Family: Euphorbiaceae

Origin, Description & Uses:
Euphorbia lathyris, commonly known as the petroleum plant or caper spurge, is native to southern Europe, North Africa, and parts of western Asia. It is a striking biennial plant that can grow up to 6 feet tall, with upright, unbranched stems and distinctive blue-green, lance-shaped leaves arranged in opposite pairs along the stem. The plant produces small, greenish-yellow flowers followed by seed capsules that can explosively disperse seeds. It has been cultivated as an ornamental for its bold architectural form and is sometimes promoted for its historical use in traditional medicine and its latex-rich sap, which has been explored as a potential biofuel source—hence the name “petroleum plant.”

Risks & Threats:
Although Euphorbia lathyris is not currently known to be naturalized in the Hawaiian Islands, it possesses several traits that raise concern. It produces abundant seeds that can be forcefully ejected and spread over distances, increasing its potential to escape cultivation. The plant also contains a toxic, milky sap that can irritate skin and eyes and may pose risks to people, pets, and livestock. In suitable climates, it can establish quickly and outcompete other vegetation, particularly in disturbed areas. These characteristics suggest it could become invasive and negatively impact native plant communities and ecosystems in Hawaiʻi. Due to these concerns, it is recommended to consider low-risk or native plant alternatives for landscaping and gardening.

High Risk Traits:

• Broad climate tolerance – USDA zones 5b–10a
  
• History of introductions – widely planted as ornamental
  
• Naturalized – North America, New Zealand
  
• Disturbance weed – gardens, roadsides
  
• Congeneric weed – genus includes leafy spurge
  
• Unpalatable – avoided by herbivores
  
• Toxic – to humans and livestock
  
• Pest host – harbors crop pathogens
  
• Self-compatible – reproduces without pollinators
  
• Short generation time – annual
  
• Intentional dispersal – sold in nurseries
  
• Multiple dispersal vectors – water, ants, ballistic
  
• Persistent seed bank – seeds viable ≥1 year
  
• Resprouts – tolerates cutting

Low Risk Traits:

• Temperate native – not tropical-adapted
  
• agricultural weed
  
• Limited ecological impact – small populations, out-competed
  
• No dense thickets
  
• Seed-only reproduction
  
• Not wind or bird dispersed
  
• Low seed production
  
• Herbicide susceptible – glyphosate effective