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Montana Field Guide

Montana Field Guides

Flat-spine Ragweed - Ambrosia acanthicarpa

Native Species

Global Rank: G5
State Rank: S3S4

Agency Status
MNPS Threat Rank:

External Links

General Description
Taprooted annual. Stems erect, simple or branched, 10–50 cm. Herbage hispid, strigose. Leaves mostly alternate; blade broadly ovate, 2–8 cm long, twice pinnatifid into ovate ultimate segments. Female heads 1-flowered. Male heads cup-shaped, black-veined, 3–5 mm wide with 6 to 12 florets, spreading on peduncles 1–2 mm long. Bur 3–5 mm long, oblong, villous with stout spines (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Range Comments
WA and AB to MN south to CA, AZ and TX (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).

Observations in Montana Natural Heritage Program Database
Number of Observations: 3

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Relative Density



(Observations spanning multiple months or years are excluded from time charts)


  • Additional ReferencesLegend:   View Online Publication
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    • Heidel, B.L. 1994. Sensitive plant species survey in Garfield and McCone Counties, Montana. Unpublished report to the Bureau of Land Management. Montana Natural Heritage Program, Helena, Montana. 58 pp. plus appendices.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
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Citation for data on this website:
Flat-spine Ragweed — Ambrosia acanthicarpa.  Montana Field Guide.  .  Retrieved on , from