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Great Ragweed Ambrosia trifida


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Great Ragweed
credit: GTBacchus/CCSA

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Family: Asteraceae, Aster view all from this family

Description Native annual; herbaceous or subshrub.
Habit: erect and coarse, with rough, hairy stems.
Height: 1-5 ft (30-150 cm) or much more.
Leaf: opposite; simple and pointed near base, palmate with 3 lobes above, eventually palmate with 5 lobes on upper stem; 1.5-12 in (40-300 mm) long by 1.25-8 in (30-200 mm) or more.
Flower: tiny greenish yellow flowerheads held in long, narrow, terminal spikes, 3-10 in (75-250 mm) long.
Fruit: spiny pyramidal seed, 1/2 in long (7 mm) or more.

Warning This plant is a major cause of hay fever allergies, and can irritate the skin if touched. Can be toxic to livestock if ingested. (Humans should generally avoid ingesting plants that are toxic to animals.)

Flower June - November.

Habitat Waste places, fields, roadsides, disturbed sites; moist.

Range Throughout North America; not reported in Nevada, British Columbia, Alaska, or far northern Canada.

Discussion Also known as: giant ragweed, horseweed. This plant is considered weedy or invasive in many areas, and is listed as a noxious weed in California, Delaware, and parts of Illinois. This is the giant among the ragweeds, reaching towering heights and possessing long flower spikes. Ragweed pollen is spread by wind rather than by insects. It is one of the main causes of late summer hay fever.