Identification and Reproduction

Identification: 

• Also known as welted thistle, this biennial plant typically grows no more than 1.5 m tall but in certain conditions it can grows up to 2 m tall. 
• Stems are covered in spines that are angled downwards. 
• Leaves are spiny and sometimes covered in hairs. They are deeply lobed with wavy edges. Along the underside the main vein will be covered in bristles. 

• Flowers are reddish-purple to pink in colour. They have a diameter of 2.5 cm at maturity and are encased in spiny bracts. 

Reproduction: 

• Spreading via seeds, each individual plant can produce 1,000 seeds annually.
• Has a high germination rate under favourable conditions (90-95%). 
• Seeds remain viable in the soil for upto 10 years.  

Habitat & Ecology

• Plumeless thistle invades open and disturbed areas: pastures, roadsides and right-of-ways. 
• They are capable of growing in different soil types. 
• Thistle spines can injure livestock and humans. 

Impacts

Social: 

• Reduces foliage availability and displaces native species. 
• Displaces desired crop species. 
• Livestock will avoid knapweed since it is spiny.

Management

Prevention is a high priority for this plant.

Mechanical/Manual Control:

• Individual plants or small patches can be hand-pulled.
• Wear gloves and long sleeves to minimize exposure to sharp spines. 
• Mowing prior to seedset may be effective, especially on larger patches. 
• If plants have bolted, cutting the stems to collect the flowerheads can prevent seed spread. 

Biological Control: 

• There are a number of biological control agents for this plant. These include weevils that attack the terminal buds, rosettes and foliage. 

Resources

Download BC's Guide to Weeds in British Columbia for Plumeless Thistle here.

For more information on historic background on Plumeless Thistle click here. 

Check out the Canadian Food Inspection Agency's datasheet on Spiny plumless thistle. 

Header photo (Syp).