Weed Seed: Centaurea stoebe (Spotted knapweed)

Family

Asteraceae

Synonyms

Centaurea maculosa

Common name

Spotted knapweed

Regulation

Prohibited Noxious, Class 1 in the Canadian Weed Seeds Order, 2016 under the Seeds Act. All imported and domestic seed must be free of Prohibited Noxious weed seeds.

Distribution

Canadian: Occurs in AB, BC, NB, NS, ON, QC, SK, YT (Brouillet et al. 2016Footnote 1).

Worldwide: Native to eastern Europe and western Asia. Introduced in other parts of Europe, as well as North America, Australia and New Zealand (CABI 2016Footnote 2, USDA-ARS 2016Footnote 3).

Duration of life cycle

Perennial

Seed or fruit type

Achene

Identification features

Size

  • Achene length: 1.8 - 3.5 mm
  • Achene width: 0.8 - 2.0 mm
  • Pappus length: 1.0 - 3.3 mm

Shape

  • Achene oblong-shaped with rounded bottom and flat top, laterally compressed; large notch near the base

Surface texture

  • Achene is smooth, dull

Colour

  • Light to dark greenish-brown achene with pale markings

Other features

  • Achene pale markings consist of: a strong central line and thin or broken lines to either side, and a pale area around the basal notch and the top

Habitat and crop association

Rangelands, grasslands, open sites, roadsides, riverbanks and disturbed areas. Reported as a weed of alfalfa but not usually associated with crop production (CABI 2016Footnote 2).

General information

Spotted knapweed was accidentally introduced into North America in contaminated alfalfa and clover seed in the late 1800s (Ochmann 2001Footnote 4).

Spotted knapweed can thrive over a wide range of climate and soil conditions (CABI 2016Footnote 2), although it prefers well-drained, light to coarse textured soils.

Similar species

Diffuse knapweed (Centurea diffusa)

  • Diffuse knapweed achenes are: a similar size, oblong shape, pappus and dark colouring with pale stripes as spotted knapweed.
  • Diffuse knapweed achenes (1.0 - 1.5 mm wide) are narrower than spotted knapweed, with strong central and side stripes. The pappus are generally shorter in diffuse knapweed (0.1 - 1.0 mm long) compared to spotted knapweed.
  • The brown colour of diffuse knapweed achenes is usually reddish while spotted knapweed is greenish.

Photos

Spotted knapweed (Centurea stoebe) achene
Spotted knapweed (Centurea stoebe) achene, top-down view
Spotted knapweed (Centurea stoebe) achene, close-up
Spotted knapweed (Centurea stoebe) achenes
Spotted knapweed (Centurea stoebe) achenes

Similar species

Similar species: Diffuse knapweed (Centurea diffusa) achenes
Similar species: Diffuse knapweed (Centurea diffusa) achene
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