Saturday, September 18, 2010

Silybum marianum – Blessed Milk Thistle

Warning: Silybum marianum has been listed as an invasive pest plant or noxious weed. It is included here only as an aid to identification and not to promote its cultivation in any way whatsoever.

Flowering plant in coastal California sage scrub. © 2010 Joseph Libertucci. Used by permission.

Silybum marianum is a biennial or winter annual originally native to the Mediterranean regions of Europe, western Asia (Israel and Turkey), and northern Africa (Egypt). It is now widely established outside of cultivation as a weed in tropical and southern Africa, northern Europe, the British Isles, Australia and New Zealand, and North and South America. It is readily identified by the spiny leaves that are conspicuously variegated, as well as by the flower heads, which are subtended by bracts whose tips expand into a leafy, spine-tipped appendage.

Foliage of young plant. © by Valérie75 2006. Used under a Creative Commons license.

Milk thistle seed extract (silymarin) has shown promise in the treatment of liver diseases, cancer, hepatitis C, HIV, diabetes, and high cholesterol, as well as protective effects against liver damage by poisonous mushrooms; however, there have been few high-quality randomized clinical trials to conclusively prove the therapeutic effects of milk thistle (see, for example, Tamayo & Diamond 2007).

Seedling. © by Beentree 2008. Used under a Creative Commons license.

Silybum marianum was touted as an ornamental garden plant in the 1980s and seeds were widely available from mailorder garden catalogs; however, its popularity has waned as gardeners realized that it dies after flowering and they must then deal with a large mass of dry and wickedly spiny foliage. It is still widely sold as an herb garden or medicinal plant but the therapeutic properties are contained in seed extracts and such extracts are easier to purchase ready-made than to make at home from seeds harvested in the garden. In several states, Silybum marianum is an invasive pest plant or a declared noxious weed. Thus, in view of its weedy nature as well as its toxicity to livestock, its cultivation should not be encouraged.

United States Distribution Map for Silybum marianum (Blessed Milk Thistle)
Map courtesy of the United States Department of Agriculture Plants Database.



  • Tamayo, C. and S. Diamond. 2007. Review of clinical trials evaluating safety and efficacy of milk thistle (Silybum marianum [L.] Gaertn.) [abstract]. Integrative Cancer Therapies 6(2): 146–157.
  • USDA, ARS, National Genetic Resources Program. Silybum marianum. Germplasm Resources Information Network - (GRIN) [Online Database]. National Germplasm Resources Laboratory, Beltsville, Maryland. Internet

Text © 2010 Rufino Osorio. All images copyrighted by their respective owners as indicated above.

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