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Class A Noxious Weed

Milk Thistle
Silybum marianum
 

Milk thistle flower

Other common names: blessed milkthistle, spotted thistle,variegated thistle
Plant family: Asteraceae (Compositaea)
Year listed:
Native to: Mediterranean region of Europe

Why is it a noxious weed?
Milk thistle threatens native vegetation and beneficial forage by its aggressiveness, like other thistles it is a prolific seeder and forms dense stands. Milk thistle is a nitrate accumulator. Ingestion of milk thistle by grazing animals causes nitrate poisoning which can be lethal.

 

How would I identify it?
General Description: Sparsely branched thistle growing up to 6 feet tall.

Leaves:

  • alternate
  • deeply lobed, pointed tips
  • shiny dark green with conspicuous white marbling
  • basal leaves can be 20 inches long and 10 inches wide

 

Flowers:

  • purple
  • broad, leathery spine tipped bracts
  • about 2 inches wide

Stems:

  • stout, rigid and branching
  • slightly cobwebby
  • stems end in solitary composite flower head

Line drawings are taken from 1,000 Weeds of North America, copyright of XID Service, Inc and used with permission. Permission granted by the author.

How does it reproduce?
Milk thistle reproduces by seed.

Where does it grow?

Milk thistle, like most thistles occur in fertile lands of improved pastures that have been overgrazed and poorly managed. It is  found in dense stands along roadsides, waste areas, pastures are invaded from roadside populations, ditches and disturbed areas.

What is the distribution in Washington State?
 
  • Field infestations are found in Clark, Cowlitz and Klickitat Counties, with ornamental
    sites occurring elsewhere.
  •  If you think you may have seen this weed, please contact your county weed coordinator or us at noxiousweeds@agr.wa.gov.

 

  • Distribution map not available at this time.

How do I control it ?

General control methods: Thistles are the most susceptible to control during the seedling stage, or as they grow from the seedling stage to the rosette state. An integrated pasture management approach is effective for thistle control.

Herbicide: Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.

Mechanical: Mowing may produce more plants the next year.

Biological: None in Washington, a weevil Rhinocyllus conicus was introduced for control in California, but was not effective.

For more information
For more information, download our Written Findings.

 

More Photos-Click photo to enlarge
Feel free to download, or contact us to receive high-resolution JPEG or TIFF formats
Milk thistle plant. Milk thistle leaves, notice the distinctive marble pattern.

 Last updated July 09, 2008