H. polysperma is an aquatic, mostly submerged, partly immersed plant that can grow to form dense stands and floating mats which cause many negative environmental and economic impacts. Some of these impacts include displacing native pl...
dwarf hygro; dwarf hygrophila; East Indian hygro; East Indian hygrophila; East Indian swampweed; green hygro; hygro; hygrophila; Indian hygro; Indian hygrophila; Indian waterweed; miramar weed; miramar-weed; oriental ludwiga
H. polysperma is an aquatic, mostly submerged, partly immersed plant that can grow to form dense stands and floating mats which cause many negative environmental and economic impacts. Some of these impacts include displacing native plant species, reducing biodiversity, decreasing water quality and flow, clogging irrigation pumps, impeding recreational activities, and diminishing aesthetic value. H. polysperma is extremely difficult and costly to control, and its ability to form new plants vegetatively facilitates its spread to new locations. The trade and potential escape of H. polysperma through the aquarium and water garden industry plays a large role in its spread to new locations, as does the transportation of this plant on recreational equipment or by wildlife moving between water bodies (DCR, 2003). H. polysperma is declared a noxious weed in the United States (USDA-NRCS, 2006), and is currently well established in Florida and parts of Texas. H. polysperma has also been recorded in Virginia, though current status of this population is unknown (Sutton, 1995). H. polysperma has recently been recorded for the first time in Europe (Hussner et al., 2007), and has the potential to spread to new locations throughout the continent.
The genus Hygrophila (family Acanthaceae) contains approximately 100 species, most of which are terrestrial and occur primarily in the tropics (Ramey, 2001). The genus name comes from the Greek hygro meaning ‘moist, wet’ and phil meaning ‘loving’, referring to the species’ affinity for a wet habitat (Ramey, 2001). Hygrophila polysperma was first named Justicia polysperma Roxb. in 1820, was revised to Hemidelphis polysperma (Roxb.) Nees. in 1832, and further revised to its current accepted scientific name, Hygrophila polysperma T. Anderson in 1867. The English common name ‘Miramar weed’ refers to the town of Miramar, Florida, where during the 1970s a naturalized population that established there first brought public and scientific attention to the expanding problem.
H. polysperma is an herbaceous rhizomatous perennial aquatic plant with squarish stems that are ascending or creeping. The stems are mostly submerged, and are usually rooted in the substrate, though can also root freely at floating nodes. The submerged stem is very brittle, and can grow over 6 feet long. The submerged leaves are opposite along the stem, and are sessile with the bases joined at the nodes by ciliated flanges of tissue. The leaves are elliptic to oblong, light green, sparsely hairy, and usually broader towards the tip. Leaves are up to 8 cm long and up to 2 cm wide (UFL-IFAS, 2005), and the leaves on the submersed stem tend to be considerably larger, wider, and lighter in color than those on immersed stem. The small bluish white flower is nearly hidden by leaves in the uppermost leaf axils, and is 2-lipped, with the upper lip being 2-lobed and the lower lip 3-lobed. The fruit is a narrow hairy capsule up to 9mm long, containing 20-30 seeds, each seed being approximately 0.4-0.62 mm long, 0.3-0.5 mm wide, and 0.002-0.06 mm thick. The seeds are compressed, obovate to elliptic to round, with the entire margin narrowly winged. The seed coating is minutely pebbled, glistening, orange-yellow to brown-yellow, and translucent where the seed is particularly thin (FNW Disseminules, 2007).
H. polysperma is native to Tropical Asia, and has been found in the regions of: India, Malaysia, Bangladesh, Bhutan, Nepal, Cambodia, Laos, Myanmar, Thailand, and Vietnam (USDA-GRIN, 1996). In India, H. polysperma is found in wet areas to an altitude of 1600m (Weeds in Florida, 2006). It is also present in southern China, and is very rare in the lowlands of Taiwan (Flora of Taiwan, 1998).
H. polysperma is currently naturalized in Florida and Texas in the southern United States (USDA-NRCS, 2006), and has also established in northern Mexico (Kasselmann, 1994). H. polysperma has been reported in the past as being established as far north as Virginia, though the current status of this population in unknown (Sutton, 1995). H. polysperma has recently been reported in Europe for the first time, where plants were found in North Rhine-Westphalia, Germany (Hussner et al., 2007).
