Family: Asteraceae

Milk thistle is a spiny herb capable of forming dense, impenetrable thickets in pastures and grasslands. A native of the Mediterranean region, it has been used medicinally for at least 2000 years. The Maui Invasive Species Committee is working to eradicate a known population from Maui. This invasive plant should not be introduced or cultivated in Hawaiʻi. Contact your local ISC if you find this plant growing.

Description and Dispersal:

• A stout herb with glabrous to slightly pubescent stems of 20-250 cm (7-100 inches) height
  
• Spiny-dentate leaves
  
• Stem leaves are sessile, smaller and less deeply divided, with yellowish white spines up to 8 mm (0.3 inch) long
  
• Reproduces by wind-dispersed seeds

High Risk Traits:

• Broad climate suitability (>5 hardiness zones and broad natural and introduced ranges)
  
• Naturalized in regions with tropical climates
  
• Naturalizing on Maui (prior to control by Maui Invasive Species Committee); widely naturalized elsewhere
  
• A disturbance-adapted agricultural and environmental weed
  
• Spiny
  
• May be high in nitrates and potentially toxic to grazing animals
  
• Tolerates many soil types
  
• Forms dense thickets that exclude other vegetation
  
• Reproduces by seeds
  
• Self-fertile
  
• Can reach maturity in 1-2 years
  
• Seeds dispersed by wind, water, in mud stuck to animals and machinery, as a contaminant and intentionally by people
  
• Prolific seed production
  
• Seeds remain viable for at least 9 years

Low Risk Traits:

• Palatable to browsing/grazing animals
  
• Requires full sun (shade limits ability to spread)
  
• Not reported to spread vegetatively
  
• Herbicides provide effective control