Hydrocotyle ranunculoides (floating pennywort)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Social Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Hydrocotyle ranunculoides L.f.
Preferred Common Name
- floating pennywort
Other Scientific Names
- Hydrocotyle adoënsis Hochst. 1841
- Hydrocotyle americana Walt. 1788
- Hydrocotyle batrachioides DC 1830
- Hydrocotyle cymbalarifolia Muhl. 1813
- Hydrocotyle natans Cirillo 1788
- Hydrocotyle nutans G. 1830
- Hydrocotyle ranunculoides f. minima Kuntze 1898
- Hydrocotyle ranunculoides var. genuina Urban 1879
- Hydrocotyle ranunculoides var. natans (Cirillo) Urban 1879
International Common Names
- English: floating marsh-pennywort; greater water pennywort; water-pennywort
- Spanish: sombrerito de agua
- French: hydrocotyle fausse-renoncule; hydrocotyle flottante
Local Common Names
- Germany: Großer Wassernabel; Wassernabel, Amerikanischer
- Netherlands: de grote waternavel
- HYDAM (Hydrocotyle americana)
- HYDRA (Hydrocotyle ranununculoides)
Summary of InvasivenessTop of page
H. ranunculoides is a perennial, aquatic plant native to the Americas. It was introduced outside of its native range through the aquatic nursery trade and has since naturalized in many countries around the world. Like many aquatic weeds, H. ranunculoides possesses a number of characteristics which contributes to its invasiveness: high growth rates, adaptability to prevailing nutrient conditions, very effective vegetative propagation, plasticity in growth response, overwintering to avoid low temperature stress, resistance to herbivory, resistance to chemical control, and absence of specific pests and diseases in introduced environments. Its rapid growth means that H. ranunculoides can produce dense, interwoven floating mats across slow-flowing waters. These mats restrict the light available for submerged macrophytes, decreases oxygen levels and therefore decrease the overall biodiversity of an area. It can also increase the risk of flooding and block channels.
H. ranunuloides is considered a serious invader in Belgium, the Netherlands, and the UK in particular and was added to the EPPO alert list in 2004 (EPPO, 2004) and Schedule 9 of the Wildlife and Countryside Act in the UK. It is also banned from sale in the Netherlands. It has spread into water bodies in a number of other European countries including France, Belgium, Germany and Italy. In 2016, the European Commission’s Implementing Regulation (2016/1141) was published, which includes H. ranunculoides among the list of 14 invasive alien plant species of Union concern.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Apiales
- Family: Apiaceae
- Genus: Hydrocotyle
- Species: Hydrocotyle ranunculoides
Notes on Taxonomy and NomenclatureTop of page
The name 'Hydrocotyle' is derived from the Greek words 'hydro' (water) and 'kotyle' (dish or plate), apparently referring to the shape of the leaves (Black, 1957).
Hydrocotyle is a widely distributed genus of more than 130 species with remarkable morphological variations both within and among species. Belonging to the Apiales, two distinct families were traditionally recognised within this order, Apiaceae and Araliaceae (Cronquist, 1988), though some unite both in one large family, a view with a long history (Harms, 1897).
Drude (1898) placed Hydrocotyle ranunculoides within Apiaceae, a family which he further divided into three subfamilies Apioidae, Saniculoideae and Hydrocotyloideae, subsequently, this method was adopted by the majority of botanists. Drude then further divided Hydrocotyloideae into two tribes, Hydrocotyleae and Mulineae, with H. ranunculides placed in the former in the sub-tribe Hydrocotylenae. Hydrocotyloideae has often been regarded as a primitive group of Apiaceae with species sharing features common to both the Araliaceae and Apiaceae. Some authors regard Hydrocotyloideae as a bridging group between the two families (Chandler and Plunkett, 2004). Much of the historic classification of Hydrocotyloideae has been based on the assumption that the sub-family was monophyletic, but recent advances in molecular studies have shown the sub-family is in fact polyphyletic with some members of the group being closely related to Apiaceae and others (Trachymene and Hydrocotyle) being more closely related to Araliaceae.
