Invasive Species: Milk Thistle

Silybum marianum

Milk thistle was first spotted in Multnomah county in 1886. Outside of Douglas County, it has limited occurrence in the Willamette Valley.

Why is it important to control?

Once established, it forms dense clumps that exclude livestock and crowd out more desirable forage species.

Close up image of milk thistle's variegated leaves

How can you identify it?

Purple milk thistle flowerClose up image of milk thistle seeds
Habitat: Full sun or part shade with fertile soils. Commonly found in degraded pastures, agricultural fields, and roadsides.
Life Cycle: Biennial
Height: Up to 6 feet
Leaf Description: Rosette with dark green and white leaves in first year, flowering stalk the second year.
Fruit & Flower Description: Bright purple flower at top of stem; spine-tipped bracts on flower bulb.
Bloom Time: April to July

How do you control it?

  • Hand-pulling works for small infestations.

  • For small infestations, cut off the flower heads and put in a tied bag and place in the garbage.

  • Herbicide control:  The PNW Pest Management Handbook has details. Take care to avoid drift and be cautious near crops. FOLLOW ALL LABEL DIRECTIONS!

  • The Oregon Department of Agriculture has approved and implemented one biocontrol, a seed head weevil and it is well established. This has had a significant impact on populations, but it needs to be kept in mind that biocontrols are not focused on eradication.

How to report it?

There are various ways you can report this noxious weed.

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