Milk thistle

Milk thistle blossom

Silybum marianum

Milk thistle brochure (PDF)

Why is milk thistle a problem?

Milk thistle is a serious threat to livestock. It accumulates nitrogen and can cause potentially lethal nitrate poisoning in animals. Spines can also cause injury to animals. Often found in dense stands, milk thistle deters livestock from grazing areas, limits usable acreage and reduces crop yield. 

This invasive is thought to be introduced to Washington through contaminated hay and medicinal gardens within the past few decades. 

Recommended control measures for milk thistle


Recognizing milk thistle

Milk thistle is a tap rooted winter annual or biennial. It is sparsely branched and can reach up to 6 feet tall, forming dense stands. In Clark County, milk thistle typically grows to 4 feet in height.

Flowers can be seen from April to October. Each solitary flowerhead can reach two inches in diameter and contains one large pink-purple flower. Broad, spiny bracts surround the flower head and spines are found along leaf edges and stems. 

Leaves are a shiny dark green and contain distinctive white marbling along veins, which distinguishes milk thistle from other types of thistle.

Seeds are dark brown, heavy (20 mg), fall close to the parent plant, and can remain viable in the soil for at least nine years. Seeds germinate after fall rains and plants then overwinter as a rosette. 

Milk thistle is typically found in full sun or partial shade pastures, on roadsides and in soils with high nitrogen content, such as dairies.