Giant ragweed (Ambrosia trifida)

Giant ragweed Common Names: Bitterweed, buffalo weed, horse-cane, horse-weed, kinghead, tall ragweed, giant ragweed

Description: Capable of producing large amounts of allergenic pollen, causing hayfever.

Habit: Annual herb with long clusters of green-yellow flowers and opposite leaves.

Leaves: Opposite, up to 12 in. long and 8 in. wide, larger leaves are divided into 3-5 deep lobes, lobe divisions are oval-lance shaped. Edges are finely toothed, surfaces variably hairy, smooth to covered in fine, stiff pubescent hairs.

Stems: Coarse, mostly smooth in the lower stalk with spreading hairs in the upper, central stem branching from nodes.

Flowers: Slender spike-like racemes, branched at the base, 3-8 in. long, borne at the tip of the central stem and branches. Male flowers are yellow to greenish in color with no petals, 1/8 in. across, hanging. Female flowers are hiding in clusters of short leafy bracts at the base of the cluster, along the stem or in axils. Densely pack but spreading out with age.

Fruit and seeds: Achene is pyramidal in shape, 6-12 mm. long, brown to gray in color, glabrous or slightly pubescent with 4-5 spines.

Habitat: Native to North America. Can be found in disturbed sites, fields, roadsides, edges of woods.

Reproduction: By seed. Capable of producing up to 10,300 seeds.

Similar species: Lanceleaf ragweed (A. bidentata), Flatspine bur ragweed (a href="">A. acanthicarpa, and Annual ragweed (A. artemisiifolia)

Monitoring and rapid response: Tilling and hand-pulling can be effective in controlling seedlings. Repeat mowing is effective in reducing seed production. Many biological control agents have been tests and has shown success including Euaresta bella and E. festiva.

Credits: The information provided in this factsheet was gathered from CABI: Invasive Species Compendium and Minnesota Wildflowers .

Individual species images that appear with a number in a black box are courtesy of the network ( Individual photo author credits may not be included due to the small display size of the images and subsequent difficulty of reading the provided text. All other images appear courtesy of Google (

Common Name:

Giant ragweed

Scientific Name:

Ambrosia trifida







USDA Symbol: