Limnobium laevigatum
(South American spongeplant)

L. laevigatum is a floating aquatic plant native to fresh water habitats of tropical and subtropical Central and South America. It has been introduced into California as an ornamental pond plant and as an aquarium plant where it spread dramatically. There are records of this species in Australia, Indonesia, Japan, and Zambia and Zimbabwe. This species has a high reproductive potential as it can reproduce both sexually by seed and vegetatively through offshoots. It also has a high propensity for dispersal. Seeds and fragments of stolons are naturally dispersed by water and wind and can also be accidentally transported by birds and as hitchhikers on watercrafts. It is possible for L. laevigatum to form dense mats on watercourses thereby decreasing biodiversity. The impact of this species is however not fully known but the dense mats are believed to disrupt light regimes and oxygen levels in the water completely altering the habitat structure of a waterbody. Although now regulated and subject to eradication in California, L. laevigatum is still grown and sold by the aquatic nursery plant trade. This species has the potential to spread into a number of states in the USA. A weed risk assessment by USDA-APHIS (2013) for L. laevigatum rated this species as ‘high risk and a ‘major invader’.

L. laevigatum originates from fresh water habitats of tropical and subtropical regions in Mexico, Central and South America and the Caribbean. It is native to Mexico, Brazil, Antigua, Cuba, Dominican Republic, Guadeloupe, Martinique, Montserrat, Puerto Rico, St. Lucia, Trinidad, Costa Rica, El Salvador, Guatemala, Nicaragua, Panama, French Guiana, Guyana, Suriname, Venezuela, Argentina, Chile, Paraguay, Uruguay, Colombia, Ecuador and Peru (Bove, 2016; USDA-ARS, 2016). There are however contradictory reports on whether it’s native to Chile or not. According to the USDA-ARS (2016) it is native to Chile however Hyde et al., (2016) state that L. laevigatum is not present in Chile.

The species has been introduced to California, USA and has escaped into greater waterways including areas surrounding Redding and Arcata, the Sacramento River delta and the San Joaquin River and ponds and irrigation canals. L. laevigatum is now present in 12 counties in California (Calflora, 2016; Encyclopedia of Life, 2016).

There are some reports on occurrences outside of the Americas. The Council of Heads of Australasian Herbaria (2016) reports that populations of L. laevigatum have naturalized in the states of New South Wales and Queensland, Australia. Hyde et al., (2016) report that L. laevigatum is rare at a national level in Zimbabwe, but abundant in some dams near Harare. There is a report of this species in Zambia (Bingham et al., 2016) and Howard et al. (2016) report more localities in both Zimbabwe and Zambia. Kadono (2004) reports on a few occurrences of L. laevigatum in Japan. 

L. laevigatum is a popular aquatic plant that is readily available for sale from nurseries and via the internet. Once introduced into a new area it is likely to spread rapidly due to its high reproductive potential and high propensity for dispersal (Anderson and Akers, 2011).

L. laevigatum is listed as a state Noxious Weed (List Q) by the California Department of Food and Agriculture (CDFA) and is subject to eradication (Calflora, 2016; DiTomaso, 2010). A weed risk assessment by USDA-APHIS (2013) for L. laevigatum rated this species as ‘high risk’ and a ‘major invader’. Although L. laevigatum is now regulated and subject to eradication in California (USDA-APHIS, 2013), it’s still grown and sold by the aquatic nursery plant trade (Hrusa et al., 2002) so new introductions are likely. It has been estimated that about 29% of the USA would be suitable for L. laevigatum to establish (USDA-APHIS, 2013). This includes states in the western coast of the USA (Washington, Oregon and California) and the southern states (Texas, Arkansas, Louisiana, Mississippi, Alabama, Georgia, South and North Carolina, Florida, Hawaii and parts of Arizona and New Mexico).

The Hawkesbury River County Counsel (2014) have also declared this species to be a Noxious Weed in an area close to Sydney, Australia and therefore requires its removal.

L. laevigatum originates from fresh water habitats of tropical and subtropical Central and South America (Encyclopedia of Life, 2016). It occurs in rivers, ponds, lakes, canals and other aquatic habitats (Cook and Urmi-König, 1983). In Puerto Rico it is reported to grow in shallow ponds, sluggish shaded rivers, fresh-water ditches and swamps at or near sea level (USDA-APHIS, 2013).