Molecular studies and morphological studies of the features of the fruit structure confirm the placement of the genus Hydrocotyle in the family Araliaceae (Chandler and Plunkett, 2004; Nicolas and Plunkett, 2009). This is consistent with observations of other morphological features shared between the groups (sclerified endocarps and broadly inserted petals lacking inflexed apices) and features not shared (lack compound umbels and vittae and carpophores) with Apiaceae.
DescriptionTop of page
H. ranunculoides is an aquatic stoloniferous plant with creeping stem with nodes at between 40- and 150-mm intervals. Profuse filiform roots occur at each node. Leaves are emergent, with the leaf stalks coming from the nodes on the horizontal stolons. Leaf matter extends up to 40 cm above the water surface and the interwoven mat of roots and stems can sink to 50 cm into the water (EPPO, 2005). Stipules are present. Leaves are reniform in shape, non sclerophyllous, less than 1 mm thick and vary in size from 20-45 mm to 100-150 mm, depending on nitrate availability, and have shallow-lobed edges. Flower stalks also derive from the nodes. The species produces very small creamy yellow flowers approximately 3 mm in diameter on a short umbel below the leaf canopy. Each umbel averages nine flowers (Klemm et al., 1993). Fruit are ovoid-ellipsoid to suborbicular, strongly flattened dorsally; mericarps with three subequal ribs; fruit wall a woody inner layer. Disseminules globose or somewhat flattened mericarps (Cook, 1990).
Plant TypeTop of page
DistributionTop of page
H. ranunculoides is considered by some to be native to North America but its wealth of closely co-evolved natural enemies in South America (Cordo et al., 1982) suggests this region to be the centre of origin, with a geographic expansion in range to Central and North America. In three of the 29 US States where it occurs (Illinois, New Jersey, New York) it is considered to be an endangered species (USDA, 2009). In Canada, the species is only recorded form British Colombia where it has all but disappeared. Across South America, the species has long naturalized and populations can largely be considered neo- indigenous in Chile, Uruguay, Paraguay, Brazil, Bolivia, Peru, Costa Rica Nicaragua, Guatemala, Mexico and Ecuador (Mathias and Constance, 1976).
H. ranunculoides has already established in at least seven countries of the EPPO region (EPPO, 2010). It was first recorded as naturalized in the south-east of the UK in the 1990s (Newman, 2003). Naturalization in the Netherlands and in Belgium was recorded in the last decade of the twentieth century (Krabben and Rotteveel, 2003; Verloove, 2006). Deleterious impacts have been reported in these three countries. The species is also recorded in France, Ireland, Italy, Germany (see EPPO, 2009) but several EPPO countries are still free from H. ranunculoides and there are concerns that it may be able to enter and establish in further countries. According to Flora Iberica, the mention of H. ranunculoides in Spain (Tutin et al., 1964-1980) could have resulted from confusions with small forms of H. vulgaris or H. verticillata.
H. ranunculoides occurs in Western Australia (Klemm et al., 1993) as a serious weed in the Canning River. It was first observed in Bannister Creek in 1983, it spread and covered large sections of the river by 1991, finding its way into the creeks that drain into the Canning River, probably by inadvertent, aquarium waste dumping. By 1993, it had established dense infestations in many freshwater areas of the River, upstream of the Kent Street Weir. It is also known as an alien plant in Japan (Hussner and Lösch, 2007).
