Washington State Weeds

Milk Thistle

milk thistle

Silybum marianum • Class A

Family Name: Asteraceae family (ass-ter-AY-see-ee)
Common: Aster, daisy, or sunflower family
Genus: Silybum (SIGH-lee-bum)
Meaning: The Greek name for some edible thistles
Species: marianum (mar-ee-AH-num)
Meaning: Of St. Mary

This sparsely branched thistle grows up to 6 feet tall. 

Leaves are dark green with distinctive white veins on them and are about 10 inches wide and 20 inches long, with bases that clasp the stem. The wavy leaf margins are edged in yellow prickles that are roughly half an inch in length.

Its leathery thorn tipped bracts at the base of each solitary purple flower head, make it unique from other thistles.

It blooms from April to August. 

 Why Is it a Noxious Weed? Milk Thistle aggressively invades and crowds out desirable vegetation. Individual plants are so large that forage displacement is high.  This weed is a nitrate accumulator. Ingestion of milk thistle by grazing animals causes nitrate poisoning, which can be lethal to cattle or sheep.
Where Does it Grow?

It is found along roadsides, waste areas. Pastures and fertile lands are invaded from roadside populations, ditches and disturbed areas. It is a prolific seeder and forms dense stands of thistle crowding out other beneficial plants. 


Do not plant Milk Thistle intentionally. While Milk Thistle does have beneficial medicinal properties, the extract is very difficult to process. It takes about 70 pounds of seeds to produce one pound of extract purified to 70-80% silymarin (of which 60% is silybin, the most active and beneficial component). In this form, it is still difficult for the body to utilize, requiring huge quantities which are not feasible for home production. 

Control Options:
  • Eradication is best achieved through prevention of seed production. It can take up to 9 years till the seeds in the soil are depleted. For small infestations dig or pull the rosettes. All buds must be removed to prevent seed production. 
  • Mowing may produce more plants the next year. If moisture is adequate, mown plants will regrow and produce seed later in the season. 

  • Spot spraying with an herbicide containing the active ingredient glyphosate most effectively kills milk thistle at the rosette stage, repeat as needed. Spot application means the herbicide is applied only to the Milk thistle plants, and not to the surrounding plants or soil. Be aware, Glyphosate is non-selective and will injure any plants that it comes in contact with, including grass. 

  • For selective control of knapweed in agricultural settings (pastures, hayfields, etc.): an herbicide containing the active ingredient aminopyralid  may be applied in the spring, when the plant is actively growing and up till the flowering stem bolts. Plants sprayed after buds develop are harder to kill and have a higher likelihood of producing viable seeds, despite damage to the plant. Applications of aminopyralid are also effective in the fall before a killing frost. Aminopyralid products will not harm grass and can be used around livestock (provided all label precautions are followed). 

  • When using herbicides, read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.) 
  • To minimize any harmful impact on bees and other pollinators, timing is important.  Ideally, treat plants before blooming.  If treatment after blooming is necessary, do control work early in the morning, or in the evening when bees are less active.
  • Because Milk Thistle is a class A noxious weed and must be eradicated completely whenever it is found, biological control is not a viable option. 

More Information:

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels


More Pictures:  

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