Diffuse Knapweed

Invasive:

• Spotted Knapweed (Centaurea stoebe) has hairy, deeply-cut leaves with the lower leaves having a black-tipped fringe, which gives it its spotted appearance. Flowers are purple (white on occasion) on a single or multiple upright stems that can grow to 1 m tall. A single plant can produce up to 140,000 seeds. Spotted Knapweed prefers moist habitats as well as disturbed areas.

Biennial or short-lived perennial that is Eastern Mediterranean in origin. It was brought to North America as a contaminant in crop seeds.

Diffuse Knapweed grows in disturbed, dry areas such as sandy areas, grasslands, pastures, woodland clearings, open forests, roadsides, right-of-ways and clear-cuts. It is intolerant of moist soils, flooding and shade.

Diffuse Knapweed is abundant in certain portions of the Sea to Sky region, but it has not yet infested all potential habitats. The goal is to contain the spread of Diffuse Knapweed to Pemberton / ISMA 3.

 

Learn how to identify Spotted Knapweed: Use the images presented on this profile page.

What to do if you spot it: You can report any Spotted Knapweed sighting by clicking here.

 

DO:

• Regularly monitor properties for weed infestations.
• Remove plant material from any equipment, vehicles or clothing used in infested areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
• Ensure soil and gravel are uncontaminated before transport.
• Minimize soil disturbances (e.g., use grazing plants that prevent soil exposure from overgrazing), and use seed mixes with dense, early colonization (e.g., alfalfa or barley) to re-vegetate exposed soil and resist invasion.
• Ensure invasive plant (particularly flowering heads or root fragments) are bagged or covered to prevent spread during transport to designated disposal sites (e.g., landfill). Do NOT compost.

DO NOT:

• Plant Diffuse Knapweed in a garden, no matter how well-contained its enclosure may seem.
• Move soil that has been contaminated with Diffuse Knapweed.
• Unload, park, or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles, or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
• Compost Diffuse Knapweed, especially the seeds!

 

Control

Mechanical Control

• Repeated hand-pulling and digging before flowering can be effective on small infestations, but must be done over several years.
• Make sure to wear gloves and a long-sleeved shirt, as Diffuse Knapweed is abrasive and can cause skin irritation.
• Mowing also prevents seed production but the remaining root will re-sprout.

Chemical Control

Foliar application of Dicamba, 2, 4-D, clopyralid, aminopyralid or glysophate are effective forms of control. Picloram is also very effective, but not appropriate to use in moist, coastal soils.

We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological Control

There are 12 biocontrol agents used in the Mainland and Coast regions of BC. Biocontrol helps decrease the size and vigour of the plants, as well as their seed production, but is not considered an eradication method.