Onopordum ancanthium • Class B

Scotch thistle is a branched plant that can grow up to 10 feet or more in height and 6 feet in width. Its leaves are up to 2 feet long and 1 foot wide and are covered with a thick mat of cotton-like hairs that give the foliage a bluish-green look and end in a very sharp yellow spine. The bright purple flower grows at the end of the leafy stalks as a single flower, or as a cluster of flowers. The bracts, which are one of the distinguishing features of the Scotch thistle, are yellow, sharp and needle-like. 

If eaten by stock the spines can cause injury to the animal particularly around the mouth. Spines and dead leaves contribute to faults in wool thereby decreasing its value. 

Scotch thistle is confined to areas with an annual rainfall of 20 to 35 inches. It drastically reduces native plant forage by outcompeting desirable species. It can be so thick that it becomes an impenetrable thorny barrier for livestock, wildlife, and people. 

Preventing seed production is the first line of defense for invasive species. Cut off all plant tops bearing flower heads or buds, and carefully bag and dispose of them in the garbage, do not compost. 

• Because it reproduces by seed, Scotch Thistle can be controlled by mechanical, chemical and cultural methods. Severing the roots of the rosette or the plant kills it. Small infestations can be pulled by hand. 

• Mowing makes the stand more uniform, which makes herbicide more effective, but mowing alone will not kill the plant. Applying herbicides to rosettes is very effective. 

• Spot spraying with an herbicide containing the active ingredient glyphosate (used in Roundup Pro, or Glyphos) is effective. Glyphosate is non-selective however, and will injure any plants it comes in contact with. Spot applications should be in spring, when plants are actively growing and up to when the flowering stem bolts. Plants sprayed after buds develop are harder to kill and may still produce viable seeds, despite plant damage. 

• Selective, translocated herbicides such as aminopyralid (used in Milestone, etc.) and 2, 4 D dicamba (used in Weedmaster) are two herbicides that can be effective on Scotch Thistle. These herbicides will not harm grass and can be used around livestock (provided all label precautions are followed).  

• When using herbicides, carefully 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.)  

• There are no biocontrols currently being used in Washington State. 

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