Hydrocotyle ranunculoides

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European and Mediterranean Plant Protection Organization

Organisation Européenne et Méditerranéenne pour la Protection des Plantes

Data sheets on quarantine pests

Fiches informatives sur les organismes de quarantaine

Hydrocotyle ranunculoides


Scientific name: Hydrocotyle ranunculoides L.f.

Synonym: Hydrocotyle natans Cirillo.

Taxonomic position: Apiaceae.

Common names: floating pennywort, marsh pennywort, greater water pennywort (English), hydrocotyle flottante (French), grote waternavel (Dutch), Grosser Wassernabel (German), sombrerito de agua (Spanish).

Notes on taxonomy and nomenclature: the plants which have been reported as invasive in Northern Europe are believed to originate in North America. However, there are also EPPO region records in Southern Italy, Palestine and Caucasus, which were referred to as H. natans Cirillo in the 19th century. This species has since been synonymized with H. ranunculoides (e.g. in Flora Europaea; Tutin et al., 1968), or referred to its var. natans (Cirillo) Urban. The present status of these forms is not clear, and there is no indication that they have ever been invasive. The European Garden Flora (Cullen, 1997) synonymizes H. ranunculoides with Hydrocotyle americana Linnaeus, implying that this species is distinct from the Mediterranean H. natans. This synonym remains controversial and Crow & Hellquist (2000) distinguish H. ranunculoides and H. americana. Since H. ranunculoides also occurs in sub-Saharan Africa, more information is needed on the similarities or differences between New World and Old World populations. Recent pilot study on barcoding Hydrocotyle species revealed that the species can be separated from other resembling Hydrocotyle species (van der Wiel et al., 2009).

Phytosanitary categorization: EPPO A2 Action list no. 334.

Plant type

H. ranunculoides is a stoloniferous, perennial, aquatic plant, with floating and emergent leaves. It is both vegetatively and seed-propagated.

H. ranunculoides is entirely glabrous, with stems floating in water or creeping onto shorelines. Stems are slender and root freely from nodes at about 4–6 cm intervals. Roots are profuse and hair-like. Leaves are alternate, emergent and held above the horizontal stem on long fleshy petioles, non-peltate, suborbicular to reniform with a cordate base, and frequently broader than long, shallowly or deeply 3–7-lobed, the lobes rounded, crenate or lobulate and subequal. They reach a diameter of 18 cm in suitable habitats in the United Kingdom. Petioles grow up to 35 cm. Flowers are hermaphrodite, white, 5–10 grouped together in a small umbel, borne on a leafless stalk, shorter than the petiole. No sepals, 5 unconnected petals, 5 stamens: ovary inferior, two-lobed, 2 styles. Fruits are nearly round and flat, brownish, with faint ribs and divided into two halves, each with a small persistent stalk (Mathias & Constance, 1976; Northern Prairie Wildlife Research Center, 1999; Huckle, 2002; Washington State Department of Ecology, 2004).
Similarities to other species

In Europe, plants are sometimes mis-identified as the native Hydrocotyle vulgaris.

Geographical distribution
H. ranunculoides is considered to be native to South America, and probably to North America (Everett 1981); collections from Ethiopia as early as the 1840’s indicate the species to be indigenous for Africa. The Schimper collections 1075 (13 November 1838) & 1482 (30 March 1842) from Ethiopia are truly H. ranunculoides (according to J van Valkenburg, pers. comm., 2009). Nevertheless, natural enemies are only reported from South America, but not from North America (Cordo et al., 1982). Some studies are in progress to determine accurately the native area of the plant (Newman, pers. comm., 2009).
EPPO region: confirmed presence in Belgium, France, Germany, Italy, the Netherlands, the UK. In Italy, present from

Toscana southwards, including Sardinia and Sicilia (Pignatti, 1982). See notes for more details.