Distribution TableTop of page
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.Last updated: 30 Jun 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Congo, Democratic Republic of the||Absent, Unconfirmed presence record(s)|
|Kenya||Present, Few occurrences||Introduced||1963||Invasive|
|Uganda||Present, Few occurrences||Introduced||Invasive||First reported: pre-1973|
|Azerbaijan||Absent, Invalid presence record(s)|
|Israel||Present, Few occurrences|
|France||Present, Few occurrences|
|Portugal||Absent, Unconfirmed presence record(s)||The EPPO PRA includes a personal communication saying not recorded in Portugal|
|Spain||Absent, Unconfirmed presence record(s)||Probably confused with other Hydrocotyle sp.|
|United Kingdom||Present, Localized||Introduced||Invasive||First reported: 198*|
|-Northern Ireland||Present, Localized||Introduced||Invasive|
|Mexico||Present, Few occurrences||Introduced|
|United States||Present, Localized|
|-California||Present, Few occurrences||Introduced|
|-Florida||Present, Few occurrences||Native|
|-Georgia||Present, Few occurrences||Introduced||Invasive|
|-Pennsylvania||Present, Few occurrences||Native|
|-South Carolina||Present, Few occurrences||Native|
|-Washington||Present, Few occurrences|
|-Queensland||Present, Few occurrences||Introduced||Invasive|
|-Western Australia||Present, Localized||Introduced||1983||Invasive|
|Argentina||Present, Few occurrences||Introduced|
|Bolivia||Present, Few occurrences||Introduced|
|Chile||Present, Few occurrences||Introduced|
|Colombia||Present, Few occurrences||Introduced|
|Ecuador||Present, Few occurrences||Introduced|
History of Introduction and SpreadTop of page
The pathways of introduction of H. ranunculoides are only known for Europe. The plant is sold as a tropical aquarium plant (often wrongly labelled as the native, H. vulgaris) in the Netherlands, Belgium and the UK. It has been grown in aquatic nurseries since the mid-1980s and was first recorded as a naturalized alien in 1990 when discovered in the River Chelmer at Chelmsford. By 1992 it was present in the Chelmer and Blackwater Navigation up to 12 km downstream of Chelmsford (Payne, 1992). By 1996 the growth of H. ranunculoides along the River Wey necessitated cutting to maintain navigation along the channel and to enable access to the water for anglers (Newman and Dawson, 1999). In the Netherlands and Belgium, Baas and Holverda (1996) reported the first occurrence in autumn 1995 as an escapee from aquatic nurseries. It has also been recorded more recently from France, Ireland, Italy and Germany.
Reported 20 years ago as being only present at 29 sites in SE England and South Wales within the UK (Newman and Dawson, 1999), there are now over 1,500 known sites of infestation in England and Wales (J. Newman, Centre for Ecology and Hydrology, UK, personal communication, 2017), although none are known in Scotland. H.ranunculoides has been discovered on the banks of the river Lagan, one of the major waterways in Northern Ireland (EPPO, 2010).
In 1983, H. ranunculoides was first observed in the urban drainage network in the Canning River Regional Park, Western Australia. By 1991 the plant had extended throughout the drainage network into the river and adjacent wetlands, disrupting the ecology and recreational uses of the waterways, and posed a threat to other waterways. It is not known to be invasive in other Australian waterways (Ruiz-Avila and Klemm, 1996).
Risk of IntroductionTop of page
The risks of its further spread in Europe, particularly in the Mediterranean and Black Sea areas, are addressed by the EPPO PRA (EPPO, 2010). As it has proved a problem in Western Australia and parts of Africa other countries in these regions may also be at risk of introduction.
HabitatTop of page
H. ranunculoides is native to the Americas, with South America being its likely centre of origin. Occurring at the margins of still or slow-flowing watercourses, McChesney (1994) identifies two distinct habitats where H. ranunculoides occurs: high altitude tropical lakes of East Africa and South America, and low altitude coastal regions of the temperate zone of USA, South America, Australia and Europe. It often forms part of floating island communities in tropical lakes.
Habitat ListTop of page
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Terrestrial||Natural / Semi-natural||Wetlands||Present, no further details||Harmful (pest or invasive)|
|Freshwater||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page
The chromosome number of H. ranunuloides is reported as 2n=24 (Federov, 1974). Moore (1971) noted a wide range of polyploids in the genus, with up to 15-ploidy, together with aneuploidy at all levels. In a short genetic study of H. ranunculoides in the UK (JR Newman, Centre for Aquatic Plant Management, Wallingford, UK, unpublished data, 2004), four groups of the species were distinguished in the UK population by AFLP analysis. One clone was found to be relatively similar to the native H. vulgaris, perhaps indicating hybridization. In the same study, the chromosome number was found to be 96, indicating tetraploidy over the same species assessed by Federov (1974). This may indicate that the horticulturally derived weed of Europe is different to the native species but further work is needed to confirm this.
