Page authors: Don Knoke, David Giblin
Ambrosia psilostachya
perennial ragweed, western ragweed

Distribution: Occurring east of the Cascades crest in Washington; British Columbia to California, east across North America to the Atlantic Coast.

Habitat: Roadsides, fields, wastelots, and other disturbed open areas.

Flowers: July-October

Origin: Native

Growth Duration: Perennial

Conservation Status: Not of concern

Pollination: Wind


Monoecious perennial from creeping rhizomes, 1-10 dm. tall.


Leaves opposite below and alternate above, usually pinnatifid, lance-ovate or elliptic in outline, 3-10 cm. long, sub-sessile.


Staminate heads small, numerous, short-pedunculate, in an open inflorescence of several spike-like flowering shoots arising from the upper leaf axils; pistillate heads borne below, in the leaf axils; fruiting involucre a short-beaked bur, 3-5 mm. long, with a few rounded projections.



Accepted Name:
Ambrosia psilostachya DC.
Publication: Prodr. 5: 526. 1836.

Synonyms & Misapplications:
Ambrosia psilostachya DC. var. californica (Rydb.) S.F. Blake
Ambrosia psilostachya DC. var. coronopifolia (Torr. & A. Gray) Farw.
Ambrosia psilostachya DC. var. lindheimeriana (Scheele) Blank.
Ambrosia rugelii Rydb.
Additional Resources:

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

WA Flora Checklist: Ambrosia psilostachya checklist entry

OregonFlora: Ambrosia psilostachya information

E-Flora BC: Ambrosia psilostachya atlas page

CalPhotos: Ambrosia psilostachya photos

10 photographs:
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