Floating pennywort (Hydrocotyle ranunculoides)

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Floating pennywort blocking ditch

Top facts

  • The floating pennywort has an extremely rapid growth rate, with a mat of vegetation being able to expand by 20 cm a day.
  • A native of North, South and Central America, the floating pennywort is considered to be an invasive species in many European countries.
  • The floating pennywort can reproduce asexually from the smallest root fragment, which enables it to colonise new areas quickly.
  • In 2007, water boards in the Netherlands spent over two million Euros to control the floating pennywort.
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Floating pennywort fact file

Floating pennywort description

KingdomPlantae
PhylumMagnoliophyta
ClassMagnoliopsida
OrderApiales
FamilyApiaceae
GenusHydrocotyle

The floating pennywort (Hydrocotyle ranunculoides) is a perennial, aquatic plant (2) (3) which typically forms dense, interwoven mats of vegetation, either on wet soil or in shallow water (2) (3) (4). The sometimes reddish stems of this species are relatively thick, and can grow up to 35 centimetres in length (3), generally floating in the water (2). Hair-like roots branch freely from these at intervals of a few centimetres (2) (4) and remain shallow in the substrate (3).

The fleshy leaves of the floating pennywort are arranged alternately along the stem (2) (3), and are almost round or kidney-shaped (2) (3) (4) (5). These leaves are either shallowly or deeply lobed (2) (3) (4) (5), and can grow to a maximum size of about 18 centimetres in diameter (2) (4), usually being broader than they are long (2). A reddish spot can sometimes be seen on the leaf at the point where it attaches to the stalk (3). The leaves of the floating pennywort grow from a branched, creeping horizontal stem and may be emergent or floating (2) (3) (4), being held above soil, vegetation or water (5).

The inflorescences of the floating pennywort are made up of a cluster of short, leafless, flowering stalks, each of which is between one and six centimetres in length, which spread from a common point (2) (3). Each flower has five unconnected petals (2) (3), which vary in colour from yellowish-white or greenish to purplish (3).

The floating pennywort produces roundish, flat fruits which are brownish and faintly ribbed (2) (3). The fruits are generally only between one and three millimetres in length (3), and are divided into two parts (2).

Also known as
floating marshpennywort, floating marsh-pennywort, greater water pennywort, Irish marsh pennywort, marsh pennywort, water-pennywort.
Synonyms
Hydrocotyle adoensis, Hydrocotyle americana, Hydrocotyle batrachioides, Hydrocotyle cymbalarifolia, Hydrocotyle natans.
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Floating pennywort biology

The floating pennywort is thought to principally reproduce vegetatively (4) (5), being capable of generating an entirely new and extensive plant from the smallest root fragment (3) (4) (5). Vegetative growth can be extremely rapid, and a floating pennywort mat can expand by up to 20 centimetres a day (2) (4), growing up to 15 metres from the bank in a single season (4). However, this species is also thought to reproduce by seed (3) (4) (5), and it is believed that plants that are rooted in the substrate reproduce via this method, whereas floating colonies reproduce vegetatively (3).

In the UK, the floating pennywort has a variable growth rate, tending to grow most quickly in the summer months (4). Seeds and stem and root fragments are dispersed with water, substrate movement, or by animals and human activity (2) (3). As the stands of floating pennywort become more dense, the plant flowers and fruits, usually between March and October (2) (3). During the autumn, when both temperature and light intensity decrease, the plants develop smaller, fresh leaves (2), and the floating pennywort then over-winters as a much flatter and smaller plant (4), with only the submerged leaves surviving the winter (2).

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Floating pennywort range

The floating pennywort is a plant species native to North, Central and South America (2) (3) (4) (5). In North America, this species is found in California, Arizona, Oregon, Washington (2) and most of the southern and eastern states of the USA as far west as Texas (3) (5), as well as in British Columbia, Canada (2). It is not known to occur in the extreme north-eastern region of the USA (3).

Outside of its native range, the floating pennywort has become established in several countries, including Australia (2) (5), France, Austria, Italy and some African countries (5). In the UK, Belgium and the Netherlands, this plant is now considered to be an invasive species (5). It is thought that the individuals which have since established in Europe originally came from North America (2).

The floating pennywort was first reported in Western Europe in 1990 (2), when it was discovered in South Essex in the UK (5). It is thought that this species was likely to have been introduced to the wild as a result of being released from aquaria and garden ponds (2) (4), and it has since become widespread and well established in England and Wales (5). The floating pennywort has now been reported in about 150 sites across the south of England and Wales (4), and appears to be spreading rapidly northwards and westwards (5).

