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Lemna minuta

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Datasheet

Lemna minuta

Summary

  • Last modified
  • 20 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Lemna minuta
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Monocotyledonae
  • Summary of Invasiveness
  • L. minuta is a small free-floating plant, no more than 3 mm in length. It is widely distributed in southern and western North America and is also found in Central and South America. It occurs in lowland ditches,...

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Pictures

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PictureTitleCaptionCopyright
Lemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
TitleHabit
CaptionLemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
Copyright©Barry Rice/Sarracenia.com/via CalPhotos - CC BY-NC-SA 3.0
Lemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
HabitLemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.©Barry Rice/Sarracenia.com/via CalPhotos - CC BY-NC-SA 3.0
Lemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
TitleHabit
CaptionLemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
Copyright©Barry Rice/Sarracenia.com/via CalPhotos - CC BY-NC-SA 3.0
Lemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.
HabitLemna minuta; habit. Angora Creek, El Dorado County, California, USA. June 2012.©Barry Rice/Sarracenia.com/via CalPhotos - CC BY-NC-SA 3.0

Identity

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Preferred Scientific Name

  • Lemna minuta Kunth, 1816

Other Scientific Names

  • Lemna minima Phil. ex Hegelm., non Thuill. ex Beauv. 1816
  • Lemna minima Phil., non Thuill. ex P. Beauv.
  • Lemna minima Philippi ex Hegelm, 1896
  • Lemna minima Philippi, 1864
  • Lemna minuscula Herter, 1954
  • Lemna valdiviana A. Lawalree non Phil.
  • Lemna valdiviana Phil. var. abbreviate Hegelm.
  • Lemna valdiviana Phil. var. minima (Phil.) Helgelm., 1868

International Common Names

  • English: dinky duckweed; least duckweed; minuscule duckweed; minute duckweed
  • French: lentille d'eau minuscule

Local Common Names

  • Austria: Winzige wasserlinke
  • Germany: Zierliche wasserlinke
  • Netherlands: dwergkroos

Summary of Invasiveness

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L. minuta is a small free-floating plant, no more than 3 mm in length. It is widely distributed in southern and western North America and is also found in Central and South America. It occurs in lowland ditches, ponds, canals, streams and rivers, and more rarely it is found in lakes (Preston and Croft, 1997). It often forms dense mats on the surface of water, reducing the light penetration and gas exchange, often causing the disappearance of submersed aquatic plants. Outbreaks are usually limited in time and space and are favoured by eutrophication. L. minuta is introduced in Eurasia (Landolt 2000) and it was first recorded in western France in 1965. From there it has spread all over Europe as far as southern Russia and Greece. It is also present in Japan (e.g. Landolt, 1986). It is considered a casual alien by Global Compendium of Weeds (2007). In many areas it is a noxious weed, as in Belgium, and it is included in the watch list with moderate impact (Branquart et al., 2007).

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Monocotyledonae
  •                     Order: Arales
  •                         Family: Lemnaceae
  •                             Genus: Lemna
  •                                 Species: Lemna minuta

Notes on Taxonomy and Nomenclature

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The family Lemnaceae has five genera and 38 species according to the last revision by Les et al. (2002). The family is divided into two subfamilies (Landolt, 1986): Wolffioideae (Wolffia and Wolffiella) and Lemnoideae (Lemna and Spirodela). Lemna is divided in 4 sections, with 15 species. Lemna minuta belongs to section Uninerves Helgem. (with just one vein in the frond) along with Lemna valdiviana, and Lemna yungensis. During the past century, the taxonomy of L. minuta Kunth has been complicated by different names used by different authors. Several of the synonyms commonly found in the literature include L. valdivianavar. minima Hegelm., L. minima Phil. ex Hegelm. and L. minuscula Herter, which were neotypified by Reveal (1990), who cleared up some of the confusion and controversy about this widespread species. Thereafter, the accepted name is Lemna minutaHumb., Bonpl. & Kunth in A. von Humboldt et al., Nov. Gen. Sp. 1: 372. 1816. The adjective minuta (Latin) means very small.

