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Milk thistle (Silybum marianum)
ODA rating: B
USDA Symbol: SIMA3
Oregon milk thistle distribution
Other common names
Blessed thistle, St. Mary’s thistle and lady’s thistle
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Images courtesy of Eric Coombes, Oregon Department of Agriculture

If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Biennial or winter annual; blooms from April to July. Grows two to six foot tall. Stems stout, rigid and generally branching. Leaves broad, clasp stem, have spiny margins and have white marbling along veins. Flower heads reddish-purple and have leathery spine-tipped bracts.
Once established, it forms dense clumps which exclude livestock and crowd out more desirable forage species. It has the potential to invade extensive acres of pasture land. Individual plants are so large that forage displacement is high. It is a nitrate accumulator, lethal when livestock ingest the plant. Milk thistle seed is valued as an herbal medicine. The seed is capable of remaining dormant in the soil for many years.  It infests roadsides, waste and disturbed areas, grazing lands and often occurs in association with Italian and slender-flowered thistles.
Milk thistle is a native to the Mediterranean region of Europe.  This plant has been known since ancient times; it is a biblical plant. It was mentioned by Theophrastus (4th century B.C.) with the name of Pternix and by Pliny the Elder (1st century A.D.) with the name of Sillybum. Von Haller (1744) in its "Medizinischen Lexicon" documented the specific use of the plant for liver disorders. In the 19th and 20th centuries many authors such as Rademacher, Schulz and Henry Leclerc mentioned the fruits of S. marianum for the treatment of liver diseases, disorders of the bile duct and spleen.
Distribution in Oregon
The first record of milk thistle in Oregon is 1886 in Multnomah.

Biological controls
One approved biocontrol agent, Rhinocyllus conicus a seed head weevil, has become established in Oregon; however, they do not always make their way to the seeds due the large receptacle on this plant.