WSU Clark County Extension
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Milk Thistle

Scientific name: Silybum marianum
Taxonomy
Family:Asteraceae
Type:Invasive plants
Native:NO
Plant Requirements
Zone:5 to 9
Plant Characteristics
Height:8 ft
Width:3 ft
Bloom:Spring flowers
Bloom Time:April to July
Bloom Color:Purple
Additional Characteristics
Trees


Fruit

Leaves


Bark
Wildlife value

Poisonous
Description Milk thistle is classified as Class A noxious weed in Washington though it has a limited distribution in the state. It is considered an invasive weed as it forms dense stands in pastures and rangelands making it difficult to control. It is toxic to livestock.

  Morphology:
The leaves are very distinctive, with white marbling on the shiny green, spiny margined leaves. It is considered a biennial or winter annual, blooming from April to July. It can grow from 2’ to 6’ in height. It features very stout stems, rigid, with a branching form. Leaves broad with clasp stem, have spiny margins and have white marbling along veins. Flower heads are reddish-purple and have leathery spine-tipped bracts. Milk thistle seed is valued as an herbal medicine.

  Adaptation:
Milk thistle can infests roadsides, waste and disturbed areas, grazing lands and often occurs in association with Italian and slender-flowered thistles.

  Pests:
None.
For assistance, contact Dr. Charles Brun (brunc@wsu.edu), (360) 397-6060 5701
Computing and Web Resources, PO Box 6234, Washington State University, Pullman WA 99164-6234