Ambrosia trifida

Scientific Name Ambrosia trifida L.
Common Names English: Giant ragweed, great ragweed; German: Dreilappige Ambrosie; French: Grande herbe à poux
Description A. trifida is an erect annual, which grows from 30 to 300 cm (average 150 cm) (0.98 - 9.8, average 4.92 ft) high and is one of the tallest annual weeds. It looks similar to a forb and forms a taproot.


Characteristic Features

Typical 3-lobed leaves with a winged petiole.

Young Plant

First leaves ovate to lanceolate, slightly coarse-lobed.


Cotyledons round, ovate, or oblong, thick, sometimes slightly indented at the tips, 2-4 cm (0.78 - 1.57 inch) long, 1-1.6 cm (0.39 - 0.63 inch) wide.


Stems coarse, single or branched, woody at the base, longitudinally black-lined, covered with soft to bristly hairs.


Leaves opposite, broad, palmately 3- to 5-lobed, 6-35 cm (2.36 - 13.7 inch) long, sparsely covered with minute, stiff hairs. Margins finely serrate.

Propagation Organs


Heads small, greenish, composed of staminate (male) or pistillate (female) disc flowers. Staminate and pistillate heads are separate on a single plant (monoecious). Terminal spikes consist of nodding staminate heads, 2-5 mm (0.079 - 0.19 inch) in diameter. Pistillate heads are clustered in the leaf axils below the spikes. Staminate head phyllaries fused, cup-like, with 3 longest lobes blackish along the midveins.

Flowering Period

July - September.


Hardened phyllaries tightly enclose a single achene to form a bur. Burs +/- obovoid.


Germination in spring. Emergence is best on soil depths to 16 cm (6.3 inch), optimal 2 cm (0.78 inch). The burs are 6-12 mm (0.24 - 0.47 inch) long, stoutly blunt-beaked at the apex, brown to gray. The beak is surrounded by a crown of 5-8 short, thick blunt teeth of vestigial spines terminating each rib.

Viability of Seeds

Very persistent in soil seed bank.


By seed. 270 seeds/plant.



Giant ragweed is found in low woods, along streams, pond margins, waste grounds, roadsides and railroads.


Dry, sunny grassy plains and sandy soils.

Additional Crop Information

Also in other broadleaf crops.

Agricultural Importance

It is extremely competitive for light and very difficult to control in many broadleaf crops. A. trifida greatly reduces yield and interferes with combine harvesting. The pollen is very allergenic and causes hay feaver and asthma; therefore eradication programs were started in Europe and the USA.


Useful non-chemical contribution to Integrated Weed Management

Repeated mowing before seed shedding will effectively reduce seed production but will not eliminate giant ragweed.

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