Family: Asteraceae

Origin, Description & Uses:
Parthenium argentatum, commonly known as guayule or Mexican-rubber, is a woody shrub native to arid regions from Texas into Mexico. It belongs to the sunflower family and is well adapted to hot, dry climates with poor soils. Guayule typically grows as a rounded shrub with silvery-gray foliage and small white flowers. Historically, it has been cultivated as an alternative source of natural rubber, particularly during periods when traditional rubber supplies were limited. In addition to rubber production, the plant has been explored for use in latex products, adhesives, biofuels, and soil stabilization in dryland landscapes.

Risks & Threats:
Although guayule is not currently known to be naturalized in the Hawaiian Islands, it does possess traits that are cause for concern. The species is hardy, drought tolerant, and capable of growing in challenging environmental conditions, characteristics that can sometimes contribute to weediness outside its native range. While there is currently limited evidence of ecological impacts in Hawaii, ongoing evaluation is needed to determine whether it could negatively affect local ecosystems, particularly in dry or disturbed habitats. Caution is recommended when considering cultivation to help ensure that this species will not negatively impact Hawaii’s native environments.

High Risk Traits:

• Grows in subtropical-temperate
  
• Other Parthenium species are invasive
  
• Possibly allelopathic
  
• Potentially toxic and allergenic sap
  
• Forms dense stands in parts of native range (although reported to be a poor competitor)
  
• Reproduces by seeds
  
• Hybridizes with Parthenium incanum
  
• Apomictic races exist
  
• May be capable of spreading vegetatively
  
• Capable of reaching maturity as early as 6 months
  
• Seeds dispersed by wind, and possibly water
  
• Intentionally dispersed by people
  
• Prolific seed production
  
• Seeds exhibit dormancy and may be capable of forming a persistent seed bank
  
• Tolerates pollarding (pruning system in which the upper branches are removed, promoting a dense head of foliage and branches)

Low Risk Traits:

• No reports of invasiveness or naturalization
  
• Unarmed (no spines, thorns or burrs)
  
• Palatable to grazing animals (despite potential toxicity/allergenicity)
  
• Source of fuel and rubber
  
• Reportedly intolerant of shade and a poor competitor with other vegetation