Scotch thistle

Onopordum acanthium

   

Family: Asteraceae

Other common names: cotton thistle, woolly thistle

Weed Class: B

Year Listed: 1988

Native to: Europe and Asia

Toxic: not known to be

Other Legal Listings: WA Quarantine list, WAC 16-750

additional photos

 

   

Why is it a noxious weed?

Scotch thistle is a problem in rangeland. Infestations of Scotch thistle reduce forage production and virtually prohibit land utilization for livestock. Dense stands of the large, spiny plants exclude animals from grazing and access to water.

How would I identify it?

General Description: Scotch thistle is a branched, biennial or annual with a broadly winged stem that can grow up to 8 feet or more in height and 6 feet in width.

Flower Description: Plants flower in mid-summer. The globe-shaped flower heads are borne in groups of 2 or 3 on branch tips. Flower heads are up to 2 inches in diameter, with long, stiff, needle-like bracts at the base. Flowers range from dark pink to lavender.

Leaf Description: Leaves are up to 2 feet long and 1 foot wide, are covered with sharp yellow spines and have a gray-green appearance from being covered with a thick mat of cotton-like or woolly hairs.

Stem Description: Stems have vertical rows of prominent, spiny, ribbon-like leaf material or wings that extend to the base of the flower heads.

Fruit/Seed Description: Seeds are smooth, slender, and plumed.

May be confused with:

How does it reproduce?

Scotch thistle reproduces by seed. Each plant can produce 8,400 to 40,000 seeds.
 

Where does it grow?

Scotch thistle will grow in wet meadows and pastures as well as dry pastures and rangelands. It may also be found alongside streams and rivers. Please click here to see a county level distribution map of Scotch thistle in Washington.
 

How do I control it?

General Control Strategy

Establishing a dense well-maintained pasture is effective in preventing a Scotch thistle infestation.

Mechanical Control

Small areas can be dug out. Mowing has limited effectiveness for controlling Scotch thistle, usually only prevents seed production.

Biological Control

Goats will graze Scotch thistle, preventing seed production.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.


 

For more information

 
See our Written Findings for more information about Scotch thistle (Onopordum acanthium).

Additional Photos


 
 

 


Scoth thistle flowerhead

 


Scotch thistle stem and leaves

 


Scotch thistle rosette

 


Scotch thistle infestation

 


Scotch thistle flowerhead

 


Scotch thistle plant

 


Scotch thistle growth

 


Scotch thistle flowerhead

 


Scotch thistle flowerhead

 


large rosette

 


leaf edge with spines

 


dense rosettes

 


seedling

 


young rosette

    

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