Latin Name: Centaurea stoebe

Origin:  Eurasia

Physical Description: It is a biennial, sometimes perennial herb from a taproot.  It has an upright branched stem that can grow up to 1.5m tall. The leaves are deeply lobed, alternate and can be slightly hairy. The flowers are typically pinkish-purple, occasionally creamy white. Each plant can produce 25 to 35 flower heads. The bracts on the flowers bases have black tips, giving it a spotted appearance. The bracts distinguish Spotted knapweed from other knapweed species.  It produced a toxin into the soil that hinders the growth or success of neighbouring desirable plant species.

Habitat: It prefers sunny locations and well drained, light to coarse textured soils. It is intolerant of shade and constant moisture. It is often found along, roadsides, pastures and rangelands.

Impacts: It can form monocultures reducing available habitat for wildlife and overall biodiversity. It can result in reduced crop yield and value, and degrade property values. It can also limit available forage for livestock and wildlife.

Reproduction: It reproduces by seed only. A single plant can produce over 140,000 seeds in a year. Seeds can remain viable in the soil for up to 8 years.

Mechanical: Cutting or pulling prior to flower can be effective for small infestations.  This must be carried out 2 to 3 times per year over several years. When pulling, remove as much of the root system as possible to prevent re-sprouting. When cutting, cut 2 to 4 inches below the surface, ensuring the root crown is removed. Mowing is not recommended. Gloves are recommended when handling the plant, to prevent skin irritations.

Chemical: Several herbicides with the following active ingredients can control Spotted Knapweed: 2,4-D, Aminocyclopyrachlor , Aminopyralid, Clopyralid, Dicamba, Picloram and Glyphosate.  For available products, contact your local agri-supply store. Prior to any herbicide application, read and follow the label instructions. 

Biological: There are several biological control agents available:

Agapeta zoegana– root feeding moth

Cyphocleonus achates– root feeding beetle (weevil)

Larinus minutus- seed feeding beetle (weevil)

Larinus obtusus– seed feeding beetle (weevil)

Metzneria paucipunctella– seed feeding month

Sphenoptera jugoslavica– root feeding beetle

Urophora affinis– seed feeding fly

Urophora quadrifasciata- seed feeding fly

Additional Resources:

Alberta Invasive Species Council’s Spotted Knapweed Factsheet

Invasive Species Council of BC’s Knapweeds Factsheet

Provincial Biological Control Agents and Host Plants-Online Resource