Asia: Lebanon (Conroy, 2006), Iran (Naqinezhad et al., 2007), Israel (old record), Syria (Mouterde, 1966), Yemen (Wood, 1997).
Africa: Angola, Ethiopia, Kenya, Malawi, Tanzania, Uganda, Democratic Republic of Congo (Gonçalves, 1978), Madagascar, Rwanda (Troupin, 1978), Zimbabwe (Chikwenhere, 2001). Possibly also Sudan.
North America: Canada (British Columbia, Quebec), Mexico, the USA (Alabama, Arizona, Arkansas, California, Delaware, Florida, Georgia, Illinois, Kansas, Louisiana, Maryland, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Washington, West Virginia). In some States (Illinois, New Jersey, New York) it is considered as an endangered species. Further details on American records can be found on the USDA website http://plants.usda.gov/java/profile?symbol=HYRA.
Central America and Caribbean: Costa Rica, Cuba, Guatemala, Nicaragua, Panama. Martin & Hutchins (1981) indicate presence in Tropical America generally.
South America: Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Paraguay, Peru, Uruguay.
Oceania: Australia (Western Australia) (Ruiz Avila & Klemm, 1996).
Note: In Italy, the first mention of the species in Italy dates from 1616 (Colonna, F., 1616. De glossopetris dissertatio) under the name Ranunculus aquaticus umbilicato folio. There is no mention of origin, as it is usually the case in these old books. Pignatti (1982) indicates it as very rare and extinct in most areas. H. ranunculoides might have been present anciently in the Mediterranean area (JP Reduron, pers. comm., 2009).

Old records in Palestine and Caucasus are also mentioned (Shiskin, 1950), originally under the name H. natans (see notes on taxonomy).

In France H. ranunculoides has become naturalised around Paris (Arnal & Guittet, 2004), where it forms dense mats (100% coverage) in small waterways (C Girod, pers. comm., 2009). It was observed in a river in Corsica but has not been seen since 1968 (Jeanmonod & Gamisans, 2007). More recently, the species was found in several other areas: in the Picardie region where it is considered as a casual alien (Toussaint et al., 2005a), in the Nord region where it is naturalised and locally invasive (Toussaint et al., 2005b) and in the Dombes where it rapidly became invasive (Reduron, 2007). Four locations are also known in South-Western France, in a ditch, in waterways and on a riverside in a estuarine situation (E Tabacchi, pers. comm., 2009).

First infestations have been reported in Germany in 2004 where the species established and started spreading (Hussner & Lösch, 2007).

In the UK it has been present since 1989 in the River Chelmer. It was sold by the horticultural industry until late 2004 when it had spread to over 100 sites. In 2009 it is known to be present in at least 150 sites.

Although previously mentioned as present in Austria, the species does not occur in this country (F Essl, pers. comm., 2009). It is as well not recorded in Denmark (H E Svart, pers. comm., 2009) and Portugal (H Marchante, pers. comm., 2009).

Biology and ecology

H. ranunculoides is a stoloniferous aquatic perennial plant with floating and emergent leaves. It roots in the shallow margins of slow-flowing waters. In the UK, the Netherlands, Belgium and Australia, it forms dense interwoven vegetation mats, rapidly covering the water surface. It has been observed to grow 20 cm per day in the UK under appropriate conditions and it is very competitive, forming monospecific stands. Leaf matter can grow up to 40 cm above the water surface and roots and stems can grow down to 50 cm (120 cm observed in November 2008 – J Newman pers comm. 2009) below the water surface (Huckle, 2002). The plant reproduces primarily by vegetative reproduction, though spread by seed has been observed through sewage treatment works. It can regenerate even from small root fragments. It flowers in July–October in its native range. H. ranunculoides has a Chromosome number: 2 n = 24. There is a wide range of polyploids within the genus Hydrocotyle, with up to 15 ploidy (Moore, 1971, Federov, 1974). Newman (unpubl.) found four distinct groups of H. ranunculoides in the UK population which can be separated by Amplified fragment length polymorphism analysis. Additionally, this study identified that the populations introduced within the UK were very similar to the ones originating from the Netherlands. There is no knowledge to what extent to which different levels of ploidy between populations influences invasiveness.


