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Milk thistle identification and control: Silybum marianum

Milk thistle identification and control

Silybum marianum


Milk thistle, a regulated Class A noxious weed, is a toxic, branching winter annual or biennial that grows 2-6 feet tall in disturbed areas, such as pastures, roadsides, ditches, and fencerows. Ingestion can cause nitrate poisoning in cattle and sheep. Young plants grow as rosettes. Shiny green leaves have distinct white marbling patterns, with spines on leaf edges and stems. In April-October, pink-purple flower heads, with broad, spiny bracts at their bases, appear singly at stem ends. Reproduces via seeds that persist in soil at least 9 years. This plant has a limited distribution in King County, concentrated in rural areas.


milk thistle flowerhead - click for larger image
Milk thistle is toxic to livestock when consumed in large quantities, and it forms dense stands in pastures and rangeland. California reports milk thistle stands of up to 4 tons per acre in heavily infested areas.

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.


Milk thistle is a large and highly distinctive thistle, with white marbling on the shiny green leaves. Flowerheads are bright magenta or purple, with thick, fleshy, spine-tipped bracts protruding around the base off the flowerhead. Leaves, stems and flowers are all armed with stiff, sharp spines.

Legal status in King County, Washington

Milk thistle is a Class A noxious weed in Washington State and eradication is required. Property owners in King County are required to remove this plant if it occurs on their property and the King County Noxious Weed Program will assist property owners with the control of this Class A noxious weed if requested.

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) 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.

For more information about noxious weed regulations, see Noxious weed lists and laws.

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

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

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).