Description

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Structurally, the Lemnaceae are the simplest of the flowering plants. The plants are not differentiated into stems and leaves; instead, the plants in the family have an undifferentiated leaf-like body commonly referred to as a frond. Fronds floating, 1 or 2-few, coherent in groups, obovate, flat to thickish (but not gibbous), 0.8-4 mm, 1-2 times as long as wide, margins entire and thin, usually pale green, shining, nearly always with a sharp ridge with white papillae; veins 1, sometimes indistinct, very rarely longer than extension of air spaces, not longer than 2/3 of distance between node and apex; with or without small papillae along midline; anthocyanin absent; largest air spaces much shorter than 0.3 mm; turions absent. Roots to 1.5 cm, tip rounded to pointed, one root per frond; sheath not winged. Stipes (stalks) white, small, often decaying. Flowers within membranous cup-like spathes (open on one side) inside budding pouches located on either side of the basal end. Ovaries 1-ovulate, utricular scale open on 1 side. Fruits 0.6-1 mm, not winged. Seeds with 12-15 distinct ribs (Landolt, 1980; Flora of North America, 2008; Armstrong, 2009).

Distribution

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L. minuta is a native of temperate and subtropical areas of North and South America. From South America extending through to Central America and West Indies extended northward along the high mountains to Mexico and California (Flora of North America, 2008; Armstrong, 2009). Very common throughout the southeastern and southwestern United States (USDA-ARS, 2009) with some scarce distribution in the centre of the country (Flora of North America, 2008). Larson and Searcy (2007) reported its presence in Massachussetts recently (2005). The species has been introduced in western Europe and Japan (Landolt, 1986; Reveal, 1990) and may well be expected to be discovered elsewhere (Reveal, 1990) because it is almost certainly under-recorded, due its similarity with L. minor (Preston and Croft, 1997).

Distribution Table

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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.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

JapanPresentIntroducedMakino, 1926; Landolt, 1986; Crawford et al., 1996
-HonshuPresentIntroducedUSDA-ARS, 2009

North America

MexicoPresentNativeUSDA-ARS, 2009Northern and Central Mexico
USAPresentPresent based on regional distribution.
-AlabamaPresentNativeUSDA-ARS, 2009
-ArizonaPresentNativeUSDA-ARS, 2009
-ArkansasPresentNativeUSDA-ARS, 2009
-CaliforniaPresentNativeUSDA-ARS, 2009
-ColoradoPresentNativeUSDA-ARS, 2009
-FloridaPresentNativeUSDA-ARS, 2009
-GeorgiaPresentNativeUSDA-ARS, 2009
-IllinoisPresentNativeUSDA-ARS, 2009
-IndianaPresentNativeUSDA-ARS, 2009
-KansasPresentNativeUSDA-ARS, 2009
-KentuckyPresentNativeUSDA-ARS, 2009
-LouisianaPresentNativeUSDA-ARS, 2009
-MichiganPresentNativeUSDA-ARS, 2009
-MissouriPresentNativeUSDA-ARS, 2009
-NebraskaPresentNativeUSDA-ARS, 2009
-NevadaPresentNativeUSDA-NRCS, 2009
-New MexicoPresentNativeUSDA-ARS, 2009
-OhioPresentNativeUSDA-ARS, 2009
-OklahomaPresentNativeUSDA-ARS, 2009
-OregonPresentNativeUSDA-ARS, 2009
-TennesseePresentNativeUSDA-ARS, 2009
-TexasPresentNativeUSDA-NRCS, 2009
-UtahPresentNativeUSDA-ARS, 2009
-WashingtonPresentNativeUSDA-ARS, 2009
-West VirginiaPresentNativeUSDA-ARS, 2009
-WyomingPresentNativeUSDA-ARS, 2009

Central America and Caribbean

GuadeloupePresentNativeUSDA-ARS, 2009
GuatemalaPresentNativeUSDA-ARS, 2009
JamaicaPresentNativeUSDA-ARS, 2009