H. ranunculoides is capable of both sexual and asexual reproduction. The mode of introduction to temperate habitats in Europe is thought to be from seed produced in tropical aquaria and released to the environment through the sewage treatment system. Vegetative reproduction takes the form of ramets detaching from parent mats to spread downstream and colonise further sites (Hussner and Lösch, 2007). It has been found to flower and fruit as early as May in the Netherlands (Meijden et al., 2001).
Physiology and Phenology
There is little information on the seed biology of H. ranunculoides. Germination takes place on mud banks with fluctuating temperature regimes in early spring with increasing daylength. Roots develop from every node of the stoloniferous growth form. H. ranunculoides grows and regenerates rapidly. In the UK, rates of 23 cm per day have been recorded (JR Newman, Centre for Aquatic Plant Management, Wallingford, UK, personal communication, 2004). Growth in waste water treatment systems have reached 19.7 tonnes per hectare (Boyd and Bayne, 1988). Studies in Germany showed increased growth under high nutrient conditions up 0.132 g g-1 dry weight d-1 (Hussner and Lösch, 2007). There are no investigations into leaf physiology, although Della Greca et al. (1993, 1994) showed production of novel antialgal oleanane glycosides and polyoxygenated oleanane triterpenes isolated from dried whole plant material.
H. ranunculoides is an obligate freshwater species, without any preferences for water velocity, water depth, bank slope, pH, dissolved oxygen or nutrients (EPPO, 2010), making it a generalist in its ecological response within the limits of cool freshwater bodies. Optimum photosynthesis occurs at temperatures between 25 and 35°C and high photon flux densities (Hussner and Lösch, 2007). It can remain dormant over winter to avoid low temperatures. In its native range floating pennywort is not considered a weed, although both H. ranunculoides and Hydrocotyle modesta Cham. and Schltdl., also native to southern South America, are regarded as indicators of eutrophication (Hauenstein et al., 2008).
Natural enemiesTop of page
Notes on Natural EnemiesTop of page
Citations for natural enemies of Hydrocotyle spp. in North America are few, and limited to generalists like Synclita obliteralis (Walker) (Lepidoptera: Crambidae), and Liriomyza munda (=L. sativae) Frick (Diptera: Agromyzidae) (Cabrera Walsh et al., 2013). The US National Fungus Collections Database (Farr and Rossman, 2011) lists the pathogens found associated with H. ranunculoides, mostly from California and southeastern states. These include Cercospora hydrocotyles, Entyloma fimbriatum, Entyloma hydrocotyles, Physoderma hydrocotylidis and Puccinia hydrocotyles (from USA and Chile). Harms et al (2012) recorded ten species of insect herbivores on H. ranunculoides in the Southern USA including Coleoptera (Scirtidae and Curculionidae), Lepidoptera (Noctuidae and Crambidae), Diptera (Chloropidae) and Hemiptera (Cicadellidae and Pseudococcidae).
In South America, however, more specialist natural enemies are recorded. Preliminary studies on the weevil Listronotus elongatus (Hustache) anticipated that it could be specific to H. ranunculoides (Cordo et al., 1982) and hold potential as a biological control agent.
Cabrera Walsh et al. (2013) listed a large suite of arthropod natural enemies and pathogens found associated with H. ranunculoides in Argentina. Collections indicated that the weevil Listronotus elongatus is the most common, widespread and damaging insect on the plant in Argentina. Other arthropod natural enemies included stolon and petiole mining diptera (Chloropidae and Ephydridae), and lepidoptera (Arctiidae, Torticidae, Noctuidae and Geometridae), whilst the rust pathogen Puccinia hydrocotyles (Pucciniaceae) and leafspot pathogen, Cercospora hydrocotyles (Mycosphaerellaceae), were also found associated with this host.
During summer, cattle will eat the plant when it grows at the water margins, but this again has not prevented the establishment of the species, and even encourages the spread of the plant due to fragmentation (EPPO, 2009).
In Germany, observations showed that coypus (Myocastor coypus) can eat H. ranunculoides (Hussner and Lösch, 2007). Some populations were partially grazed by this mammal, which exclusively eats the leaf lamina of these plants. However, grazing does not prevent the establishment of the species.
