Family: Hydrocharitaceae

Origin, Description & Uses
Hydrocharis spongia (commonly known as American spongeplant or frog’s-bit) is native to the central and eastern United States, where it grows in slow-moving freshwater habitats such as ponds, marshes, and quiet streams. This free-floating aquatic plant forms small rosettes of rounded, spongy leaves that rest on the water’s surface, with dangling roots that hang beneath. It reproduces readily through vegetative means, forming new plantlets along stolons that allow it to spread across still water. While it is sometimes appreciated in water gardens or aquatic plant collections for its attractive floating foliage and ability to provide shade and habitat cover, it is not widely used in large-scale landscaping.

Risks & Threats
Although Hydrocharis spongia is currently not known to be naturalized in the Hawaiian Islands, it possesses traits that are cause for concern. Its rapid vegetative reproduction and ability to form dense floating mats could allow it to spread quickly if introduced into suitable freshwater systems. Such growth can block sunlight, reduce oxygen levels in the water, and displace native aquatic plants and habitat for fish and invertebrates. Dense mats may also impede water flow, interfere with drainage and irrigation systems, and create stagnant conditions that can support mosquito breeding. Given these potential impacts, this species is considered high risk for tropical island ecosystems, and choosing low-risk or native Hawaiian aquatic plants is strongly recommended.

High Risk Traits:

• Grows in temperate to subtropical climates
  
• Naturalized outside native range
  
• Weedy smothering growth, with possible negative impacts on agriculture or the natural environment
  
• Limnobium laevigatum also reported to be invasive
  
• Tolerates shade
  
• Forms dense, smothering mats in fresh water bodies
  
• Reproduces by seeds and vegetatively
  
• May be dispersed by external attachment to boats or other water craft
  
• Sold in aquarium trade
  
• Seeds dispersed by waterfowl and possibly other animals

Low Risk Traits:

• Unarmed (no spines, thorns or burrs)
  
• Non-toxic
  
• Ornamental
  
• Several herbicides provide effective control
  
• May be controlled mechanically