South America

ArgentinaPresentNativeUSDA-ARS, 2009
BoliviaPresentNativeUSDA-ARS, 2009
BrazilPresentPresent based on regional distribution.
-Mato GrossoPresentNativePott and Cervi, 1999
-Mato Grosso do SulPresentNativePott and Cervi, 1999
-Minas GeraisPresentNativeUSDA-ARS, 2009
-Rio Grande do SulPresentNativeUSDA-ARS, 2009
-Santa CatarinaPresentNativeUSDA-ARS, 2009
-Sao PauloPresentNativeUSDA-ARS, 2009
ChilePresentNativeUSDA-ARS, 2009
ColombiaPresentNativeUSDA-ARS, 2009
EcuadorPresentNativeUSDA-ARS, 2009
ParaguayPresentNativeUSDA-ARS, 2009
PeruPresentNativeUSDA-ARS, 2009
UruguayPresentNativeUSDA-ARS, 2009
VenezuelaPresentNativeUSDA-ARS, 2009

Europe

AustriaPresent, few occurrencesIntroducedDAISIE, 2008; NOBANIS, 2009
BelgiumWidespreadIntroduced1983Branquart et al., 2007; DAISIE, 2008; NOBANIS, 2009
FrancePresentIntroducedThiébaut, 2007; DAISIE, 2008; EPPO, 2014
GermanyPresentIntroduced1973Preston and Croft, 1997; NOBANIS, 2009
GreecePresentIntroducedLandolt, 1986; Preston and Croft, 1997
HungaryPresentIntroducedPreston and Croft, 1997; Mesterházy et al., 2009; USDA-ARS, 2009
IrelandPresentIntroducedCotton, 1999; DAISIE, 2008Widespread in west coast of the country
ItalyPresentIntroduced1989Desfayes, 1997; Merluzzi et al., 2003; DAISIE, 2008
NetherlandsPresentIntroduced1988DAISIE, 2008
PolandPresentIntroduced2007Banaszek and Musial, 2009
PortugalPresentIntroducedGalán et al., 2006
RomaniaPresentIntroduced2005DAISIE, 2008
Russian FederationPresentPresent based on regional distribution.
-Southern RussiaPresentIntroducedLandolt, 1986
SlovakiaPresentIntroducedFerakova and Onderikova, 1998; DAISIE, 2008
SpainPresentIntroducedGalán and Castroviejo, 2005; DAISIE, 2008
-Balearic IslandsPresentIntroducedFraga et al., 2003; DAISIE, 2008
SwedenPresentIntroducedRyman and Anderberg, 1999
SwitzerlandPresentIntroducedPreston and Croft, 1997; DAISIE, 2008
UKWidespreadIntroduced Invasive Landolt, 1979; DAISIE, 2008Widespread in south England
-Channel IslandsPresentIntroducedDAISIE, 2008
UkrainePresentIntroducedReveal, 1990; Preston and Croft, 1997; USDA-ARS, 2009

History of Introduction and Spread

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L. minuta was first recorded, as L. valdiviana, in southwestern France in 1965 (Jovet and Jovet-Ast, 1966; Thiébaut, 2007) and subsequently found in Germany in 1966, as well as in Switzerland, Spain, Portugal, Sweden, Ukraine, Ireland and Hungary, spreading as far as southern Russia and Greece (Landolt, 1986). L. minuta is widespread in Belgium; the first observation was in 1983, and it is included in the watch list as having moderate impact (Branquart et al., 2007). It is also widespread in the British Isles, where it was first reported in 1977 (Landolt, 1979). It has spread rapidly since the 1980s (Leslie and Walters, 1983; Bramley et al., 1995) causing weed problems. The rate of spread of L. minuta in Britain in the past two decades has been considerably high, and it is now widespread in south and central Britain (BSBI, 2009). More recently, in 2007, it has been found in southwestern Poland, probably introduced from Germany (Banaszek and Musial, 2009).

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
Poland Germany 2007 Yes Banaszek and Musial (2009)

Risk of Introduction

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L. minuta has been accidentally introduced with ornamental aquatics from America into Europe; from garden ponds and parks it has escaped into watercourses and ponds (DAISIE, 2008). Like other aquatic plants, it has also been introduced accidentally along with shipments of commodities such as fish and aquarium cultures (Armstrong, 2009). It is also likely that L. minuta is spread long distances by water currents (Armstrong, 2009).