The plant is native to South and probably North America. Intended habitats in areas where the species is intentionally imported are aquaria and garden ponds. H. ranunculoides is found in static, slow-flowing and occasionally flowing water bodies, especially ditches, canals, lakes and ponds. In the Netherlands, the species is found over a broad range of water quality conditions: from mesotrophic pools to the eutrophic lake IJsselmeer margins. Eutrophic conditions are preferred (Newman, 2002) as the species grows best at high concentrations of nitrate and phosphate, and/or organic matter. Two factors, contribute to the rapid establishment of H. ranunculoides: high nutrient levels through agricultural, urban and industrial run-offs favour the rapid growth of the species and impoundment of waters by creating dams, altering hydrological regimes to create favourable flow conditions. It has only been observed to grow at less than 1500 m altitude (Washington State Department of Ecology, 2004; Newman & Dawson, 1999; van der Krabben & Rotteveel, 2003).

Environmental requirements

The areas H. ranunculoides currently invades differ strongly in humidity and temperature, indicating that the species is very adaptable. Low frost tolerance seems to be a limiting factor for the distribution of this plant, though other plants (e.g. Glyceria maxima) provide shelter from frost for H. ranunculoides in margins and on non-mown banks.

Climatic and vegetational categorization

H. ranunculoides is typically associated with climates Af, Cf and Dfa in Köppen’s classification, i.e. warm to hot summer, cool to hot winter, wet year round. It may also survive in climate Cs (hot dry summer, cool wet winter). It is hardy to zone 7 (-15°C). It is associated with the vegetation zones: temperate to tropical deciduous forests, temperate steppes, Mediterranean sclerophyllous forests.
Natural enemies

In Germany, observations showed that coypus (Myocastor coypus) can eat H. ranunculoides (Hussner & 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.

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 (Newman, pers. comm., 2009).

History of introduction and spread

The main pathway of introduction is via the aquatic nursery trade. H. ranunculoides was first recorded in the wild in the UK in 1989. According to Newman & Dawson (1999), it was present at 29 sites in the South-East of England and in Southern Wales, in a wide range of water body types and had increased to 71 recorded observations by 2001 (Huckle, 2002). A first infestation in the Netherlands was detected and controlled in 1995 (Baas & Duistermaat, 1999). However, in the Netherlands, the plant is now present in all provinces, and only absent from the Wadden islands that are separated from the main land by sea water (J van Valkenburg, pers. comm., 2009), and the spread of this plant is now regarded as unstoppable (van der Meijden et al., 2001). In France, H. ranunculoides has been reported in pools in the Essonne valley south of Paris and in the Dombes area (near Lyon); invaded pools are completely covered by the end of the season, but there is currently no spread to other water bodies. The species has been present in some Southern European countries since at least the 1970s. Herbarium collections for Toscana (Momte Pisano) go back to the 19th century (1836/1860/1868). The infestation in the UK is assumed to have originated from a single clone which was sold by aquatic garden centres and nurseries (Newman & Dawson, 1999).

In 1983, the plant was observed for the first time in the urban drainage network in the Canning River Regional Park in Western Australia. Eight years later, it had spread throughout the drainage system into the river and nearby wetlands (Ruiz Avila & Klemm, 1996).

Pathways of movement

Flooding and summer storm events are important for transporting fragments within the same system and between different parts of the same catchment (Newman, pers. comm., 2009). Waterfowl may spread viable fragments of the plant (Huckle, 2002), but it is not considered to be an important pathway for spread. The main pathway of movement is considered to be distribution and sale by the aquatic nursery trade. This may also involve contamination of other aquatic plants with fragments of H. ranunculoides, as has been observed in aquatic plant sales within the EPPO region (J van Valkenburg, pers. comm., 2009). Spread from the intended to the unintended habitat then occurs primarily as a result of cleaning aquaria and garden ponds where H. ranunculoides has been grown. Trying to remove the plant mechanically is the most important cause of spread in the Netherlands (Pot, 2000).

