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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


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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 et al. 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 et al. 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)


  • Literature Cited AboveLegend:   View Online Publication
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • 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 Unpublished report to the Bureau of Land Management.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.
    • Quire, R.L. 2013. The sagebrush steppe of Montana and southeastern Idaho shows evidence of high native plant diversity, stability, and resistance to the detrimental effects of nonnative plant species. M.Sc. Thesis. Bozeman, MT: Montana State University. 124 p.
  • Web Search Engines for Articles on "Flat-spine Ragweed"
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Citation for data on this website:
Flat-spine Ragweed — Ambrosia acanthicarpa.  Montana Field Guide.  .  Retrieved on , from