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Photo Credit: Guido Gerding

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Photo Credit: H. Zell

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Photo Credit: Forest and Kim Starr

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QUICK FACTS

  • HPRWA SCORE 17
  • FAMILY Asteraceae
  • CATEGORY Grasses & Groundcovers

milk thistle, Mary's thistle (Silybum marianum)

Milk thistle is a spiny medicinal herb capable of forming dense, impenetrable thickets in pastures and grasslands.

  • A stout herb with glabrous to slightly pubescent stems of 20-250 cm (7-100 inches) height
  • With spiny-dentate leaves
  • Stem leaves are sessile, smaller and less deeply divided, with yellowish white spines up to 8 mm (0.3 inch) long
  • A native of the Mediterranean region that has been used medicinally for at least 2000 years
  • Reproduces by wind-dispersed seeds
  • A priority pest controlled by the Maui Invasive Species Committee

Harm

  • Produces copious amounts of seeds (6,000 per plant) and has large leaves which will shade out other plants
  • Contains high levels of nitrate which are poisonous to ruminant animals such as cattle and sheep
  • Seeds remain viable in the soil for up to 9 years and the plant thrives in disturbance, making eradication difficult
  • Thistle seeds have tiny tufts of hair (like dandelion seeds) which aid in wind dispersal
  • It can be spread accidentally in cattle feed, water, mud, vehicles, machinery, erosion and by animals.