E-Flora BC: Electronic Atlas of the Flora of British Columbia

Ambrosia psilostachya DC.
western ragweed (Cuman ragweed)
Asteraceae (Aster family)

Introduction to Vascular Plants
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Distribution of Ambrosia psilostachya
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This monoecious weedy species is a late spring emergent that sets seeds in late summer and early fall. It is a species of disturbed ground. Hybrids between this species and the common ragweed Ambrosia artemisiifolia) have been observed. The two species can be separated by leaf morphology. Plants can go unnoticed because of their overall green colour and lack of showy flowers.

Species Information

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Perennial from creeping rhizomes; stems slender, stiff-hairy, branched, at least above, 30-100 cm tall.
Alternate above, opposite below, triangular to rounded in outline, 4-10 cm long, nearly unstalked or short-stalked, usually only once-pinnately cut, often toothed, stiff-hairy, uppermost leaves narrower and less divided.
Male (sterile) heads numerous in terminal, elongate, often panicled racemes; female (fruiting) heads solitary or clustered in upper leaf axils; fruiting involucres short-beaked, 3 mm long, stiff-hairy and glandular, bearing 1-6, abruptly sharp-pointed, short, conic tubercles at the apex.
Achenes egg-shaped, about 3 mm long, hairy, glandular, with short, conic tubercles around apex of body; pappus lacking.

Source: The Illustrated Flora of British Columbia

Habitat / Range

Dry roadsides and waste areas in the lowland and montane zones; rare in S BC east of the Coast-Cascades Mountains; introduced from SW U.S.

Source: The Illustrated Flora of British Columbia


The climate type for this species, as reported in the: "British Columbia plant species codes and selected attributes. Version 6 Database" (Meidinger et al. 2008), is not evaluated, unknown or variable.

Synonyms and Alternate Names

Ambrosia californica Rydb.
Ambrosia coronopifolia Torr. & A. Gray
Ambrosia cumanensis auct. non Kunth
Ambrosia psilostachya var. californica (Rydb.) Blake
Ambrosia psilostachya var. coronopifolia (Torr. & A. Gray) Farw.
Ambrosia psilostachya var. lindheimeriana (Scheele) Blank.
Ambrosia rugelii Rydb.

Taxonomic and Nomenclatural Links

Additional Photo Sources

General References