Weed Seed: Ambrosia trifida (Giant ragweed)



Common Name

Giant ragweed


Primary Noxious, Class 2 in the Canadian Weed Seeds Order, 2016 under the Seeds Act.


Canadian: Occurs in AB, MB, NB, NS, ON, PE, QC and SK. Reported as ephemeral in BC (Brouillet et al. 2016Footnote 1).

Worldwide: Native to North America and introduced to Asia (China, Georgia, Israel, Japan) and Europe (USDA-ARS 2016Footnote 2).

Duration of life cycle


Seed or fruit type

Achene within a bur

Identification features


  • Bur length (not including apical spine): 4.0 - 7.0 mm
  • Bur width: 4.0 - 6.0 mm
  • Bur apical spine length: 1.8 - 2.5 mm


  • Ovate to obovate burs with a narrow base and a large apical spine

Surface Texture

  • Burs woody and dull


  • Burs dull greyish-brown, yellowish-brown or brown

Other Features

  • A ring of 5 or 6 small spines is found near the top of the bur
  • Thin ribs extend down the sides of the bur

Habitat and Crop Association

Old fields, field margins, pastures, gardens, fencerows, shores, ditches, roadsides, railway lines, and disturbed areas (Darbyshire 2003Footnote 3, CABI 2016Footnote 4). A weed of many broadleaved crops (CABI 2016Footnote 4).

General Information

Dispersal of giant ragweed can occur through agricultural transport or roadside mowing (Von der Lippe et al. 2013Footnote 5) and it can also be a contaminant of bird and livestock feed (European Food Safety Authority 2010Footnote 6).

Giant ragweed is an aggressive weed of grain crops and no-tillage fields (Regnier et al. 2008Footnote 7). It is a major cause of hay fever (CABI 2016Footnote 4).

Similar species

Common ragweed (Ambrosia artemisiifolia)

  • Common ragweed burs are a similar obovate shape and have a similar arrangement of spines as giant ragweed.
  • Common ragweed burs (length: 2.5 - 3.5 mm; width: 2.0 - 2.5 mm) are generally smaller and have less prominent spines than giant ragweed.


Giant ragweed (Ambrosia trifida) burs
Giant ragweed (Ambrosia trifida) bur
Giant ragweed (Ambrosia trifida) bur
Giant ragweed (Ambrosia trifida) burs
Giant ragweed (Ambrosia trifida) bur, cross-section

Similar species

Similar species: Common ragweed (Ambrosia artemisiifolia) bur
Similar species: Common ragweed (Ambrosia artemisiifolia) burs
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