In Brazil L. laevigatum occurs in a wide range of habitats from the Amazon Rainforest, Caatinga (xerophilous thorny forest and scrub of the drylands of north-eastern Brazil), Central Brazilian Savanna, Atlantic Rainforest, Pampa (grasslands from Southern Brazil) and Pantanal (periodically flooded grasslands by the rivers Paraná and Paraguay in Central-Western Brazil) (Bove, 2016).

Reproductive Biology

L. laevigatum can reproduce sexually through flower pollination and seed production (Encyclopedia of Life, 2016). The species is monoecious (there are separate male and female flowers on the same plant) (Acevedo-Rodriguez and Strong, 2005) and plants are autogamous (pollen from male flowers pollinate female flowers on the same plant). After pollination the pedicel of female flowers bends downward, forcing the fruit to develop in the water or in the mud (Cook and Urmi-König, 1983). The seeds can germinate underwater (Cook and Urmi-König, 1983). The survival time of seeds is not known. But at a pond in Redding, California seedlings of this species were appearing despite populations being almost completely suppressed for three years. This suggests that there may be a seed bank (Akers, 2010). It can also reproduce vegetatively through fragmentation of stolon segments which connect rosettes (Cook and Urmi-König, 1983).

Population Size and Structure

L. laevigatum reproduces sexually by seed and vegetatively through offshoots and as such this species can rapidly form dense floating or rooted mats on the water surface (USDA-APHIS, 2013). In one place in California 2,500 plants m^-2 were recorded (Akers, 2010).

Associations

L. laevigatum often forms sub-communities with Eichhornia azurea (Murphy et al., 2003) and often grows and floats with Eichhornia crassipes (Howard et al., 2016).

Environmental Requirements

L. laevigatum originates from fresh water habitats in the tropics and subtropics. The USDA-APHIS (2013) reports on a minimum annual precipitation of 250 mm. But as an aquatic plant, precipitation is probably not a direct limiting factor. Cold temperatures are more likely to limit its distribution. It grows in full sun, but in Puerto Rico it is also reported to grow in shady streams (Acevedo-Rodriguez and Strong, 2005; USDA-APHIS, 2013). The preferred water temperature is between 15-28°C and the preferred water’s acidity is between 6-8 pH. Although it originates from fresh water habitats, it also grows in slightly saline waters and even outcompetes other species in this environment (Perryman, 2013). In South America this species has been recorded at altitudes of more than 2,000 m (Acevedo-Rodriguez and Strong, 2005).

General

• Pet/aquarium trade

With respect to its habit and morphology, L. laevigatum is very similar to Eichhornia crassipes (water hyacinth), which is widely recognised as a significant invader (Pieterse and Murphy, 1990).

L. laevigatum is also similar to the closely related species L. spongia. It can be distinguished from L. spongia by its flowers and leaf characteristics. In addition to this their distributions are different as L. spongia is not known to occur in Western States unlike L. laevigatum (Encyclopedia of Life, 2016).

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.

SPS Measures

L. laevigatum islisted as a noxious weed in California and has been classified as having high invasiveness by the California Invasive Plant Council (Cal-IPC) inventory (Cal-IPC, 2015). In New South Wales, Australia, this species is listed as a Prohibited Weed (NSW Department of Primary Industries, 2016). This means that it must be eradicated from land.

Mechanical Control

For small infestations hand removal of L. laevigatum is possible. The use of choppers and shredders however often result in the small seedlings breaking off and spreading into new areas (DiTomaso et al., 2013).

Chemical Control

A number of aquatic herbicides have been recommended for control of this species. These include diquat, glyphosate, Imazapyr and 2,4-D (DiTomaso et al., 2013; Encyclopedia of Life, 2016).

There is currently very little information on the growth rates, nutrient requirements and cold tolerance of L. laevigatum which would help to provide information on the likelihood of this species becoming invasive in other countries. In addition to this, there have been a limited number of studies on the direct impact that this species has on the environment.

06/02/2016 Original text by:

Ymkje van de Witte, Consultant, Wageningen, The Netherlands