Page authors: Don Knoke, David Giblin
Ambrosia trifida
giant ragweed

Distribution: Occurring chiefly east of the Cascades crest; Alaska to California, east across North America to the Atlantic Coast.

Habitat: Roadsides, fields, wastelots, and other disturbed sites, often where moist.

Flowers: July-October

Origin: Native

Growth Duration: Annual

Conservation Status: Not of concern

Pollination: Wind


Monoecious, coarse annual from a few decimeters to several meters tall with short, stiff hairs above.


Leaves opposite, petiolate, ovate or sub-orbicular, serrate, palmately 3- to 5-lobed, or without lobes in small plants, the blade 2 dm. long or more, rough on both sides.


Staminate heads small, numerous, short-pedunculate, in an elongate, bractless, spike-like inflorescence, the involucres 3-nerved; pistillate heads borne below, in the leaf axils, the involucres 5-10 mm. long in fruit, several-ribbed, each bearing a short spine at the summit.

Accepted Name:
Ambrosia trifida L.
Publication: Sp. Pl. 2: 987. 1753.

Synonyms & Misapplications:
Ambrosia aptera DC.
Ambrosia trifida L. var. integrifolia (Muhl. ex Willd.) Torr. & A. Gray
Ambrosia trifida L. var. texana Scheele
Ambrosia trifida L. var. trifida [HC]
Additional Resources:

PNW Herbaria: Specimen records of Ambrosia trifida in the Consortium of Pacific Northwest Herbaria database

WA Flora Checklist: Ambrosia trifida checklist entry

OregonFlora: Ambrosia trifida information

E-Flora BC: Ambrosia trifida atlas page

CalPhotos: Ambrosia trifida photos

10 photographs:
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