Potential pathway(s) of introduction: Unauthorized release, hitchhiking/fouling Hygrophila polysperma is listed as a Federal Noxious Weed and is prohibited in Illinois and Minnesota (Great Lakes Panel on Aquatic Nonindigenous Species 2012).
Hygrophila polysperma has a moderate probability of introduction to the Great Lakes (Confidence level: High).
Established in North America, but not including the Great Lakes. The availability of and accessibility to H. polysperma in the aquarium industry may increase its potential to be introduced to the Great Lakes; after purchase and usage for aquariums, owners may dispose of the plant into waters connected to the Great Lakes basin.
Hygrophila polysperma has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).
This perennial aquatic plant inhabits freshwater lakes and streams. The literature predicts that H. polysperma has the potential to overwinter in the Great Lakes (Rixon et al. 2005). This species can tolerate temperatures of 4°C (Kasselmann 1995) to 30°C (GISD 2005). It inhabits waters at depths of 1.5-2.0 m and can photosynthesize in lower light levels than most native aquatic plant species (Spencer and Bowes 1984). Hygrophila polysperma is able to draw carbon dioxide from both water and atmosphere (Doyle et al. 2003). This species grows best at pH 5-7 (Spencer and Bowes 1985).
Hygrophila polysperma does not currently occur near waters connected to the Great Lakes basin. Its nonindigenous occurrences in the U.S. have somewhat similar climates and abiotic conditions as the Great Lakes. Suitable habitats for H. polysperma are likely somewhat available in the Great Lakes. The effects of climate change on the Great Lakes, such as warmer water temperatures and shorter duration of ice cover, may improve habitat suitability for this species.
Although it can produce seeds, Hygrophila polysperma primarily propagates vegetatively, and forms many adventitious roots at nodes along the stems, which aids the rooting of dispersed fragments (Spencer and Bowes 1985). Hygrophila polysperma has a high regrowth potential from stem fragments (Spencer and Bowes 1985). Growth rate of H. polysperma is enhanced by higher flow rates (Van Dijk et al. 1986). It can spread rapidly to form dense monoculture stands; it expanded from 0.04 ha to over 0.41 ha in one year (Vandiver 1980). Hygrophila polysperma has spread extensively in the southeastern U.S. and parts of Mexico.
Hygrophila polysperma may compete with native species. When H. polysperma was experimentally grown with Ludwigia repens, L. repens experienced slower growth rate, produced fewer and shorter stems, and produced fewer branches per stem than when grown without the presence of H. polysperma (Doyle et al. 2003). In addition, H. polysperma exhibits competitive ability when grown with L. repens; H. polysperma produces fewer but longer highly branched stems. This species forms dense monocultures that exclude native plants and is a superior competitor because of its low requirements for light and rapid growth (GISD 2005, Nault and Mikulyuk 2009, Spencer and Bowes 1985, Robinson 2003).
Hygrophila polysperma has the potential for moderate environmental impact if introduced to the Great Lakes.
Hygrophila polysperma can have negative impacts on native species and the ecosystem. This species grows quickly into dense mats that can reduce light availability and dissolved oxygen levels. It shades out native submerged plants (Ramey 2001) and can displace native plants when it occupies the entire water column. Surveys conducted shown that H. polysperma can spread rapidly to become one of the most abundant species where it has been introduced, displacing native species (Owens et al. 2001, Vandiver 1980). In Texas, it rapidly spread to occupy 20% of the Comal River’s area, where it is thought to displace native macrophytes (Doyle et al. 2003). When H. polysperma was grown with Ludwigia repens, L. repens exhibited slower growth rates compared to when grown alone, suggesting that H. polysperma has superior competitive abilities (Doyle et al. 2003). Hygrophila polysperma can create anoxic conditions once decomposition occurs (Owens et al. 2001, Robinson 2003). The dense mats of H. polysperma can trap sediments and reduce water flows (Robinson 2003).
Hygrophila polysperma has the potential for high socio-economic impact if introduced to the Great Lakes.
Dense mats of Hygrophila polysperma can provide breeding grounds for mosquito populations. The mosquito, Coquillettidia perturbans, reportedly attaches to submerged roots of H. polysperma to complete development and is a vector of eastern and western equine encephalomyelitis (Cuda and Sutton 2000). This species is problematic, as it clogs irrigation and flood control canals and interferes with water control pumping stations (Cuda and Sutton 2000). It is costly to control H. polysperma infestations; in 2006, Florida spent $14,000 to control H. polysperma covering 206 acres (FL DEP 2007). Infestations of H. polysperma make navigation difficult and inhibit recreational use (Cuda and Sutton 2000, Robinson 2003). The presence of widespread, dense mats of H. polysperma can hinder fishing, boating, and swimming activities, causing a reduction lake property value (Robinson 2003).
Hygrophila polysperma has the potential for moderate beneficial impact if introduced to the Great Lakes.
In India, H. polysperma seeds are used as a medicine (Spencer and Bowes 1985). Hygrophila polysperma is commercially valuable as an ornamental plant and aquarium species (Cuda and Sutton 2000). Hygrophila polysperma is advertised for beginner aquarists because it is hardy and easy to grow. This species may increase water clarity when abundant (Osceola County 2012).