Means of Movement and DispersalTop of page
Natural dispersal by water occurs within contiguous systems once the plant has become established by other means. It is not thought to be responsible for introduction to new areas isolated from existing populations.
Birds roost on mats of H. ranunculoides and may transport fragments to other locations.
Transport on machinery used to clear watercourses may be a factor in local spread. It is also likely to be introduced locally into water systems following cleaning of aquaria and ponds.
H. ranunculoides has been intentionally introduced as an ornamental tropical aquarium plant in the UK, Netherlands and Belgium. The viability of seeds is unknown and it is likely that the majority of sites where the species occurs outside the natural range are the result of deliberate introductions through cultivation or the aquarium trade. H. ranunculoides has also been considered for phytoremediation as it accumulates heavy metals and phosphorus (EPPO, 2010).
Pathway CausesTop of page
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Stems (above ground)/Shoots/Trunks/Branches|
Impact SummaryTop of page
|Fisheries / aquaculture||Negative|
Economic ImpactTop of page
The economic impacts of H. ranunculoides mainly concern the causation of flooding events where channels and water control structures are blocked. Prevention of navigation is also a problem in some watercourses. Costs of control are another economic factor. An outbreak on the Canning River in Western Australia in 1991 reportedly cost AUS$2 million and the species is still present in Australia (EPPO, 2010).
H. ranunculoides has already cost millions of Euros to control in Europe (EPPO, 2010), and increasingly frequent flooding is transporting the plant to new systems. The invasion of this invasive weed will severely limit the chance of water bodies reaching good ecological status as defined by the Water Framework Directive. EU withdrawal of chemicals licensed near water and ineffective mechanical control means the invasion has gone beyond any containment and eradication stage and longer-term solutions, such as biological control, are being investigated in the UK. In the Netherlands, some water boards faced a doubling of costs each year during the 1990s, and, in 2000, the total annual control costs were around €1 Million (Krabben and Rotteveel, 2003). In 2007, in the Netherlands, 11 water boards out of 26 responded to an inquiry stating that they spent an additional €1.8 million for the management of H. ranunculoides over and above normal operating costs for this plant (EPPO, 2009).
In Flanders, the estimated cost for the management of H. ranunculoides was €1.5 million per year (needed during 3 years from 2009) (EPPO, 2009).
Annual costs to the UK economy in terms of management, disposal and its effect on tourism were estimated in a review of non-native species impacts in the UK (Williams et al., 2010) and total £25,467,000 for H. ranunculoides. This cost is likely to have increased given the ongoing spread of H. ranunculoides in the UK and the lack of control achieved using available management options.
Environmental ImpactTop of page
Impact on Habitats
H. ranunculoides occupies a relatively empty niche in invaded environments, and therefore the impacts on existing species in the marginal zone are limited. However, the growth form of floating mats reduces light and restricts growth of submerged macrophytes and has been shown to greatly reduce the number of native plants in an affected area in Belgium (EPPO, 2010). It also forms a major part of floating island communities in tropical African lakes where oxygen depletion can occur under the mats, reducing the habitat availability for fish. Severe oxygen depletion also occurs under dense mats in the UK (JR Newman, Centre for Aquatic Plant Management, Wallingford, UK, unpublished data, 2004).
Impact on Biodiversity
The threat posed to habitats in the UK have led H. ranunculoides to be added to Schedule 9 of the Wildlife and Countryside Act. Large infestations have been observed on canals with 100% cover for 25 km or more which will have severe impacts on biodiversity. The plant is perennial and is present all year round in the UK. The impact on biodiversity has not been adequately assessed. In several unpublished studies, the UK Environment Agency has measured invertebrate diversity in mats of H. ranunculoides and found it to be similar to other types of vegetation, but with a shift in the type of invertebrate recorded.
Many of the severe infestations occur in endangered and sensitive areas with a high conservation interest both in the UK and Europe (e.g. Pevensey Levels (SSSI), Seasalter Levels (SSSI/Ramsar/SPA) and Natura 2000 in Belgium). Rare (Utricularia vulgaris) and vulnerable (Hydrocharis morsus-ranae) red list species were found to be absent in heavily invaded plots (Van Landuyt et al., 2006) in Belgium.