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
H. polysperma was imported to the United States as ‘oriental ludwigia’ through the aquarium trade from India and Malaysia in 1945, and was first collected in 1965 as an escape from cultivation along a roadside north of Tampa, Florida (Les and Wunderlin, 1981), though it wasn’t correctly identified until 1977. In 1979, reports of populations came from Able Canal, which drained the Caloosahatchee River in western Florida, as well as from Miramar and City of Margate Canals, which are part of the Everglades drainage in eastern Florida (USGS-NAS, 2003). During the 1980s, populations were found at 29 additional sites, including the Loxahatchee River in 1986, and Withlacoochee River in 1989. By 1989, the range of H. polysperma extended northward through central Florida to the Santa Fe River, and also disjunctively spread westward to Tallahassee in the Florida Panhandle (USGS-NAS, 2003). In 1999, H. polysperma was known from at least 22 rivers/streams, 13 lakes, 2 ditches, and 7 canal systems in Florida, distributed over 20 counties and 17 water drainages in the state. H. polysperma is replacing the extremely aggressive non-native Hydrilla verticillata in some southeast Florida canals, due to the tolerance of H. polysperma towards herbicides and grass carp that are usually used to control hydrilla in these locations (Duke et al., 2000).
In Texas, specimens were first collected in the San Marcos River in 1969, and additional locations in the San Marcos drainage, including Sessoms Creek, were recorded in the 1970s (USGS-NAS, 2003). The herbarium specimens were incorrectly identified as Hygrophila lacustris (Schlecht. and Cham.) Nees or Ludwigia repens Forst., and weren’t correctly identified as Hygrophila polysperma until 25 years later (Angerstein and Lemke, 1994). In 1994, H. polysperma was recorded in spring fed portions of the Comal River system, and in 1998 was recorded at San Felipe Springs in western Texas (USGS-NAS, 2003).
H. polysperma was reported as being introduced in the Richmond, Virginia area during the 1950s, and “quickly established itself” for 15-20 years, until extremely cold winter temperatures occurred during the 1970s. The current status of this population is unknown (Sutton, 1995).
H. polysperma was first recorded in Europe very recently, and a population was located in the Kasterer Muhlenerft in Germany (Hussner et al., 2007).
H. polysperma is continuing to expand its range and become more abundant. H. polysperma is a very popular aquarium and water garden plant, and the ability to order this plant over the internet and through mail order gives it the ability to travel to all parts of the world (Kay and Hoyle, 2001; Ramey, 2001). It has escaped confinement and has been intentionally or accidentally introduced on several occasions outside of its native range. In the locales to which it has been introduced, it has often become the dominant plant species, outcompeting both native and previously established exotic species in addition to displacing other species which depend on the ecosystem. H. polysperma is a highly competitive plant which is capable of rapid growth and spread. In one case, H. polysperma grew from 0.1 acre to over 10 acres in one year (Vandiver, 1980). H. polysperma has been shown to be less susceptible to herbicides and grass carp grazing than the extremely invasive Hydrilla verticillata, and in parts of Florida H. polysperma has replaced Hydrilla as the major aquatic nuisance weed (FNW Disseminules, 2007).
H. polysperma can grow submersed in water up to 3m (10 ft) deep and as an immersed plant along banks, preferring flowing waters, but also found growing in slow-moving systems such as lakes, marshes, canals, rivers, swamps, wetlands, and irrigation ditches (FNW Disseminules, 2007). Rarely, a terrestrial growth form can grow in moist soil (Ramey, 2001). The leaves of H. polysperma are uniquely adapted to draw carbon dioxide directly from either the water or the atmosphere (Bowes, 1987), allowing the plant to inhabit a wide range of amphibious conditions. It prefers warmer climates and tends to grow much more vigorously in flowing water, producing approximately 5 times more biomass than that observed in static water (Van Dijk et al., 1986). Ambient temperature, nutrients in the sediments, and day length are the major factors that influence the growth of H. polysperma (Cuda and Sutton, 2000).
H. polysperma has a chromosome number of n=16 (Löve, 1980).
H. polysperma has the ability to prolifically reproduce vegetatively through brittle stem fragments and even detached leaves which are capable of rooting and developing into new plants (Kasselmann, 1994; Sutton, 1995). H. polysperma also can reproduce sexually, though production of viable seeds does not appear to have been reported in North America (Doyle et al., 2003), and the extent of the role that seeds play in population expansion is uncertain (Sutton, 1995).