Habitat

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L. minuta grows in slow-moving, calm, freshwater ponds and marshes (Armstrong, 2009), and stagnant freshwater habitats such as wetlands like in the Pantanal (Brazil) (Pott and Cervi, 1999). It occurs in temperate to subtropical regions with relatively mild winters (Flora of North America, 2008) and prefers a Mediterranean climate (Landolt, 1986) with high water temperature in summer. L. minuta is found in its introduced areas in sluggishly moving waters of ponds, pools, lakes, swamps, streams, drainage ditches, canals, and sloughs (Preston and Croft, 1997).

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial
 
Terrestrial ‑ Natural / Semi-naturalRiverbanks Secondary/tolerated habitat Harmful (pest or invasive)
Wetlands Principal habitat Natural
Freshwater
 
Irrigation channels Secondary/tolerated habitat Harmful (pest or invasive)
Lakes Secondary/tolerated habitat Harmful (pest or invasive)
Reservoirs Principal habitat Natural
Rivers / streams Principal habitat Harmful (pest or invasive)

Biology and Ecology

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Genetics

Chromosome data for Lemnaceae are not helpful systematically due to the wide range in variation of euploid and aneuploid numbers that occurs even within single species (Les and Philbrick, 1993). Lemna, Spirodela, Wolffia, and Wolffiella all contain species with counts ranging from 2n=20–80. The populations in Europe are tetraploid (2n=40) (Urbanska-Worytkiewicz, 1980).
 
Reproductive Biology
 
There are difficulties in achieving sexual reproduction in some species of Lemna due the simple floral structures which are reduced to 2 stamens (androecium) and one pistil (gynoecium), all enclosed in a membranous sac-like spathe, which appear within budding pouches at the edge of the plant body in genera Spirodela, Landoltia and Lemna (Armstrong, 2009). The plant produces an ephemeral membrane-bound flower although rarely seen (Flora of North America, 2008). Duckweed may occasionally flower and produce seed, but very rarely (Holm et al., 1997). Since the flowers are typically protogynous with the stigma receptive before the anther is mature, the plants must be cross-pollinated during the summer months. Lemnaceae pollen has been detected on flies, aphids, mites, small spiders, and honey bees on the surface of dense duckweed layers (Armstrong, 2009) The seeds are rarely produced by clonal populations of some species and, when they are produced, all Lemnaceae investigated so far tolerate drying for at least a few months to several years (Armstrong, 2009).
 
L. minuta propagate almost exclusively by vegetative reproduction, a process called budding; flowering is rare (Landolt, 1986). This form of growth allows very rapid colonisation of new water and re-colonisation after treatment in previously infested areas.
 
Physiology and Phenology
 
Several species usually overwinter on the bottom of the pool, to surface again the following spring. L. minuta, however, remains green throughout the winter (Preston and Croft, 1997) and there is an assured starter population for each spring, the season when the plant begins again to develop and to multiply vegetatively. Reproduction starts in summer, allowing the colonization of large surfaces. The plant mass can double in size every two or three days in optimum conditions over summer. Flowering (very rarely) from late spring to early autumn in its native area (Flora of North America, 2008). Flowering has not been observed in Britain (Preston and Croft, 1997) but in the south of France a small proportion of the fronds produce flowers and fruits during the winter months (Jovet and Jovet-Ast, 1966).
 
L. minuta shows high plasticity, which is related to environmental factors affecting the size and shape of the fronds. In full sunlight growing in high abundance, the fronds reach no more than 1-2 mm with solitary fronds or clusters of two; in shady conditions, the fronds can reach 4 mm long and can be clustered in four segments (Landolt, 1980; Armstrong, 2009). It should be noted that in running water in temperate regions, floating plants, such as L. minuta and L. minor, while able to maintain themselves in low-flow conditions, are usually washed away when the flows increase and are therefore dependent upon continual recolonization from sources upstream (Hynes, 1970). Although vegetative plant bodies are unable to withstand desiccation for more than a few hours, they may survive days (or weeks) embedded in wet mud and debris, for more information to see Landolt (1997).
 
