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Washington State Weeds

Scotch Thistle

scotch thistle

Onopordum ancanthium • Class B

Family Name: Asteraceae family (ass-ter-AY-see-ee)
Common: Aster, daisy, or sunflower family
Genus: Onopordum  (On-OH-por-dum)
Meaning: Donkey fodder or donkey flatulence
Species: ancanthium (a-KANT-tee-um)
Meaning: From the Greek word meaning  thorny, spiny

Scotch thistle is a branched plant that can grow up to 8 feet or more in height and 6 feet in width.

Stems have vertical rows of broad, spiny wings that extend to the base of the flower heads.

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.

Its 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. 

 Why Is it a Noxious Weed?

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. 

Where Does it Grow?

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. 

Facts: It is responsible for lost wildlife habitats and recreation areas. Access to trails, stream banks and fishing areas can be completely cut off by the spread of scotch thistle. 

Control Options:
  • 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 will 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 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 containing the active ingredients such as aminopyralid, 2, 4 D, or dicamba  are also 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.)  
  • 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.

More Information:

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


More Pictures:  

scotch thistlescotch thistlescotch thistlescotch thistle