Physiology and Phenology
H. polysperma grows year round in south Florida (Sutton, 1996). Plant growth begins in the spring (March-May), and submersed shoots reach the surface in late spring. The growth rate of H. polysperma increases in relation to water temperature and daylight, and maximum biomass occurs in the summer and early fall (June-October). Throughout the summer, fragments with numerous adventitious roots break away from the mats, and during extremely hot weather in late summer (August), the whole shoot will break off near the root crown, creating large floating mats of vegetation. Flowers form in the autumn (late October) and continue throughout the winter (October-March) with a high percentage of seed set in Florida populations (UFL-IFAS, 2005). There is significantly less biomass present in winter, though enough present so that H. polysperma is able to occupy its niche year round in the systems that it inhabits.
It is possible that chemical treatments for the control of non-native Hydrilla verticillata may leave an open niche for H. polysperma invasion (Spencer and Bowes, 1985).
The optimum temperature for H. polysperma is 22-28°C (71-82°F), with a minimum temperature of 4°C (39°F), and maximum temperature of 30°C (86°F) (Kasselmann, 1995; Ramey 2001). H. polysperma is most commonly found in waters with pH between 5-7 (Spencer and Bowes, 1985; Doyle et al., 2003), while sources dealing with aquaria specimens find that H. polysperma can tolerate a pH range of 6.5-7.8 and water hardness conditions of 30-140 ppm (FNZAS, 1988). H. polysperma prefers a light intensity of 110 micro-einsteins/m2/h (Cobb and Haller, 1981).
Surveys of the natural enemies of H. polysperma are needed because no information is currently available (Buckingham, 1994; Pemberton, 1996). It is possible that H. polysperma could be controlled by the larva of an agromyzid fly Melanagromyza sp., which bores into the stems of H. auriculata (Schumach.) Heine (Lucknow), visibly damaging the plant (Sankaran and Rao, 1972; Sankaran, 1990).
Hydrochory, the dispersal of disseminules by water currents, seems to be the main dispersal mode of vegetative fragments within a watershed.
Vector Transmission (Biotic)
H. polysperma can be transported with wildlife and carried to new locations (DCR, 2003).
H. polysperma can be spread accidentally to new locations by the movement of boats, trailers, nets, sea planes, and other recreational equipment between water bodies (DCR, 2003). It is also possible for H. polysperma to be a ‘hitchhiker’ plant with other species ordered through water garden catalogues. H. polysperma has been introduced through hobbyists emptying unwanted aquarium species directly into surrounding waterways, and can alsobe accidentally introduced by ornamental ponds flooding into surrounding natural waterways.
The trade of this plant as an aquarium plant through the Internet and mail order has greatly increased its availability and ease of spread into new environments. In a study examining the top 100 Internet websites associated with H. polysperma, 30 of them were commercial in nature, which was three times higher than that seen for any of the other eleven highly invasive species examined. Twenty-four of the 100 websites were associated with hobbyists dealing with the sale and swapping of aquatic plants, which was also approximately three times higher than that seen for the other species (Kay and Hoyle, 2001).
H. polysperma has limited water flow in irrigations channels and flood-control systems (UFL-IFAS, 2005). H. polysperma is also reported as being a threat to rice fields (Krombholz, 1996). In addition, the loss of recreational and aesthetic value associated with H. polysperma can also cause a decline in waterfront property values, as well as possible declines in tourism related revenue for the community (DCR, 2003).
Herbicides typically used in controlling H. polysperma are estimated at costing between US$988 to US$1482 per hectare (US $400 to US $600 per acre), and total costs are even higher when labour and equipment are included (Cuda and Sutton, 2000). In an extreme case involving the use of fluridone in flowing water, control was achieved for a period of 20 months at a cost of US $34,580 per hectare (Sutton, 1996).
The dense stands and mats of vegetation that are characteristic of this species when introduced outside of its native range can decrease the oxygen levels by limiting water circulation and increased decomposition of dead plants. Increased sediment levels are observed with increasing H. polysperma abundance (DCR, 2003). Dense mats of H. polysperma also have the ability to change water hydrology and quality, negatively affecting the ecosystem in which it occurs. Due to the relatively low seasonality of H. polysperma, it is able to maintain shoot biomass and occupy its niche throughout the entire year (ISSG, 2005).
Impact on Biodiversity
H. polysperma reduces biodiversity by competing with and displacing native vegetation, and is capable of changing the fauna and flora of an ecosystem. H. polysperma can form dense monocultures which exclude all native plants and do not provide habitat or food for wildlife. H. polysperma is an excellent competitor due to its low light compensation and saturation points, which allow it to start growing in low light conditions before other native plants do. H. polysperma is also able to rapidly change resource acquisition in response to changing environmental conditions, allowing it to outcompete many other species (Spencer and Bowes, 1985). Decomposing mats of H. polysperma also have the ability to cause fish kills by creating low oxygen levels in the water (DCR, 2003).
