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. 2016^Footnote 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 2016^Footnote 2, USDA-ARS 2016^Footnote 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 2016^Footnote 2).

General information

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

Spotted knapweed can thrive over a wide range of climate and soil conditions (CABI 2016^Footnote 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

Similar species