Family: Arecaceae

Origin, Description & Uses
Washingtonia filifera (California fan palm) is native to the desert oases of southern California, southwestern and central Arizona, and northwestern Mexico (Baja California and western Sonora). This tall, single-trunked palm is recognized by its large, fan-shaped leaves with distinctive thread-like fibers along the leaf margins and its impressive crown that can reach over 60 feet (18 m) tall. It is widely cultivated as an ornamental tree in warm climates because of its stately appearance, tolerance of drought once established, and ability to thrive in a variety of soils. California fan palm is commonly planted along streets, in parks, resorts, and residential landscapes.

Risks & Threats
California fan palm is naturalized in Hawaiʻi, including on Oʻahu and Maui, and is considered potentially naturalizing on Molokaʻi. It produces abundant fruit that is readily dispersed by birds and other animals, allowing seedlings to establish beyond cultivated areas. The species possesses traits that are cause for concern and could detrimentally impact tropical island ecosystems by invading disturbed sites, riparian areas, and other suitable habitats where it can compete with native vegetation and alter plant community structure. Plant Pono recommends choosing a low-risk or native alternative when suitable substitutes are available. Where California fan palm is already widespread, landowners and landscape managers are encouraged to prevent further spread by avoiding new plantings near natural areas, removing unwanted seedlings, and managing existing populations in areas where the species is not yet widely established.

High Risk Traits:

• Naturalized or potentially naturalizing on O'ahu, Maui, and Molokai (Hawaiian Islands)
  
• Garden weed (seedlings prolific)
  
• Congener (W. robusta) is an environmental weed
  
• Petioles have sharp thorns
  
• Fruit extracts have allelopathic properties (lab)
  
• Dead leaf thatch creates a fire hazard
  
• Forms dense thickets in moist areas
  
• Self-compatible (self-pollinating)
  
• Seeds dispersed by birds and mammals (gut passage enhances germination)
  
• Seed bank persists up to 6 years
  
• Fire promotes regeneration and growth

Low Risk Traits:

• Narrow climate tolerance (arid/Mediterranean; susceptible to root rot in wet conditions)
  
• Non-toxic to animals and humans (edible fruit)
  
• No vegetative reproduction (seed only)
  
• Long generation time (~19 years to flowering)