Family: Poaceae

Origin, Description & Uses
Poa annua, commonly known as annual bluegrass or annual meadow grass, is a small cool-season grass native to temperate regions of the Old World and tropical mountain environments. It is a low-growing grass with soft, bright green leaves and distinctive pale seed heads that can appear throughout much of the year. Because it germinates quickly and tolerates frequent mowing and foot traffic, it has been widely used in lawns, parks, golf courses, athletic fields, and pasture areas. The species is now naturalized across the Hawaiian Islands, including Mokupāpapa (Kure Atoll), Kuaihelani (Midway Atoll), Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Maui, and Hawaiʻi Island.

Risks & Threats
Although often considered a common lawn grass, Poa annua possesses traits that are cause for concern in Hawaiʻi and other tropical island ecosystems. This species produces abundant seed, spreads rapidly, and can establish dense populations in disturbed areas, trailsides, lawns, agricultural lands, and natural habitats. Its ability to quickly colonize open ground allows it to compete with native plants and alter ground-layer vegetation communities. Because it is already naturalized throughout the Hawaiian Islands, continued spread could detrimentally impact sensitive ecosystems, particularly in cooler upland and montane environments. To help protect Hawaiʻi’s unique native habitats, Plant Pono recommends choosing low-risk or native grass alternatives whenever possible.

High Risk Traits:

• Elevation range exceeds 1000 m, demonstrating environmental versatility
  
• Broad climate suitability
  
• Naturalized in areas with subtropical climates
  
• Widely naturalized, including all main Hawaiian Islands except Ni'ihau and Kaho'olawe
  
• A weed of crops, cultivated and wasteland, paths, roadsides, etc.
  
• Other Poa species are invasive
  
• Host of crop pests
  
• Allergenic (causes hayfever)
  
• Shade-tolerant
  
• Tolerates many soil types
  
• Reproduces by seed
  
• Self-pollinated
  
• Able to reproduce vegetatively by rooting at nodes
  
• Annual forms reach maturity in <1 year
  
• Seeds dispersed by wind, water or attached to animal fur, livestock and vehicles and as a seed contaminant
  
• Viable seeds also dispersed internally by grazing animals
  
• Prolific seed production
  
• Seeds may persist in the soil for 4-6 years
  
• Tolerates mowing and trampling

Low Risk Traits:

• May only be a threat to higher elevation areas with tropical to subtropical climates
  
• Unarmed (no spines, thorns or burrs)
  
• Palatable to browsing and grazing animals
  
• Used as a turfgrass
  
• Some herbicides may provide effective control, although herbicide resistance is reported