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Integrated Crop Management
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Giant ragweed

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 are rough, leaves are 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 for giant ragweed foliage. Giant ragweed biotypes have been identified as resistant to Group 2 (ALS) and Group 9 (glyphosate) herbicides.

cotyledons on giant ragweed
Large, ovate cotyledons on giant ragweed.
giant ragweed leaves
Leaves have 3 to 5 lobes.

 

giant ragweed leaves without lobes
Occasionally the first true leaves of giant ragweed lack lobes.
erect giant ragweed
Plants are erect reaching heights up to 8 feet.

 

male flowers
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
Authored by:
Bob Hartzler
Bob Hartzler
  • Retired Weed Scientist and Professor of Agronomy
Dr. Bob Hartzler is a former professor of agronomy and a former extension weed specialist with a Ph.D. in Crop Physiology from Iowa State University. He conducted research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also taught undergraduate classes in weed science and weed identification.
Meaghan Anderson
Meaghan Anderson
  • Field Agronomist in Central Iowa
Meaghan Anderson is a field agronomist serving nine counties in central Iowa and an extension specialist with Iowa State University Extension and Outreach. Areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and integrated pest management.

Meaghan holds a bachelor's degree in agronomy and a master's degree in crop production and physiology (focus: weed science) from Iowa State University.
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