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.

Milk thistle photos - click thumbnail for larger image

The weed program is closely working with landowners to remove this plant and to keep it from spreading to other sites. If you think you have milk thistle, or for control information, please contact our office at 206-296-0290 or send us an email.

If you find milk thistle in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for milk thistle.

For information on milk thistle identification and control, download our milk thistle weed alert (190 KB Acrobat file) or for comprehensive biology and control information, download the Milk Thistle Best Management Practices (386 KB Acrobat file).

For additional information about milk thistle, please follow this link to the WA State Noxious Weed Control Board.