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Milk Thistle

Milk Thistle

Milk Thistle

(Silybum marianum)

Priority: -  Prevent

Tags: Terrestrial | Toxic

Identification and Reproduction


  • 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. 


  • 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.



  • 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. 


  • 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. 


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. 


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

Header photo (Zeynel Cebeci).