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Noxious weeds

King County, Washington

To offer a suggestion or report an error on the King County Noxious Weeds website, please contact Sasha Shaw, education specialist.

Milk thistle
Silybum marianum

milk thistle flowerhead - click for larger imageThis 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. The species is on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington. 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 leaves by fence - click for larger image

Additional information on milk thistle

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

milk thistle large rosette - click for larger image milk thistle flowering - click for larger image milk thistle in cow pasture - click for larger image milk thistle fall rosette and seedlings - click for larger image milk thistle flowerhead - click for larger image
milk thistle rosette - click for larger image milk thistle comparison - click for larger image milk thistle mound - click for larger image milk thistle seeds - click for larger image milk thistle plants - click for larger image
milk thistle in seed - click for larger image milk thistle seedling - click for larger image milk thistle rosette - click for larger image milk thistle seed heads - click for larger image milk thistle plant - click for larger image

Report milk thistle in King County, Washington

Locate milk thistle in King County, Washington

Related information

Related agencies


Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).