Weed Seed: Centaurea diffusa (Diffuse knapweed)

Family

Asteraceae

Common name

Diffuse 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, ON, QC, SK (Brouillet et al. 2016^Footnote 1).

Worldwide: Native to southeastern Europe and western Asia and it has expanded beyond its native range into central and western Europe and has also been introduced into North America and Argentina (CABI 2016^Footnote 2).

Duration of life cycle

Biennial or short-lived perennial

Seed or fruit type

Achene

Identification features

Size

Achene length: 1.4 - 3.0 mm
Achene width: 0.7 - 1.5 mm
Pappus length: 0.1 - 1.0 mm

Shape

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

Surface texture

Achene has smooth, dull surface texture

Colour

Pale, reddish-brown, occasionally dark greenish-brown achene with pale markings: a strong central stripe within lines to either side and a pale area around the basal notch

Habitat and crop association

Fields, rangelands, grasslands, dunes, open forests, roadsides, railway lines, gravel pits, industrial sites and other disturbed areas (Darbyshire 2003^Footnote 3, CABI 2016^Footnote 2). A serious weed of alfalfa in its native range but mainly a weed of rangelands (CABI 2016^Footnote 2).

General information

Diffuse knapweed can be found over a wide range of soil and environmental conditions but cannot tolerate cultivation or excessive moisture (CABI 2016^Footnote 2). Diffuse knapweed produces large numbers of seeds that are dispersed by wind, water, and animals. Plant stems may break off to become tumbleweeds, dispersing seeds over large distances (CABI 2016^Footnote 2).

Similar species

Spotted knapweed (Centurea stoebe)

Spotted knapweed achenes have a similar size, oblong shape, pappus and dark colouring with pale stripes as diffuse knapweed.
Spotted knapweed achenes are wider, with a prominent central stripe and a pale area around the top compared to diffuse knapweed. The brown colour of diffuse knapweed achenes is usually reddish while spotted knapweed is greenish. The pappus is also longer by up to 2.0 mm in spotted knapweed.