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. 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.
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
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