Family: Poaceae

Origin, Description & Uses:
Sporobolus pyramidatus, commonly known as giant rat’s tail grass, is a perennial bunchgrass native to the southern United States, Central America, South America, and other tropical and subtropical regions of the Americas. It forms dense clumps of narrow leaves and produces tall flowering stems bearing numerous seed heads. Although it has limited forage value when young, mature plants are generally avoided by livestock because of their tough, coarse foliage. The species is commonly found in disturbed areas, pastures, roadsides, and open landscapes in warm climates. In Hawaiʻi, it has become naturalized across much of the archipelago, including Kauaʻi, Oʻahu, Molokaʻi, Maui, Hawaiʻi Island, Kahoʻolawe, Midway Atoll, Kure Atoll, Laysan Island, and French Frigate Shoals.

Risks & Threats:
Sporobolus pyramidatus is considered a high-risk species because it is naturalized in Hawaiʻi, possesses traits that are cause for concern, and could detrimentally impact tropical island ecosystems. The species produces abundant seeds that can be dispersed by wind, animals, vehicles, and contaminated equipment, enabling it to spread rapidly into new areas. Dense infestations can outcompete desirable vegetation, reduce pasture productivity, alter habitat conditions, and increase management costs. Its ability to establish and persist in disturbed environments makes it difficult to control once established. Because this species is already widespread in Hawaiʻi, management efforts should focus on preventing its spread into areas where it is not yet well established and reducing existing populations where feasible. When selecting plants for landscaping, restoration, or agricultural use, low-risk or native alternatives are recommended whenever suitable options are available.

High Risk Traits:

• Naturalized on Mokupāpapa (Kure Atoll), Kuaihelani (Midway Atoll), Kaua'i, O'ahu, Molokai, Kamole (Laysan Island), Kaho'olawe, Lalo (French Frigate Shoals), Maui, and Hawai'i.
  
• Congeneric weeds (S. indicus complex) are serious pests in Australia
  
• Tolerates wide range of soils (sands, saline clay, alkaline soils)
  
• Produces viable seed; matures in <1 year
  
• Sticky seeds dispersed by animals, vehicles, machinery, clothing
  
• Seed contaminant in hay and produce
  
• Dispersed by water and external animal attachment
  
• Seeds survive gut passage (e.g., cattle)
  
• Prolific seed production (30,000–150,000 seeds/m²)
  
• Persistent seed bank (viable up to 10 years)
  
• Allelopathic potential (lab evidence)

Low Risk Traits:

• Not a weed in garden, agricultural, or environmental categories
  
• No spines, thorns, burrs, or parasitic habit
  
• Not toxic to animals or humans
  
• Not shade-tolerant, climbing, or smothering
  
• No vegetative spread