Identification and Reproduction

Identification: 

• Italian thistle is an annual plant. It grows between 0.3-1.2 m tall, condition dependent. 
• Winged, spiny slender stem. 

• They produces leaves that are green from above but are white and covered in hairs below. These hairs are often referred to resemble cobwebs. 
• Leaves are pinnately lobed with spines becoming more prominent near the tip.

• Flowerheads are purple, narrow, and appear in clusters of 2-5. The bracts are stiff and upright with hairs. It flowers from September through December. 

Reproduction: 

• This plant only reproduces by seed. 
• A single plant can produces 20,000 seeds in one growing season. 
• They produce two types of seeds; brown and silver seeds. Brown seeds remain in the flowerhead and fall at the end of the growing season. Silver seeds are carried by wind and can remain viable in the soil longer (upto 10 years). 
• Seed germination is very high; 83-96%. 

Habitat & Ecology

• Italian thistle takes over pastures and rangelands. 
• Prefers droughty conditions. 
• Any disturbance activities will trigger aggressive growth of this plant. 

Impacts

Social:

• Reduces foliage availability and displaces native species. 
• Displaces desired crop species. 
• Can limit the mobility and injure livestock. 
• Leaves have also been prone to contaminating wool. 

Ecological:

• It overwinters in a rosette form and a patch can have a blanketing effect. They will out-crowds and prevent spring establishment of native species. 

Management

Prevention is a high priority for this plant. 

Mechanical/Manual Control: 

• Hand-pulling is effective on small patches. Roots should be removed in their entirety or cut at least 10 cm below the ground to prevent regrowth. 
• Any activity should take place in the summer, prior to seedset.
• For medium sized patches, hand-hoeing may help in severing roots and slowing the spread. 

Biological Control: 

• There has been success with biological control with insects and fungi. Currently agents are being researched. 
• Several weevils slow the spread but have not bee successful in eradicating Italian thistle. 
• Puccinnia cardui-pycnocephali is a rust that targets this weed. This fungus slows the growth of the rosette stage and impacts flower and seed production. 

Resources

For more for details check out the Invasive Species Compendium datasheet on Carduus pycnocephalus.  

For more information on thistle identification and comparisons between look-alike species check out this resource by the University of California: Agriculture and Natural Resources.

Header photo (Harry Rose).  

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