Family: Asteraceae

Origin, Description & Uses
Onopordum acaulon, commonly known as horse thistle, stemless onopordon, or stemless thistle, is native to the western Mediterranean region. This striking biennial plant forms a low-growing rosette of large, silvery-green leaves covered in sharp spines and woolly hairs, with purple thistle-like flowers appearing close to the ground. It is adapted to dry, sunny environments and poor soils, making it tolerant of drought and disturbance. The species has occasionally been cultivated as an ornamental for xeriscapes, naturalistic gardens, and specialty collections due to its dramatic appearance and resilience.

Risks & Threats
Although Onopordum acaulon is not currently known to be naturalized in the Hawaiian Islands, it possesses traits that are cause for concern and could detrimentally impact tropical island ecosystems if introduced and allowed to spread. Like other invasive thistles, it produces spiny growth that can deter grazing animals, reduce recreational access, and outcompete native vegetation in open or disturbed habitats. Its ability to thrive in harsh conditions and produce numerous seeds increases the risk of establishment in dry or degraded areas. Preventing the introduction and spread of potentially invasive thistles is important for protecting Hawaii’s native ecosystems and biodiversity.

High Risk Traits:

• Naturalized outside native range
  
• Recorded as a garden/amenity weed
  
• Produces spines or thorns
  
• Unpalatable to grazing animals
  
• Toxic to animals
  
• Host for pests/pathogens
  
• Shade tolerant
  
• Tolerates wide range of soil conditions
  
• Produces viable seed
  
• Minimum generative time: 1 year
  
• Propagules dispersed unintentionally (traffic, produce contamination)
  
• Propagules dispersed intentionally by people
  
• Propagules dispersed by wind and water
  
• Persistent seed bank (>1 year)

Low Risk Traits:

• No history of repeated introductions outside native range
  
• No vegetative fragmentation
  
• No dispersal by birds, other animals, or gut passage
  
• Well controlled by herbicides