Effects on plants

The potential of H. ranunculoides to become invasive was first stated in a publication dating from 1936 ((Mathias, 1936). Due to its vigorous growth, H. ranunculoides causes loss of light and reduction in dissolved oxygen content. Due to the high Leaf Area Index of up to 5.47 +-0.2, the species is able to outcompete submerged vegetation (Hussner & Lösch, 2007) and as a result ecosystem functions can be altered. Rare and endangered species and biodiversity can be reduced. In the EPPO region, where present, H. ranunculoides competes with many plant species in the habitats it invades. These may include water-edge plants such as species of Carex, Juncus, Rorippa amphibia, Myosotis palustris (syn. M. scorpioides), Nasturtium officinale (A. Hussner, pers. comm., 2009). In Germany, the native Myriophyllum spicatum, Callitriche spec. and Potamogeton crispus were displaced (Hussner, 2008). Many more species can be outcompeted due to H. ranunculoides' capability to build floating carpets that shade out other plants. In Belgium, H. ranunculoides has been observed to reduce the total number of native aquatic plant species by more than 50% and 100% of the submerged species. Cover of native species is reduced from 50% to 10% in invaded habitats (Nijs et al., 2009).

Environmental and social impact

H. ranunculoides is a popular plant in aquaria and garden ponds, but other species could adequately replace it. When it spreads to unintended habitats, it presents a lasting threat to the native flora. Though Baas & Holverda (1996) initially stated that H. ranunculoides would cause no significant harm in the Netherlands, this statement had to be retracted only three years later by Baas & Duistermaat (1999), as they then expected this plant to cause major impact on plants in nature reserves and recreation areas. H. ranunculoides can damage waterworks and, as well as displacing native flora through competition, can affect fauna by habitat modification (van der Krabben & Rotteveel, 2003). Indirect effects on other biota and food web (phytoplankton, zooplancton, fishes) is caused by its summer biomass and by decay (lowering of oxygen) and alteration of detritus (impact on macroinvertebrates) (Nijs et al., 2009; Triest, pers. comm., 2009). Dense mats reduce penetration of light to the water below and oxygen shortage may induce high fish mortality. Strongly invaded waters loose their attractiveness and safety for recreation. H. ranunculoides increases flood risk (Newman & Dawson, 1999) which may result in blockage of agricultural drainage networks, raising the ground water level which causes impacts on plant communities and grazing pastures, as well as potential economic impacts on crops (Kelly, 2006). Plants accumulate heavy metals, making disposal of plant material problematic (Pinochet et al., 2002). In 2007, in the Netherlands, 11 water boards out of 26 responded to an inquiry stating that they spent an additional 1.8 million euros for the management of H. ranunculoides over and above normal operating costs due to this plant (van Valkenburg, pers. comm., 2009). In Flanders, the estimated cost for the management of H. ranunculoides is 1.5 million euros per year (needed during 3 years from 2009) (Triest, pers. comm., 2009). In the UK, the estimate for control of the total area infested by H. ranunculoides using herbicides was between £250.000 and £300.000 per year (Harper, 2002). In 2008, £1.93 million was spent on the management and disposal of H. ranunculoides in the UK (Newman, pers. comm.., 2009). From 2002 to 2008, the costs were multiplied 7 times.

H. ranunculoides can present a barrier zone to fishing, navigation and swimming. As waterways covered with H. ranunculoides are not attractive for recreation and may hinder the movements of boats, some profit losses have been observed in the Netherlands (van Valkenburg, pers. comm., 2009). Dense vegetation mats can present a direct safety risk to the public and livestock. Cattle have drowned in the UK (Newman, pers. comm., 2009). Loss of aesthetic value in nature reserves has been reported in Belgium (Triest, pers. comm., 2009).