Data from Belgium suggested that the number of native aquatic plant species is reduced by more than 50% and that of submerged species up to 100% whilst in Germany, the native Myriophyllum spicatum, Callitriche sp. and Potamogeton crispus were displaced (EPPO, 2009). Stiers et al. (2011) found qualitative evidence that sites invaded by H. ranunculoides, have a negative impact on both native plants and macroinvertebrates, significantly reducing native plant species richness, with fewer species in heavily invaded plots. Native submerged and floating species occupy the same position in the water column as the invader and therefore competition is expected to be intense, but floating pennywort’s rapid growth rate will allow it to capture the available resources more efficiently and the subsequent canopies will further displace native species as sunlight is blocked and oxygen exchange/water quality is compromised.
Social ImpactTop of page
The presence of H. ranunculoides in watercourses creates a severe flood risk and impact on recreational use of water bodies and aesthetic value. These increased flooding risks and impacts on natural capital cannot be easily quantified but should not be underestimated. The Office for National Statistics (ONS) developed initial estimates for UK freshwater ecosystem assets and ecosystem services and found the monetary value of UK freshwaters was £37 billion in 2012 (of which outdoor recreation accounted for £14 billion (Office for National Statistics, 2015).
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Highly likely to be transported internationally deliberately
- Difficult/costly to control
UsesTop of page
As of April 2014, in England and Wales, H. ranunculoides has been banned from sale. This species is also listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore, it is also an offence to plant or otherwise cause to grow these species in the wild. H. ranunculoides is also one of 14 invasive plant species subject the EU Invasive Alien Species (IAS) Regulation (1143/2014). The Regulation imposes strict restrictions on a list of species known as “species of Union concern”. These are species whose potential adverse impacts across the European Union are such that concerted action across Europe is required.
The European Commission published the implementing regulation (2016/1141) and as such, H. ranunculoides cannot be imported, kept, bred, transported, sold, used or exchanged, allowed to reproduce, grown or cultivated, or released into the environment.
This plant has been tested as a treatment for the removal of wastewater nutrients (Boyd and Bayne, 1988) with some success. Its rapid growth rate provides a suitable livestock food source with high crude protein and digestibility indices. Della Greca et al. (1993, 1994) showed production of novel antialgal oleanane glycosides and polyoxygenated oleanane triterpenes isolated from dried whole plant material.
Uses ListTop of page
- Pet/aquarium trade
DiagnosisTop of page
DNA barcoding of H. ranunculoides is under development. It is possible to distinguish this species from closely related congeners using a single plastid DNA sequence (Wiel et al., 2009).
Similarities to Other Species/ConditionsTop of page
Comparisons with the similar species H. vulgaris and H. umbellata are included below:
H ranunculoides - Leaf margin split to central petiole. Overlapping marginal lobes, deeply indented margin. Vein pattern highly branched. Mid-green in colour, ranging in size from 2 cm to over 10 cm in eutrophic conditions. Found rooted at the margins of lakes, ponds, ditches and slow-flowing canals and drainage ditches. Emergent leaves to 30 cm from water surface. Rooting at every node.
H. vulgaris - Leaf margin entire, with small indentation on one side, if at all. No overlapping lobes. Thick leaf, vein pattern distinct to every marginal indentation, pronounced central pale disc on upper surface of leaf where petiole joins below. Leaves very dark green, ranging in size from 1 to 3.5 cm. Occurs on damp ground near water, not often found in water.
H. umbellata - Leaf margin entire, with small indentation on one side, if at all. Leaves are 2-3.5 cm, on long (10-25 cm) stems. Regular marginal indentations, no overlapping lobes. Flowers held above leaves. Occurs on damp ground, rarely in water.
Prevention and ControlTop of page
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.
Mechanical removal of floating mats is practised in the UK and The Netherlands. Estimates of biomass of 70-80 kg wet weight m-2 have been measured in UK situations (Appleby, 1997). Manual removal by volunteer groups has proved a successful method of management, particularly for smaller sites; mechanical removal followed by hand picking four times a year during the growing season is an accepted practice in the UK. Hand pulling works very well in small infestations and as a follow-up after major mechanical removal.
