milk thistle

Silybum marianum


Family: Asteraceae

Other common names: blessed milkthistle, spotted thistle, variegated thistle

Weed Class: A

Year Listed: 1989

Native to: Western Asia, Europe and Northern Africa

Toxic: livestock

additional photos



Why is it a noxious weed?

It is an aggressive invader that threatens native vegetation and beneficial forage. It is a prolific seeder and forms dense stands. It is a nitrate accumulator. Ingestion of milk thistle by grazing animals causes nitrate poisoning which can be lethal.

How would I identify it?

General Description: Milk thistle is a sparsely branched thistle growing up to 6 feet tall with distinctively patterned green and white leaves.

Flower Description: Flowerheads are large and rounded, occurring at stem tips. Flowers are purple and all tubular (all disk flowers). Flowerheads with leathery, spine-tipped bracts at the bases. Bracts in many overlapping rows with spreading tips.

Leaf Description: The leaves are alternate, deeply lobed, have pointed tips. They are shiny dark green with conspicuous white marbling. Basal leaves can be 20 inches long and 10 inches wide.

Stem Description: The stems are stout, rigid, branching and slightly cobweb-like. They end in a solitary composite flowerhead.

Fruit/Seed Description: Seeds are hairless and about ¼ inch long.

May be confused with:

How does it reproduce?

Milk thistle reproduces by seed.

Where does it grow?

Milk thistle occurs on fertile lands of improved pastures that have been overgrazed and poorly managed. It’s found in dense stands along roadsides, waste areas, pastures are invaded from roadside populations, ditches and disturbed areas. Please click here to see a county level distribution map of milk thistle in Washington.

How do I control it?

General Control Strategy

Thistles are most susceptible to control during the seedling stage, or as they grow from the seedling stage to the rosette state. An integrated pasture management approach is effective for thistle control.

Mechanical Control

Hand-pull (wearing good gloves!) or dig up young plants, and be sure to clip flowerheads of mature plants and dispose of them in the trash before the seeds ripen. Mowing is not recommended, as it may produce more plants the next year.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.


For more information

See our postcard for early detection information about milk thistle.
See our Written Findings for more information about milkthistle (Silybum marianum).

Additional Photos


Milk thistle plants in flower

Milk thistle leaves with white marbeling

Milk thistle infestation

in bloom and developing seed

flowerhead in bloom

leaf marbling

side view of flowerhead

top view of flowerhead

leaf shape and marbling

plant in cultivated garden

plant with blooming flowerhead

flowerhead on short flower stem

flowerhead on long flower stem


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