Summary of invasiveness

H. ranunculoides is a New World aquatic plant which has been on sale in Europe as a garden or aquarium plant for at least 20– 30 years and has become established in non-contained conditions in several Western European countries. In the UK and the Netherlands, it has shown its capacity to invade water courses by forming extensive floating mats of vegetation, excluding native floating and submerged aquatic plant species, and interfering with human use of the water courses. Its potential spread to other water bodies and catchments is almost entirely dependent on human assistance. H. ranunculoides has significant negative impacts principally on biodiversity and the environment, tourism and transport. H. ranunculoides is included in the EPPO A2 List and is considered as invasive by the PRA conducted by EPPO on the basis of the work performed by Schrader et al. (2005).

Mechanical control

In the case of mechanical removal, the areas of concern should be fenced or netted off, to reduce the risk that water downstream is infested. Also, all cut plant material should be removed from the water, otherwise spread and impact can even be increased due to the high regeneration capability of the species. The first infestation in Dutch urban waterways in 1995 was apparently controlled successfully by mechanical removal of plant material and a severe winter. Now the situation has changed and the plant proved to be hardy to severe frosts when protected by surrounding vegetation. Baas & Duistermaat (1999) conclude that H. ranunculoides in the Netherlands is now very unlikely to be controlled by man. Follow up operations using manual hand-picking of fragments results in very good control and can even lead to eradication at certain sites

Chemical control

According to Newman & Dawson (1999), H. ranunculoides is most susceptible to the herbicide 2,4-D amine applied at 4.23 kg/ha active substance, and is resistant to glyphosate applied at 2.16 kg/ ha active substance, probably due to insufficient uptake through the leaf cuticle. Further work has shown that improved control with glyphosate can be achieved using very low volume applications (less than 50 L/ha water volume), and even better control can be achieved by using adjuvants approved for use in water. In some countries (e.g. Germany), treatment of water bodies with herbicides is prohibited.

Biological control

Up to now, there are no biological control measures applicable in Europe. Listronotus elongatus (Curculionidae, syn Lixellus) has been found to feed exclusively on Hydrocotyle species in Argentina (Cordo et al., 1982). Further research on this potential biological control agent has been carried out and limited non-target feeding tests showed good host specificity under UK conditions. Further work is necessary (Newman, 2003).

Possibilities for eradication

Eradication is possible only in the very early stage of invasion. For initial infestations some water districts in the Netherlands have shown that containment is possible in fully controlled systems where water levels are artificially regulated. Central organization and funding are crucially important, and so is legislation. In later stages, eradication becomes very difficult and its success partly depends whether legislation allows herbicides to be applied. An example of control costs is the investment by the Western Australian government to control H. ranunculoides in the Canning River. The species became a serious problem in 1992 and after declaration as a P2 plant (a plant to be eradicated), a program costing over 200 000 AUD in the first year was implemented (AFFA, 2003).

Regulatory status

In 2001, the Dutch Ministry van Landbouw, Natuurbeheer en Visserij prohibited the sale and possession of H. ranunculoides. Due to its high invasiveness, the Royal Horticultural Society in the UK prohibited this plant at its shows and gardens (Shaw, 2003). In 2005, EPPO added this plant to its A2 action list, and EPPO member countries are thus recommended to regulate it. Suggested measures are related to EPPO Standard PM 3/67 (OEPP/EPPO, 2007), with emphasis in particular on: prohibition of import, sale, holding, planting and movement; the obligation to report findings; publicity; surveillance; establishment of an action plan for eradication when the plant is found. In the UK the species is included in the review of the Wildlife and Countryside Act 1981 (Defra 2007) as a banned species which is prevented from sale.


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North America

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The Netherlands

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