Milk thistle is a Class A noxious weed in Washington State and eradication is required. Leaves, stems and flowers are all armed with stiff, sharp spines. Flowerheads are bright magenta or purple, with thick, fleshy, spine-tipped bracts protruding around the base off the flowerhead. Milk thistle is a large and highly distinctive thistle, with white marbling on the shiny green leaves. Early detection and rapid, effective response is of the highest priority for this noxious weed. The largest infestations in the state are in pastures in the southeastern section of King County but infestations are occasionally found elsewhere. California reports milk thistle stands of up to 4 tons per acre in heavily infested areas. Overview Milk thistle is toxic to livestock when consumed in large quantities, and it forms dense stands in pastures and rangeland. This plant has a limited distribution in King County, concentrated in rural areas. Reproduces via seeds that persist in soil at least 9 years. In April-October, pink-purple flower heads, with broad, spiny bracts at their bases, appear singly at stem ends. Shiny green leaves have distinct white marbling patterns, with spines on leaf edges and stems. Ingestion can cause nitrate poisoning in cattle and sheep. 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.
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