Slenderflower Thistle

slenderflower Thistle

Carduus tenuiflorus • Class A

Family Name: Asteraceae family (ass-ter-AY-see-ee)
Common: Aster, daisy, or sunflower family
Genus: Carduus (KARD-yoo-us) 
Meaning: From the Greek word kardos (thistle)
Species: tenuiflorus (ten-yoo-ee-FLOR-uh)
Meaning: Slender flowered

Slenderflower thistles grow about 3 feet tall with prickly winged stems. The stems and leaves are spiny. Its leaves are deeply cut into 2 to 5 pairs of lobes and are wooly looking with light-colored veins. It may have many stems branching from the lower part of the plant with clusters of 5 to 20 flowers. Flowers range in color from pink to purple with usually more than five heads per cluster and the bracts are not hairy. 

 Why Is it a Noxious Weed?

Slenderflower thistle is extremely competitive and outcompetes native plant species and desirable vegetation. They form dense stands, which severely limit grazing of cattle or foraging of wildlife.This species is also a nitrate accumulator; ingestion by grazing animals may cause nitrate poisoning, which can be fatal to cattle or sheep. 

Where Does it Grow?

They infest roadsides, natural clearings, clear-cuts, industrial sites, and waste areas. They like dry, open areas such as pastures, ranges, and right-of-ways and are generally found in disturbed areas, and vacant lots. 


Germination of seeds is rapid and occurs at high rates. Seed dispersal is by wind, water, and birds. Seeds remain viable in the soil for as long as 7 years. It produces 2 kinds of seeds, each about 3/4 of an inch long. Seeds produced by the central flowers in the head are cream colored, grooved and sticky with a plume of dirty white barbed hairs. This pappus is easily dispersed. The outer ring of flowers in the head produces seeds that are smooth, darker, non sticky and lack a pappus, have no obvious means of dispersal and are often dormant. 

Control Options:

Preventing the establishment of populations of Slenderflower Thistle is the most time and cost effective way of controlling this species. Above all, any plants with flower heads or buds should be cut off, bagged up   and disposed of in the garbage (not composted) to prevent seed production. 


  • Hand pulling or digging can be effective for isolated plants or small patches, especially when performed in the seedling stage. Mechanical controls like tilling or digging will kill Slenderflower Thistle. 

  • MCPA (Weed-B-Gone, Weed-n-Feed) is recommended for application during the seedling or rosette stage. Other growth regulator type herbicides 2, 4 D or dicamba (Outlaw, Weedmaster) may be effective too. 

  • 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.) 


More Information:

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


More Pictures:  

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