweed

Encyclopedia Article

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.

Large, ovate cotyledons on giant ragweed.

Leaves have 3 to 5 lobes.


Occasionally the first true leaves of giant ragweed lack lobes.

Plants are erect reaching heights up to 8 feet.


The male flowers (staminate) are produced on terminal branches, female flowers are found in leaf axils.  The male flowers release large amounts of allergenic, wind-dispersed pollen.

large seed of giant ragweed is encased in a crown-shaped husk
Large seed of giant ragweed is encased in a crown-shaped husk