Associations
 
Lemna often grows with the other duckweeds (e.g., Spirodela, Landoltia, Wolffia, Wolffiella) and occupy a similar niche to Azolla filiculoides (Armstrong, 2009). In the Amazon region it has been observed with Lemna gibba, L. valdiviana, Wolffia brasiliensis, and Spirodela intermedia (Landolt, 2000). L. minuta in the UK is usually associated with L. minor (Lucey, 2003), L. gibba and sometimes with Azolla filiculoides, Spidorela polyrhiza and Riccia fluitans and can be much more abundant than any competitor, forming dense stands completely covering the water surface (Preston and Croft, 1997).
 
Environmental Requirements
 
In general, the Lemnaceae grow very fast in eutrophic water - lakes, reservoirs, ponds and ditches from mesotrophic to eutrophic, from sea level to 2200 m (Flora of North America, 2008), often in shady conditions - and are strongly resistant to pollution. In the native range, when the water is low in nutrients, in general the Lemnaceae are rare (Landolt, 2000), but when the conditions tend more towards eutrophication, they become more abundant, as in Europe. For example, in Ireland, where L. minuta was found in the Womanagh River, the pH of the water at the site was 7.75 and conductivity, 543 µS/cm indicating relatively hard-water conditions and high phosphorus levels of 0.05-0.77 mg/L P (Lucey, 2003).

Climate

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ClimateStatusDescriptionRemark
Af - Tropical rainforest climate Preferred > 60mm precipitation per month
Am - Tropical monsoon climate Preferred Tropical monsoon climate ( < 60mm precipitation driest month but > (100 - [total annual precipitation(mm}/25]))
Aw - Tropical wet and dry savanna climate Preferred < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
BS - Steppe climate Preferred > 430mm and < 860mm annual precipitation
Cf - Warm temperate climate, wet all year Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year
Cs - Warm temperate climate with dry summer Tolerated Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
60 40

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Lemnaphila scotlandae Whole plant to genus Buckingham, 1989
Tanysphyrus lemnae Herbivore Leaves to genus Gassmann et al., 2006

Notes on Natural Enemies

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With the exception of the shore-fly, Lemnaphila scotlandae, which has been reported as attacking Lemna sp. in Florida (Buckingham, 1989), little is known about the natural enemies of L. minuta in its natural range in North America. The weevil, Tanysphyrus lemnae, which is already present in the British Isles, creates transparent leaves similar to those caused by L. scotlandae (Gassmann et al., 2006).

Means of Movement and Dispersal

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Natural Dispersal (Non-Biotic)

L. minuta is likely to be spread by surface drift (Landolt, 1986) and can easily be dispersed over short distances (Landolt, 2000) and long distances by water currents, sometimes by the wind (Armstrong, 2009).
 
Vector Transmission (Biotic)
 
It is likely that L. minuta will be spread by birds and mammals (DAISIE, 2008).
 
Accidental Introduction
 
It has been accidentally introduced with ornamental aquatics from America into garden ponds and has escaped into watercourses and ponds (DAISIE, 2008; Flora of Northern Ireland, 2009); also L. minuta has been introduced with other aquatic plants in shipments of commodities such as fish and aquarium cultures (Armstrong, 2009).

Impact Summary

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CategoryImpact
Cultural/amenity Negative
Economic/livelihood Positive and negative
Environment (generally) Negative

Economic Impact

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The plants can clog the intakes of potable water supplies and irrigation pumps. Slow-moving canals are particularly susceptible to invasion (Briggs, 1992); it chokes waterways in the UK.

Environmental Impact

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Impact on Habitats

It affects the ecology of its habitat by forming mats on the water surface, reducing sunlight penetration and oxygen exchange. In Poland, it has been found in a nature reserve (Banaszek and Musial, 2009). Mats of free-floating plants such as L. minuta are well known to cause physico-chemical changes in the water beneath them (e.g. Pokorny and Rejmankova, 1983).
 
