Milk thistle
Silybum marianum

This Class A noxious weed has a very limited distribution in Washington State, and eradication is required. The largest infestations in the state are in pastures in the southeastern section of King County but infestations are occasionally found elsewhere. Early detection and rapid, effective response is of the highest priority for this noxious weed. Although occasionally found in gardens, it is illegal to sell or buy milk thistle in Washington State and all existing plantings should be removed in order to prevent accidental spread.

Milk thistle is toxic to livestock when consumed in large quantities, and it forms dense stands in pastures and rangelands. California reports up to 4 tons per acre in heavily infested areas. The leaves are very distinctive, with white marbling on the shiny green leaves.

Additional information on milk thistle

• Washington State Noxious Weed Control Board (external link)
• Milk thistle weed alert (190 KB Acrobat file)
• Milk Thistle Best Management Practices (386 KB Acrobat file

What to do if you find this plant in King County, Washington

Please notify us if you see milk thistle in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as milk thistle in order to help us and others locate new infestations in time to control them.

Milk thistle photos - click thumbnail for larger image