Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Onopordum acaulon
(horse thistle)



Onopordum acaulon (horse thistle)


  • Last modified
  • 25 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Onopordum acaulon
  • Preferred Common Name
  • horse thistle
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • Onopordum acaulon, stemless thistle, reduces the carrying capacity of the land it infests and seriously harms ...

  • Principal Source
  • US Federal Noxious Weed List (draft fact sheet 2011)

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Preferred Scientific Name

  • Onopordum acaulon L.

Preferred Common Name

  • horse thistle

International Common Names

  • English: stemless onopordon; stemless thistle
  • Portuguese: cardo

Summary of Invasiveness

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Onopordum acaulon, stemless thistle, reduces the carrying capacity of the land it infests and seriously harms livestock that try to eat it, resulting in impaction and liver damage. A pest risk assessment finalized by APHIS (Lehtonen, 2004) ranked the weed with medium consequences of introduction and high likelihood of introduction resulting in a ranking of medium-high overall risk potential in the USA. O. acaulon was added to the Federal Noxious Weed List and seed list in 2010.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Onopordum
  •                                 Species: Onopordum acaulon


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O. acaulon is nearly stemless, with the stalks rarely exceeding 10 cm in length. The short-stalked leaves are 15-38 cm long and deeply dissected into spine-toothed lobes and are narrow and oblong, grey or white-hairy on both surfaces. The flower heads are white or purple, and form at the centre of the basal rosettes of foliage (Everett, 1981). Seeds are grey-brown and about 4 mm long. They are four-sided, pitted and ridged lengthwise, with a pappus of cream-coloured barbed hairs about 25 mm long. In Australia, stemless thistles germinate from seed mainly in the autumn. Plants develop during winter into small rosettes, which grow rapidly in spring. They flower from October to November dying in summer. Some seeds germinate during winter and spring, but most of the resulting plants do not develop sufficiently to flower before dying. Under favourable conditions, some of these plants do survive and flower in the second summer. Reproduction is by seeds only.

Plant Type

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Seed propagated


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Stemless thistle is believed to have originated in northern Africa and southwestern Europe. It was introduced to Australia as an ornamental and as an agricultural contaminant. Since then, it has naturalized and become a major pest along roadsides and wastelands, and specifically in cultivated lands and pastures.  Stemless thistle is not known to be present in the United States, but there is a high likelihood of its introduction. The plant thrives in warm temperate regions that receive less than 50 cm of annual precipitation. It would probably establish itself in the southwestern United States, including Texas, California and Arizona.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Last updated: 10 Feb 2022
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes







Habitat List

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Terrestrial ManagedCultivated / agricultural land Principal habitat
Terrestrial ManagedManaged grasslands (grazing systems) Principal habitat
Terrestrial ManagedDisturbed areas Principal habitat
Terrestrial ManagedRail / roadsides Principal habitat
Terrestrial Natural / Semi-naturalNatural grasslands Principal habitat
Terrestrial Natural / Semi-naturalArid regions Principal habitat

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Horticulture Yes

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Plants or parts of plants Yes Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
True seeds (inc. grain) weeds/seeds Yes

Impact Summary

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Livestock production Negative

Risk and Impact Factors

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  • Proved invasive outside its native range
Impact outcomes
  • Negatively impacts animal health
Impact mechanisms
  • Competition - monopolizing resources
  • Produces spines, thorns or burrs

Uses List

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  • Ornamental


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Department of Primary Industries (DPI). “Stemless thistle (Onopordum acaulon) (Nox).”  Ministries of Agriculture and Energy and Resources for Victoria, Australia. Last updated 10 June 2009, accessed 12 June 2009.

Everett, Thomas H. 1981. The New York Botanical Garden Illustrated Encyclopedia of Horticulture Volume 3. General Publishing, Inc. New York & London.

USDA, ARS 2009. Germplasm Resources Information Network (, 12 June 2009). 10300 Baltimore Blvd, Beltsville, MD 20705, USA.

Links to Websites

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GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gateway source for updated system data added to species habitat list.

Principal Source

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US Federal Noxious Weed List (draft fact sheet 2011)

Distribution Maps

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