Impact on Biodiversity
 
A blanketing growth over a large area leads to deoxygenation and fish-kills in hot weather, as well as a decrease in invertebrate diversity (Bramley et al., 1995). These thick, floating mats of vegetation could reduce submerged plant diversity by selecting a few tolerant species (Janes et al., 1996).

Social Impact

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Carpets (mats) can occasionally impede activities (navigation, bathing, fishing) and are sometimes perceived by the local residents as a symbol of contamination of waters

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Highly adaptable to different environments
  • Is a habitat generalist
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Pioneering in disturbed areas
  • Tolerant of shade
  • Highly mobile locally
  • Benefits from human association (i.e. it is a human commensal)
  • Fast growing
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
  • Reproduces asexually
Impact outcomes
  • Altered trophic level
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Increases vulnerability to invasions
  • Infrastructure damage
  • Modification of hydrology
  • Modification of natural benthic communities
  • Modification of nutrient regime
  • Modification of successional patterns
  • Monoculture formation
  • Negatively impacts tourism
  • Reduced amenity values
  • Reduced native biodiversity
  • Soil accretion
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - shading
  • Competition
  • Filtration
  • Interaction with other invasive species
  • Rapid growth
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field

Uses

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Economic Value

Members of the Lemnaceae have considerable economic importance as a source of green fertilizer (green manure), which can be added to fields in many under-developed and developing nations, where great masses are grown in sewage ponds. They can also be fed to livestock, rabbits, poultry and have significant aquaculture applications (Skillicorn et al., 1993). The utilization of duckweeds as food for animals is summarized by Landolt and Kandeler (1987).
 
Social Benefit
 
Their small size and rapid, predominantly vegetative reproduction forming genetically uniform clones make them valuable research organisms for studies in plant physiology, genetics, ecology and environmental monitoring. Due to their high sensitivity to organic and inorganic substances Lemna species are also used as test organisms for water quality assessments as well as for ecotoxicological studies regarding adverse effects of herbicides, pharmaceuticals and heavy metals on aquatic plants (e.g. Hillman, 1961; Kandeler, 1985; Landolt and Kandeler, 1987).
 
Environmental Services
 
Like the other genera in the Lemnaceae, Lemna is a valuable food for waterfowl, as evidenced by their common name, ‘‘duckweeds’’ (Tarver et al., 1986), they also provide food and habitat for fish (Sculthorpe, 1967).
 

Similarities to Other Species/Conditions

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Since flowers and fruits are rarely observed, most taxonomic keys to the Lemnaceae are based on relatively few diagnostic vegetative characteristics that may vary under different environmental conditions. This often makes precise identification of some species difficult, or in some cases, practically impossible. With the exception of L. trisulca, which is attached to the parent plant by a narrow stipe, identification of the various species of Lemna is often difficult.

Though generally smaller in size, but with some overlap, it is most likely to be confused with L. minor (common duckweed), which also has a single root. The only reliable diagnostic character is vein number: L. minuta has just one but is not easy to see in the field, whereas L. minor has three (Leslie and Walters, 1983). In the field, however, separation by this characteristic can be difficult, except sometimes in decaying fronds (Rich and Jermy, 1998). Even then it is sometimes not possible, due to faintness of vein and/or frond pigmentation, to discern between these species (Leslie and Walters, 1983). The leaves of L. minuta tend to be more ellipsoidal than those of L. minor and the plants do not stick together on the surface, behaving like individuals rather than as a mat.
 
L. minuta is difficult to distinguish morphologically from L. valdiviana (Landolt, 1986). The taxa are very similar, and it has been suggested they are closely related (Crawford et al., 1996), the best distinguishing trait in both being the length of its single nerve in the frond relative to the extension of the air spaces; but identification is not possible in the field (Landolt, 1986). Note that the difficulty to distinguish between these species increases when L. minuta is grown in shady habitats. The shade form of L. minuta can be separated from L. valdiviana by the extent of the nerve. The obscure nerve of L. minuta only extends about half the distance from the node to apex (Armstrong, 2009).

Prevention and Control

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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.

