Identification and Reproduction

Identification

• Spotted knapweed grows from a slender stem and with maturity will divide and become heavily branched. It can reach heights up to 1.5 meters tall. 
• Leaves are deeply lobed, hairy, grayish-green leaves. Form as rosettes in their first year.

• Flowers are small white, pink or purple. The flowers form from a flower head bracts that have black tips, making it appear "spotted". 

Reproduction

Spotted knapweed is a biennial plant that only reproduces by seed. It is a prolific seed producer as each single plant will result in over 140,000 seeds per year. 

Habitat & Ecology

Spotted knapweed thrives in exposed areas with well-drained soil. It is commonly found on dry roadsides, gravel pits, disturbed sites, and in fields

Impacts

Ecological: 

• Chemical properties of spotted knapweed alters soil habitat, displacing native and agricultural vegetation. 
• It reduces forage quality and availability for livestock, contaminates crops, and creates a fire hazard.
• Large infestations can increase sedimentation and runoff. 

Management

Mechanical/Manual Control:

• Prior to seed production, pulling, cutting or mowing will be effective. 
• If done while flowers are present be sure to properly dispose of plants, preventing dispersal of viable seeds. 
• The root system should also be fully removed to reduce re-establishment. 

Biological Control: 

Currently 12 biocontrol agents have been used in the Mainland/Coast Region for the spotted knapweed and is being monitoried by the Ministry of Forests and Range. Bio-controls help decrease the knapweed size, growth and seed production. 

Chemical Control: 

• Currently picloram, dicamba, 2,4-D, clopyralid, aminopyralid and glyphosate are registered as effective herbicides on knapweed. 
• It is recommended to wick or select spot spray plants. 
• Please carefully read herbicide labels prior to application. 

Resources

Download A Guide to Weeds in British Columbia for Spotted Knapweed here.

Header photo (Matt Lavin).