H. polysperma can form dense mats that impede recreational activities such as boating, fishing, swimming, water skiing, canoeing, and kayaking. In addition, unsightly mats of vegetation decrease aesthetic values. These declines in recreational and aesthetic values can decrease tourism, which can be a major source of livelihood within the community. Surface mats may also provide habitat for mosquitoes to breed, which could potentially transmit diseases that could have public health implications (Cuda and Sutton, 2000).
Ornamental plants of H. polysperma are sold for aquariums and water ponds (USDA-GRIN, 1996), though the specific economic value of this particular species in the ornamental plant trade is undocumented.
The seeds of H. polysperma are said to be used as a medication in India (Ramey, 2001). Hygrophila has also been utilized in studies of apical dominance and in grafting experiments (Spencer and Bowes, 1985).
In severely disturbed ecosystems where exotics are the only plants capable of surviving, removal of plants such as H. polysperma can further degrade the habitat.
Infestations of aquatic invasive species are often first reported at boat launches, and these areas should be monitored frequently in order to eradicate or control new invasions at an early stage. Users should inspect all recreational equipment before leaving any water body, and any visible plants, animals, or sediment should be removed. In addition, rinsing gear with hot water or steam may help in removing any additional non-visible organisms.
H. polysperma may be confused with other small, opposite-leaved plants that are sometimes found submersed. Ludwigia repens has a 4-petaled yellow flower, blunt leaf tips, often has a purple pigment in the submersed leaves, and lacks flanges at the nodes (DCR, 2003). Hygrophila costata is entirely emersed or terrestrial, larger and taller, with flowers along the entire stem. Hygrophila lacustris (Schlecht. & Cham.) Nees is larger and more erect in habitat, with larger flowers in axillary clusters along the upper stems (UFL-IFAS, 2005). H. polysperma is also similar to Alternanthera philoxeroides (Mart.) Griseb., though the large white papery flowers distinguish the species from the subtle blue flowers of H. polysperma (Ramey, 2001). Diodia spp. has flat-bristled flanges (UFL-IFAS, 2005).
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
As with all weed management, prevention is better and more cost-effective than control.
Early detection and eradication are essential in the prevention of future invasions and spread of H. polysperma. Smaller, localized populations have better success at being controlled than those which have the opportunity to spread and become well-established.
Several publications have been produced in areas with H. polysperma populations regarding the impacts of invasive species such as H. polysperma, and the steps that lake recreationists need to take in order to prevent introducing and spreading aquatic invasives.
Cultural Control and Sanitary Measures
In several regions where aquatic invasives have established, governmental organizations have started requiring that recreationists drain all water and clean off all gear (boats, trailers, fishing equipment, etc.) used on water bodies in order to minimize the chance of spreading aquatic invasive species, such as H. polysperma, to other areas.
Control of H. polysperma has had limited efficacy due to its ability to propagate vegetatively through fragments. Attempts to mechanically harvest only serve as means of creating and introducing more plant fragments, and potentially aiding in dispersal to new locations (Ramey, 2001).
The sensitivity of H. polysperma to water level fluctuation requires investigation as a possible control methodology. A biomass decline was observed after severe spring flooding of the Suwannee River in north Florida, and H. polysperma populations did not recover to pre-flood conditions (Spencer and Bowes, 1985).
Several countries have banned the importation or sale of exotic plants, such as H. polysperma in attempts to minimize the chance of introduction to non-native regions.
Triploid grass carp, Ctenopharyngodon idella, will feed to a limited extent on submersed H. polysperma in the absence of preferred food plants, though very high stocking rates of large fish is necessary (Sutton, 1995). However, introduction of grass carp negatively impacts the coexisting native submerged vegetation, and introduction is even prohibited in some countries.
H. polysperma is very difficult to control with herbicides currently used in the control of hydrilla (e.g. fluridone), and is resistant to many other herbicides registered for aquatic use (Sutton, 1996). Temporary control of both the submersed and immersed forms of H. polysperma has been achieved with endothall, but regrowth occurs 4 to 8 weeks after treatment during peak biomass production, and multiple applications are required to keep populations under maintenance control (Sutton, 1995).