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 largest annual weeds. It is similar to a forb and forms a taproot.

Descriptions

Characteristic Features

Typical 3-lobed leaves with a winged petiole.

Young Plant

First leaves ovate to lanceolate, slightly coarse-lobed.

Cotyledons

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

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

Leaves

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.

Propagatio Organs

Flowers

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.

Fruit

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

Seeds

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 seed bank.

Propagation

By seed. 270 seeds/plant.

Occurrence

Habitat

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

Soil

Dry, sunny grassy plains and sandy soils.

Additional Crop Information

Also in 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.

Control

Integrated Crop Management

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

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