Family: Poaceae

Origin, Description & Uses: Lolium perenne (perennial ryegrass) is a cool-season grass native to Macaronesia, North Africa, Europe, and parts of Asia extending to Siberia and the Himalayas. It is a fast-growing, tufted grass commonly used worldwide for lawns, pasture forage, erosion control, athletic fields, and temporary landscape cover. Perennial ryegrass is valued for its rapid germination, dense growth, and ability to tolerate mowing and grazing. In Hawaiʻi, it has been introduced for turf, forage, and soil stabilization purposes and is now naturalized on Kauaʻi, Oʻahu, Maui, and Hawaiʻi Island, with reports suggesting it may also be naturalizing on Lānaʻi.

Risks & Threats: Perennial ryegrass is naturalized in the Hawaiian Islands and possesses traits that are cause for concern in tropical island ecosystems. Its rapid growth, prolific seed production, and ability to establish dense ground cover can allow it to compete with native vegetation and alter plant community composition, particularly in disturbed areas, pastures, roadsides, and open habitats. It may also contribute to changes in fire behavior and ecosystem processes where grasses accumulate as fine fuels.

High Risk Traits:

• Elevation range exceeds 1000 m, demonstrating environmental versatility
  
• Broad climate suitability
  
• Able to grow in areas with tropical climates
  
• Naturalized on Hawaii, Maui and Oahu islands, and widely naturalized elsewhere
  
• A disturbance adapted grass with negative environmental impacts
  
• Other Lolium species are invasive
  
• May contain endophytic fungi that can be toxic to livestock
  
• Pollen regarded as a severe allergen
  
• Increases fire risk
  
• Tolerates many soil types
  
• Forms dense swards that displace native grass and forb species and reduce species richness
  
• Reproduces by seed
  
• Hybridizes with other grasses
  
• Able to reach maturity in <1 year
  
• Seeds dispersed internally and externally by animals, by water, and both intentionally and accidentally by humans
  
• Prolific seed production
  
• Tolerates mowing, tilling, grazing and fire

Low Risk Traits:

• Unarmed (no spines, thorns or burrs)
  
• Provides fodder for livestock (palatable despite reports of toxicity)
  
• Ornamental
  
• Shade-intolerant
  
• Not reported to spread vegetatively (does not produce stolons or rhizomes)
  
• Self-incompatible
  
• Seeds not persistent in seedbank
  
• Several herbicides provide effective control