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. 2016^Footnote 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 2016^Footnote 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 1980^Footnote 3, Darbyshire 2003^Footnote 4). Reported as a weed of alfalfa (Watson 1980^Footnote 3).

General Information

Russian knapweed was introduced into Canada in the early 1900s as a contaminant in alfalfa seed (Watson 1980^Footnote 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 1980^Footnote 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 1980^Footnote 3). A dense patch of plants can spread up to 12 square metres in 2 years (Watson 1980^Footnote 3). Leaves and stems are also reported to be toxic to horses (Watson 1980^Footnote 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.