Hydrocotyle ranunculoides (floating pennywort)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- 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
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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
- Spanish: sombrerito de agua
Local Common Names
- Germany: Wassernabel, Amerikanischer
- Netherlands: de grote waternavel
- HYDAM (Hydrocotyle americana)
- HYDRA (Hydrocotyle ranununculoides)
Summary of InvasivenessTop of page
The characteristics that indicate its invasiveness are typical of many aquatic weeds: 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.
In Belgium, the Netherlands, and the UK in particular, it is considered a serious invader having escaped into the wild following its introduction to Europe in the 1980s through the aquatic nursery trade (often wrongly labelled as the native, H. vulgaris). This species was added to the EPPO alert list in 2004 (EPPO, 2004) and the serious threat it poses to habitats has led to it being added to Schedule 9 of the Wildlife and Countryside Act in the UK, and its ban in the Netherlands. It has spread into water bodies in a number of other European countries including France, Germany and Italy. A PRA for the EPPO region is available fromhttp://www.eppo.int/INVASIVE_PLANTS/ias_plants.htm. Control measures have been applied in Western Australia (Klemm et al., 1993).
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 Aquatic
DistributionTop of page
Hydrocotyle is not well known as a weed genus throughout the world, although H. umbellata is recorded as a weed in the southwestern USA (Anderson, 1990) and other species are listed as potential weeds in central USA (Steward, 1990). Fernandez et al. (1990) recorded H. ranunculoides as present in Chile, but its importance as a weed was not known. It occurs in Western Australia (Klemm et al., 1993) as a serious weed in the Canning River and has already established in at least seven countries of the EPPO region (EPPO, 2010).
H. ranunculoides is native to southeastern North America and possibly Central America (Mathias, 1936; Cook, 1974).
History of Introduction and SpreadTop of page
The pathways of introduction are only known for Europe. The plant is sold as a tropical aquarium plant in the Netherlands, Belgium and the UK. It has been grown in aquatic nurseries since the mid-1980s and was first noted in the UK in the river Chelmer in 1991 (Payne, 1992). 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 from France, Ireland, Italy and Germany.
There are over 150 known sites of infestation in England and Wales, although none are known in Scotland. Hydrocotyle ranunculoides has been discovered on the banks of the river Lagan, one of the major waterways in Northern Ireland (http://www.bbc.co.uk/news/uk-northern-ireland-11693434).
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 a native of southern North America and possibly Central America (Mathias, 1936; Cook, 1974), 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
|Freshwater||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
|Wetlands||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page
The chromosome number 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.
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.
This species 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 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 colonize 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).
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.
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Notes on Natural EnemiesTop of page
Herbivory has been observed on Hydrocotyle species by the co-evolved, leaf-feeding weevil Lixellus elongatus [Listronotus elongatus] in Argentina (Cordo et al., 1982). A large suite of other arthropod natural enemies have been recorded from Argentina and neighbouring countries, including stolon mining diptera, lepidoptera (Arctiidae, Torticidae, Noctuidae and Geometridae). 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).
Means of Movement and DispersalTop of page
Natural Dispersal (non-biotic)
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.
Vector Transmission (biotic)
None known, although birds roost on mats and may transport fragments to other locations.
Transport on machinery used to clear watercourses may be a factor in local spread.
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).
This species is likely to be introduced locally into water systems following cleaning of aquaria and ponds.
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 this species 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).
Floating pennywort 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 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.
Annual costs to the UK economy in terms of management, disposal and its effect on tourism have been estimated in a recent review of non-native species impacts in the UK (Williams et al., 2011) and total £25,467,000 for H. ranunculoides.
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).
An integrated management strategy for H. ranunculoides, using a combination of physical, chemical and ecological techniques was developed for the Canning River in Australia by a group of state and local government and community members. The initial short-term control phase was completed successfully. An assessment of water quality and aquatic invertebrates during the initial phase showed only short-term disruption of river ecology following physical and chemical control but the long-term eradication phase is on-going, as infestations continue to threaten the river system in the Regional Park.
Impact on Biodiversity
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.
Social ImpactTop of page
The presence of H. ranunculoides in watercourses creates a severe flood risk and may impact on recreational use of water bodies and aesthetic value.
Risk and Impact FactorsTop of page Invasiveness
- 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 H. ranunculoides is sold as a tropical aquarium plant in the UK, and also as an outdoor pond plant. It is likely, without the introduction of restrictive legislation and enforcement, that further occurrences will be noted. Similar patterns of introduction are noted for other aquatic plants in most countries with an aquatic horticultural trade.
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 Hydrocotyle is one of the few genera of the Apiaceae possessing entire leaves, most others having much divided pinnate leaves. A distinguishing feature is the presence of only one ovule in each mericarp (Tseng, 1967). Comparisons with the similar species H. vulgaris and H. umbellata are included below:
Hydrocotyle 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.
Hydrocotyle 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.
Hydrocotyle 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
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). Mechanical removal followed by hand picking four times a year during the growing season is now the accepted practice in the UK.
The plant is susceptible to diquat applied to the emergent foliage and is used in confined locations. Repeat applications are required to achieve eradication.
The application of 2,4-D amine, at intervals of 3 weeks, delays regrowth of the plant by up to 12 weeks and was found to be the optimum chemical control (Newman et al., 1998) although this herbicide is not translocated in the plant and unaffected parts tend to overgrow the treated biomass, reducing efficacy. Low volume applications of glyphosate have been used to limit regrowth, although further development of the technique is required.
The most effective method of control is mechanical removal followed by chemical spot treatment or hand pulling.
A CABI project sponsored by the UK Government and Environment Agency has been investigating the potential for biological control of H. ranunculoides using the host specific weevil, Listronotus elongatus (Cortat et al., 2010).