Family: Euphorbiaceae

Origin, Description & Uses:
Crown of thorns (Euphorbia milii) is a succulent shrub native to Madagascar. It is widely cultivated in tropical and subtropical regions as an ornamental plant, valued for its bright, long-lasting flowers and tolerance of heat and drought. The plant features thick, spiny stems and small green leaves, with colorful bracts—often red, pink, or yellow—surrounding its tiny true flowers. It is commonly grown in gardens, containers, and as a landscape accent, and is especially popular for xeriscaping due to its low water requirements and resilience in sunny, dry conditions.

Risks & Threats:
Euphorbia milii is considered a low-risk species in Hawai‘i. While it can reproduce vegetatively and by seed, it is not known to spread aggressively or form dense, invasive populations. However, like other members of the Euphorbia genus, it produces a milky latex sap that can irritate skin and eyes and is toxic if ingested, so care should be taken when handling. Its sharp spines also pose a minor physical hazard. Overall, it presents minimal ecological threat, especially when properly maintained in cultivated settings, making it a suitable choice for ornamental use.

High Risk Traits:

• Naturalized in tropical regions, China, and Florida
  
• History of repeated introductions as an ornamental
  
• Genus includes other weedy species (E. helioscopia, E. hirta)
  
• Produces sharp thorns
  
• Toxic, milky sap causes dermatitis and poisoning
  
• Unpalatable to grazing animals
  
• Host for fungal and bacterial pathogens
  
• Reproduces vegetatively via stem cuttings
  
• Actively spread by people through horticulture

Low Risk Traits:

• Not documented as a environmental weed
  
• No allelopathic or parasitic traits
  
• Requires full sun; not shade tolerant
  
• Restricted to well-drained soils; no wet tolerance
  
• Non-climbing shrub
  
• Rarely produces seed without human intervention
  
• Self-incompatible; limits seed set in isolation
  
• Slow growth rate
  
• Lacks seed dispersal adaptations (wind, water, animal, bird)
  
• Seeds infrequently produced in cultivation