Control

Cultural control and sanitary measures

Duckweed can be introduced into garden pools unknowingly, brought in with newly-acquired water plants. It is, therefore, good practice to quarantine purchases for a few weeks before introducing them.
 
Destroying the duckweed layer with herbicides does not solve the problem of excess nutrients in the water. Because of the exponential growth rate of Lemnaceae, herbicides must be used repeatedly (perhaps several times a year). Ideally, it is best to eliminate the inflow of nutrients, and the repetitive removal of the duckweed layer will greatly reduce the growth of duckweeds (Armstrong, 2009).
 
Physical/mechanical control
 
Mechanical removal is often possible in small ponds by dragging or using harvesting machines in larger lakes. Continuous removal of this plant is often necessary. Dispersion could be prevented by stretching a boom across inflows to catch any plants coming in from upstream (Newman, 2009).
 
Environmental controls can also be used as duckweeds prefer still water, and increasing the disturbance of the water surface can reduce the amount of duckweed. This can be achieved by the use of a fountain or recirculating pump system. Lemna does not compete well with other floating leaved plants and planting species with floating leaves can substantially reduce the nuisance level of duckweeds (Newman, 2009).
 
Dragging a floating boom across the affected area can remove large amounts of the weed and booms stretched across inlets can prevent infestation from upstream. There are machines which can be used on larger lakes. Collected weed can be composted very successfully. Continuous removal is often necessary and re-colonisation can be rapid, but the method can be adequate in some situations and is useful in combination with methods of control. It has been observed in the UK that removal has only been partially successful (Briggs, 1992). Complete control is impossible and growth should be controlled before it reaches nuisance levels.
 
Biological control
 
Biological control using ducks, fish, turtles and crustaceans (water shrimp, crayfish, ostracods, freshwater prawns, daphnia, amphipods, etc.) may also help to control duckweed populations. There are a number of species of freshwater fish that eat duckweeds to supplement their diets, including grass carp (Ctenopharyngodon idella) for example. Duckweeds are also eaten by pacu (Colossoma bidens), a freshwater fish native to the Amazon River (Armstrong, 2009).
 
Chemical control
 
In general, the Lemnaceae are very sensitive to herbicides. In fact, duckweeds are often used to test the toxicity of herbicides and to detect the presence of herbicides in water. According to Landolt (1986), heterocyclic compounds (e.g. 6-methylpurin), urea derivatives, and quaternary ammonium compounds (e.g. diquat and paraquat) are the most toxic substances for the Lemnaceae. Lemna species are also susceptible to herbicides containing terbutryn and glyphosate but L. minuta is resistant to glyphosate treatment. Where dense mats are present and after mechanical treatment, or in places with difficult acess as margins or under trees, it is recommended to use terbutryn for control of L. minuta (Newman, 2009).

Gaps in Knowledge/Research Needs

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L. minuta, for which very little is known in their respective native range (Gassmann et al., 2006). Little is yet known of the plant's biology and attempts to control it with herbicide and mechanical methods (Briggs, 1992). Better knowledge on distinguishing the different species of Lemna in the field would contribute to better distribution data.

References

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Armstrong WP, 2009. Wayne's Word Lemnaceae On-Line. http://waynesword.palomar.edu/1wayindx.htm

Banaszek A; Musial K, 2009. The new kenophyte in Poland - Lemna minuta Humb., Bonpl., & Kunth. Acta Societatis Botanicorum Poloniae, 78(1):69-72.

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Links to Websites

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WebsiteURLComment
GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gatewayhttps://doi.org/10.5061/dryad.m93f6Data source for updated system data added to species habitat list.
Global register of Introduced and Invasive species (GRIIS)http://griis.org/Data source for updated system data added to species habitat list.
Wayne's Word Lemnaceae on-line maintained by Wayne Armstronghttp://waynesword.palomar.edu/indxwayn.htm

Contributors

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25/11/09 Original text by:

Manuel A. Duenas, Universidad de Cordoba, Dept. de Botanica, Ecologia y Fisiología Vegetal. Edificio C-4, Celestino Mutis, Campus de Rabanales, 4071-Cordoba, Spain

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