Family: Asteraceae

Ambrosia psilostachya, commonly called western ragweed, is a perennial plant in the sunflower family (Asteraceae). Native to much of North America, it grows from spreading rhizomes and forms dense colonies. The plant is typically found in open, disturbed habitats such as roadsides, pastures, and abandoned fields, where it can quickly spread and dominate an area. Western ragweed produces inconspicuous flowers, but its pollen is highly allergenic and a well-known cause of hay fever in the regions where it grows.

In Hawaiʻi, Ambrosia psilostachya is only found in cultivation and has not yet naturalized. However, its ability to spread aggressively through underground rhizomes and produce large amounts of wind-dispersed pollen suggests it could become invasive if it escapes into the wild. Once established, it can be difficult to manage due to its persistent underground root system and prolific seed production.

High Risk Traits:

• Broad climate suitability and environmental versatility
  
• Naturalized in regions with subtropical climates
  
• Widely naturalized (but no evidence from Hawaiian Islands to date)
  
• Disturbance and agricultural weed
  
• Other Ambrosia species are invasive
  
• Allelopathic
  
• Unpalatable at certain stages of growth
  
• Alternate host of grape pathogenic bacteria
  
• Allergenic pollen
  
• Fire hazard
  
• Shade-tolerant
  
• Tolerates many soil types
  
• Forms dense stands
  
• Geophyte life form
  
• Reproduces by seeds and vegetatively by rhizomes
  
• Reaches maturity in 2nd growing season
  
• Seeds dispersed externally in mud stuck to equipment, vehicles and animals
  
• Seeds persist in the soil for many years
  
• Able to resprout after cutting and fire

Low Risk Traits:

• Unarmed (no spines, thorns, or burrs)
  
• Limited seed production
  
• Herbicides provide effective control