Family: Poaceae

Origin, Description & Uses
Megathyrsus maximus (Guinea grass, green panic grass) is a large, fast-growing perennial grass native to tropical and southern Africa, the western Indian Ocean region, and the Arabian Peninsula. It forms dense clumps with tall leafy stems that can reach several feet in height. Because of its rapid growth and high biomass production, Guinea grass has been widely introduced throughout the tropics as forage for livestock, pasture improvement, erosion control, and hay production. It is now naturalized across the Hawaiian Islands, including Niʻihau, Midway Atoll, Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Kahoʻolawe, Maui, and Hawaiʻi Island.

Risks & Threats
Guinea grass is naturalized in Hawaiʻi and possesses traits that are cause for concern in tropical island ecosystems. This aggressive grass grows rapidly, spreads readily by seed, and can form dense stands that crowd out native vegetation and reduce biodiversity. It is especially problematic in dry and disturbed habitats, where it alters natural plant communities and increases competition for water and nutrients. Guinea grass also produces large amounts of flammable material, greatly increasing wildfire risk in areas not historically adapted to frequent fire. These fires can further damage native ecosystems and promote additional spread of invasive grasses. Because of these impacts, Megathyrsus maximus could detrimentally affect tropical island ecosystems, and choosing low-risk or native alternatives is recommended when suitable options are available.

High Risk Traits:

• Broad climate suitability (dry to wet, sea level to 1,200 m)
  
• Naturalized throughout the tropics, including Hawaii
  
• Major agricultural weed in several countries (Hawaii, Australia)
  
• Congeneric weeds exist (P. fasciculatum, P. repens)
  
• Allelopathic
  
• Creates fire hazard (fire-tolerant, regenerates rapidly)
  
• Tolerates wide range of soils (pH 5.2–6.9)
  
• Forms dense, rapid-growing thickets
  
• Reproduces vegetatively (seeds + underground stems)
  
• Minimum generative time ≤ 1 year
  
• Propagules dispersed intentionally (cattle grazing)
  
• Propagules likely as produce contaminant (very small seeds)
  
• Wind-dispersed seeds
  
• Prolific seed production (1.7–3.1 million/kg)

Low Risk Traits:

• No spines, thorns, or burrs
  
• Palatable and non-toxic to animals and humans
  
• No serious pest/pathogen issues
  
• Shade intolerant
  
• No persistent seed bank (short viability)