Milk Thistle
Silybum marianum
Washington State Classification
Class A — Eradication Required Statewide
It is an aggressive invader that threatens native vegetation and beneficial forage.
Quick Reference
Identification
Growth Habit
Milk thistle is a sparsely branched thistle growing up to 6 feet tall with distinctively patterned green and white leaves.
Leaves
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.
Flowers
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.
Fruit & Seeds
Seeds are hairless and about ¼ inch long.
Impact
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.
Ecology & Spread
Habitat
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.
Spread Mechanisms
Reproduction
Milk thistle reproduces by seed.
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Control Methods
Mechanical
- 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.