Hygro, East Indian hygrophila, Indian swampweed, Miramar weed, East Indian swampweed, Dwarf hygrophila, Justicia polysperma (Roxb.), Hemidelphis polysperma (Roxb.) Nees in Wall.

Leaves: It has elliptical leaves that are up to 8 cm long and 2 cm wide, and taper to a sharp point (Langeland and Burks 1999). Attachment of leaves is sessile, with the bases joined at the nodes by ciliated flanges of tissue. The leaf arrangement is opposite.

Flowers: Flowers are small and solitary in the uppermost leaf axils, and are nearly hidden by leaves. The calyx is 5-lobed, the corolla is bluish-white and 2-lipped, and there are 2 fertile stamens.

Fruit/Seeds: The fruit is a narrow capsule, which splits lengthwise to release tiny round seeds.

Look-a-likes: Ludwigia repens creeping primrose-willow, Alternanthera philoxeroides alligatorweed, and Hygrophila costata lake hygro. The key difference is that H. polysperma is mainly submersed.

Nonindigenous Occurrences: It occurs in Australia (Romanowski 2011), Kasterer Muhlenerft in Germany (Hussner et al. 2007), and Tamaulipas in Mexico (Mora-Olivo et al. 2008). In the US, H. polysperma occurs in Florida (USDA NRCS 2014), South Carolina (SC DNR 2010), Texas (Angerstein and Lemke 1994), and Virginia (USDA NRCS 2014).  Hygrophila polysperma has been documented near the Catskills region, New York (Harman 2011).

In North America, H. polysperma has a specific life cycle, starting with a rooted stage in hydro-soil in dense stands of shoots, some with large leaves reaching up to the canopy, and some emergent ones with smaller leaves. Shoots on moist banks are very small, and resemble the submerged form after banks are flooded. Shoots begin elongating in March as the water temperature rises, then they reach the surface in late spring. In summer, they break off into mats and float away, and take root as soon as they come into contact with soil. The whole shoot of the plant breaks off near the root crown in August and forms very dense floating mats, which can sink piece by piece, or all at once to form a new colony; new shoots regrow from the roots, and they grow slowly in winter (Hall et al. 2003).

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), but can be ordered on the internet (Kay and Hoyle 2001). Surveys conducted by Rixon et al. (2005) found that H. polysperma was available for purchase in 25% of the pet and aquarium stores surveyed near Lakes Erie and Ontario. Maki and Galatowitsch (2004) had evidence that H. polysperma is available for purchase from vendors across the U.S. with delivery service to Minnesota. In addition, it is sometimes sold under the incorrect name of Alternanthera sessilis (Maki and Galatowitsch 2003). 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 was cultivated in Ohio at the end of WWII by an aquarium dealer (Reams 1953), but there is no indication that this species is still cultivated there.    

Hygrophila polysperma spreads via vegetative fragmentation. Hygrophila polysperma has a high regrowth potential from stem fragments (Spencer and Bowes 1985). It can get entangled and be transported to the Great Lakes on boats and recreational gear. Hygrophila polysperma fragments can also be dispersed by water; however, because this species does not currently occur near waters connected to the Great Lakes basin, it is unlikely that fragments will be transported by water to the Great Lakes.

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. Rixon et al. (2005) predict that H. polysperma has the potential to overwinter in the Great Lakes. 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). 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). A recent study found that non-native populations in the U.S. and Mexico were very genetically similar, suggesting that Hygro was propagated clonally in its invasive range (Mukherjee et al. 2016). 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 outcompete 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 produces fewer but longer highly branched stems when grown with L. repens. This species forms dense monocultures that exclude native plants and is a superior competitor because of its low requirements for light as well as its rapid growth (GISD 2005, Nault and Mikulyuk 2009, Spencer and Bowes 1985, Robinson 2003).

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

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 displacing 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 present 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).

This species is prohibited in parts of the Great Lakes including Chicago, Illinois, and Minnesota. It is not prohibited in Indiana, Michigan, New York, Ohio, Ontario, Pennsylvania, Quebec, or Wisconsin (Great Lakes Panel on Aquatic Nuisance Species 2012). Hygrophila polysperma is listed as a Federal Noxious Weed, so it is illegal to import, sell, or purchase this species in the United States (APHIS 2012).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological
Triploid grass carp (Ctenopharyngodon idella) is used in combination with mechanical or chemical methods in Florida, but there are no records indicating that it was successful in eliminating H. polysperma (Cuda and Sutton 2000).

Physical
Mechanical harvesters are ineffective against H. polysperma and may encourage its spread by vegetative fragmentation (Cuda and Stutton 2000).

Chemical
Invert herbicide applications of endothal (7-oxabicyclo [2,2,1] heptane-2,3, dicarboxylic acid) plus copper have not shown to be effective in eradicating H. polysperma (Spencer and Bowes 1985).

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

It is now causing problems in areas formerly dominated by Hydrilla (Cuda and Sutton 2000).