Cutting and removal is also a very good method of management, but it will not control or reduce the vigour of the plant. The cut or dredged material should be left on site at the top of the bank,well away from water. Regular cutting from May to October can prevent complete dominance of this species but cut material should be removed from the water immediately and followed by hand pulling or by spot treatment with chemicals to reduce the risk of regrowth.
Use of chemicals on or near water is heavily restricted and since the withdrawal of diquat, the only licenced chemical available for use is glyphosate. Applying glyphosate (and the adjuvants TopFilm and Codacide Oil to improve efficacy) at 6 litres product in 400 litres of water per hectare is the most effective treatment with this chemical. Repeat and regular treatments will be necessary throughout the growing season as soon as regrowth occurs. In general the plant does not rot down very quickly after chemical treatment and treated vegetation in flood-risk areas should be removed after two to three weeks if possible. Follow-up spot treatment after mechanical removal is recommended. Applications of glyphosate to dense infestations appear to have little effect on the plant, possibly because the waxy leaf surface restricts uptake and because of the multi layered growth of the plant (rafts) in the summer. H. ranunculoides has been found to have a survival strategy which includes poor translocation of chemicals, excretion of glyphosate through its roots and apical bud resistance (J. Newman, Centre for Ecoology and Hydrology, UK, personal communication, 2017).
The most effective method of control is mechanical removal followed by chemical spot treatment or hand pulling.
Investigations into the potential for classical biological control of H. ranunculoides were initated by CABI in 2006 with exploratory surveys to Argentina. The weevil, Listronotus elongatus, was subsequently prioritised for comprehensive host range studies against a list of carefully selected test plant species of relevance to the UK and North Western Europe. Host range testing undertaken in the Argentina against 36 species of plants (Cabrera Walsh and Maestro, 2017) showed the weevil to hold great potential and the weevil’s significant preference for its host, H. ranunculoides, has been further confirmed in UK studies. A pest risk assessment outlining all of the research conducted on the weevil was submitted to UK regulators in 2017 and a decision is pending.
Whilst research into specialist natural enemies has largely focused on South American species of insects and pathogens, research in the southern USA has also been conducted by Harms et al. (2012). They found two species, Eugaurax floridensis and E. basigera (Diptera: Chloropidae), exhibited the ability to severely damage plants in the laboratory, although their effect on field populations of H. ranunculoides was not known. Observations of plants infested with high levels of E. floridensis in the field tended to appear “droopy” and yellow, similar to senescing plants. Specificity studies initiated in the UK with E. floridensis suggested specificity was not sufficiently restricted to the target host, H. ranunculoides (D. Djeddour, CABI, UK, personal observation) to warrant further investigation.
ReferencesTop of page
Agnew ADQ, Agnew S, 1994. Upland Kenya wild flowers: a flora of the ferns and herbaceous flowering plants of upland Kenya, Ed. 2:vi + 374 pp
Anderson LWJ, 1990. Aquatic weed problems and management in western United States and Canada. In: Pieterse AH, Murphy KJ, eds. Aquatic Weeds: the Ecology and Management of Nuisance Aquatic Vegetation. Oxford, UK: Oxford University Press, 371-391
Appleby N, 1997. Survey report on the existence of floating pennywort (Hydrocotyle ranunculoides) along the River Wey and Wey Navigation. Centre for Aquatic Plant Management Report. Wallingford, UK: CEH Wallingford
Aulbach-Smith CAS, Kozlowski SJ de, Dyke LA, 1990. Aquatic and wetland plants of South Carolina. South Carolina, USA: South Carolina Aquatic Plant Management Council
Baas WJ, Duistermaat LH, 1999. The invasion of floating pennywort (Hydrocotyle ranunculoides L.f.) in the Netherlands, 1996-1998. (De opmars van Grote waternavel (Hydrocotyle ranunculoides L.f.) in Nederland, 1996-1998.) Gorteria, 25(4):77-82
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30/05/17 Updated by:
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