Weed Seed: Rhaponticum repens (Russian knapweed)

Family

Asteraceae

Common Name

Russian knapweed

Regulation

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

Distribution

Canadian: Occurs in AB, BC, MB, ON, SK (Brouillet et al. 2016Footnote 1).

Worldwide: Native to eastern Europe and temperate Asia and introduced in North America, Argentina, Australia, and beyond its native range in Europe (USDA-ARS 2016Footnote 2).

Duration of life cycle

Perennial

Seed or fruit type

Achene

Identification features

Size

  • Achene length: 2.5 - 4.0 mm
  • Achene width: 1.5 - 2.5 mm

Shape

  • Obovate-shaped achene, compressed

Surface Texture

  • Achene has shallow longitudinal ridges

Colour

  • Achene is bone-white to pale straw colour

Other Features

  • Pappus present only on immature achenes
  • Small (0.5 mm) notch at or near the base of the achene
  • Short peg at the top of the achene

Habitat and Crop Association

Cultivated fields, old fields, pastures, rangelands, ditches, roadsides, railway lines and disturbed areas (Watson 1980Footnote 3, Darbyshire 2003Footnote 4). Reported as a weed of alfalfa (Watson 1980Footnote 3).

General Information

Russian knapweed was introduced into Canada in the early 1900s as a contaminant in alfalfa seed (Watson 1980Footnote 3). This species is found on a variety of soil types, and has been noted to invade areas where it was not directly introduced (Watson 1980Footnote 3).

Russian knapweed seed can persist in the soil for up to 75 years, and the plants also readily reproduce through stem buds borne on the roots (Watson 1980Footnote 3). A dense patch of plants can spread up to 12 square metres in 2 years (Watson 1980Footnote 3). Leaves and stems are also reported to be toxic to horses (Watson 1980Footnote 3).

Similar species

Gumweed (Grindelia squarrosa)

  • Gumweed achenes may be similar obovate size, longitudinal ribs and pale colour as Russian knapweed.
  • Gumweed achenes are more angular-shaped than Russian knapweed, and do not have a notch at the base of the achene. Gumweed also lacks a pappus.

Photos

Russian knapweed (Rhaponticum repens) achenes
Russian knapweed (Rhaponticum repens) achene
Russian knapweed (Rhaponticum repens) achene, side view
Russian knapweed (Rhaponticum repens) flower head

Similar species

Similar species: Gumweed (Grindelia squarrosa) achenes
Similar species: Gumweed (Grindelia squarrosa) achene
Date modified: