Latin Name: Centaurea diffusa

Physical Description: Diffuse knapweed is a biennial or short-lived perennial with a well-developed taproot and highly branched stems. It was introduced to North America from Eurasia near the beginning of the 20th century, likely through alfalfa seeds (Roche and Roche 1999). It grows to 60 cm tall and has white, lavender, or purple composite flowers.

Type: Biennial or short-lived perennial

Habitat & Impacts: It is found in a variety of dry habitats particularly in disturbed areas and does not tolerate shade. It is generally not found in cultivated or irrigated fields. Diffuse knapweed is unpalatable to livestock and can cause serious loss of forage production on heavily infested rangelands.

Method of Spread: Diffuse knapweed reproduces exclusively from seeds and a single plant can produce up to 18,000 seeds. Most seeds fall close to the parent plant, but adult plants may distribute seeds when they break off at the root collar or lower stem and tumble for a considerable distance blown by the wind. Seeds can remain viable in the soil for many years.

Mechanical: Cutting or mowing prior to seed-set can reduce seed production, but cut plants may survive, re-bolt, and produce additional seed heads. Hand pulling can be effective on small infestations but must be repeated depending on the size of the initial population.

Chemical: Chemical treatments consist of Picloram, 2, 4-D, Aminopyralid, and Glyphosate, and applied before flower production.

Biological: Successful release and establishment of a number of biocontrol agents have reduced the vigour and abundance of diffuse knapweed in other regions of British Columbia. Larinus obtusus, Larinus minutus, are seed weevils, and Cyphocleuns achates is a root weevil that have been released in the region.

CCCIPC Priority & Treatment Strategy: 

Priority 1 (new invader) in Nazko, Chilcotin, and central Coast.

Priority 2 in all other areas (containment).

Local Level: Hand-pulling small sites, herbicide for larger sites

Landscape Level: Herbicide, biocontrol if larger infestations occur.