Identification and Reproduction

Identification: 

• Milk thistle is a large conspicuous thistle that reaches heights between 0.6 and 2 m tall. 
• Rosettes will have large, thick, succulent leaves that have white variegation and short yellow prickles. 

• Leaves are shiny, lobed, spiny and green with white variegation. Basal leaves are are large, reaching up to 70 cm long. Leaves along the stem are clasping. 

• Purple flowers form at the end of stems, are between 2-6.5 cm wide and is surrounded by projecting spines. Flowers appear between April and July. 

Reproduction: 

• Reproducing by seed, each plant produces over 6,000 seeds annually. 
• Seeds can remain viable in the soil for up to 9 years. 
• Seeds have a long white pappus that allows for easy wind dispersal. 

Habitat & Ecology

• Milk thistle is commonly found along roadsides, in ditches, fertile fields and other disturbed areas. 
• It typically is only found on high fertility soils.

Impacts

Social: 

• This plant is poisonous to livestock. Toxicity increases as the plants begin to wilt after being cut or damaged during plowing or when soil moisture is too high. 
• Can also injure livestock from the presence of sharp spines. 
• Reduces forage opportunities and crop yields. 

Ecological: 

• Can form dense stands that restrict large wildlife movement. 
• Can be very competitive under certain conditions, out-shading and outcompeting native species for water and nutrients. 

Management

Prevention is a high priority for this plant.

• Learn to identify this plant and remove it off your property if you spot it.
• Clean clothing, pets, equipment and vehicles prior to leaving an infested site. 

Mechanical/Manual Control: 

• Cultivation can help destroy establishing seedlings. 
• Mowing plants prior to seedset can reduce seed dispersal. Be cautious as this may increase the danger of livestock poisoning. 
• Hand pulling should only be done with small patches. 
• Be sure to wear gloves, long sleeves and pants when dealing with plants as they are covered in spines. 

Chemical Control: 

• The most effective herbicide on milk thistle is 2,4-D. Apply between seeding and rosette stage of growth for optimal efficacy. 

Resources

For more details check out BugwoodWiki's datasheet on Silybum marianum. 

Header photo (Zeynel Cebeci).