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Floating pennywort habitat

An aquatic plant (2) (3), the floating pennywort is a freshwater species (5) which can be found in stagnant and slow-flowing water such as rivers, streams, ponds, lakes, canals and ditches (2) (3) (4) (5). This species tends to colonise the shallow margins or banks of such water bodies (2) (3) (4). Once it has become established in such areas, the floating pennywort is then able to spread to deeper water by forming extensive floating carpets of vegetation (2).

The floating pennywort grows on all types of soil, but is known to grow best in high-nutrient sites, and is tolerant of organic pollution. Although it tends to prefer sunny conditions, this species is resistant to the cold European winters, and is even able to survive frequently fluctuating water levels within its habitat (2). The floating pennywort has been recorded at elevations up to 1,500 metres (3).

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Floating pennywort status

The floating pennywort has yet to be classified on the IUCN Red List.

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Floating pennywort threats

In areas of its natural range, floating pennywort colonies are often considered to be desirable components of aquatic ecosystems, and are commonly sold as ornamental pond plants (3). Although the floating pennywort is considered to be endangered in some north-eastern states of the USA, in many countries where this species has become established it is now considered to be invasive, including in the UK, France, Germany and Belgium (2). In Australia, this species is capable of doubling its biomass in just three days (4), and in southern and western regions of the country it is a government-listed noxious weed (3).

Once established, the floating pennywort can cause major problems in nature reserves, recreation areas and managed waterways (2). This invasive plant competes with native species, which can become shaded out by the floating pennywort’s extensive mats of vegetation (2) (5). It has been reported that the species richness of native aquatic plants may be reduced by a worrying 50 percent due to the presence of floating pennywort, and submerged species may be extirpated entirely (2).

In addition to blocking out light which plants need for photosynthesis, the floating pennywort also reduces the amount of oxygen present in the water (2) (5), which in turn can lead to fish mortality (2). Other environmental impacts as a result of the spread of the floating pennywort include the disruption of the movement of animals, an increase in the prevalence of mosquito breeding areas, and increased nutrient loads in the water (5). Due to causing die-off of other aquatic plants as well as animals, the floating pennywort can facilitate the infilling of shallow waters, as decomposing material gathers on the bottom of the water body and alters the substrate composition. This can affect the drainage of flowing waters (2).

Once introduced, the floating pennywort is capable of thriving in a wide range of conditions, and is able to spread rapidly (2). Its creeping nature has led to problems in irrigation and drainage ditches (3), with reduced drainage having a negative knock-on effect on agricultural lands within the floodplain (2). Additionally, floating pennywort has a negative impact on tourism, fishing and water sports (5), with some areas potentially becoming unsafe for recreational activities (2).

A major concern with the floating pennywort is the ease with which the species can be spread. Any activities which lead to the fragmentation of the species, such as boating or water sports, can facilitate the expansion of the floating pennywort’s range (2). Small fragments can be carried downstream along waterways, or upstream attached to boats (5), while flooding enables the species to become established widely in river valleys (2).

It is feared that, with its proven ability to re-grow from small fragments and rapidly establish itself in a variety of aquatic habitats, the floating pennywort is likely to spread further around the UK, posing a major threat to the ecology and use of water bodies (4). There are also concerns that increasing temperatures in Europe will enable this species to thrive and become more invasive in the central and northern parts of the continent (2).

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Floating pennywort conservation

As the floating pennywort is an invasive species in many countries, there are currently no known conservation measures in place for this species. However, there is legislation in place to prevent its further spread. For example, the sale and possession of the floating pennywort has been prohibited in the Netherlands since 2001, and distributing it is prohibited in Switzerland (2). The Great Britain Programme Board for non-native species has asked that an invasive species action plan be developed for the floating pennywort (6), and the Royal Horticultural Society has prohibited growing the plant in Great Britain (2).

In addition, recommended targeted actions which can be taken to avoid further spread of the species include mechanical control, whereby the floating pennywort is cut back using weed-cutting buckets or boats. However, this generally only offers short-term relief, as the species can grow back so quickly from a small fragment of plant. It is recommended that affected areas being treated mechanically should be fenced off to avoid any fragments drifting downstream and establishing elsewhere (5).

Chemical control is a further option, and glyphosate has been identified as a relatively effective herbicide for the floating pennywort (2) (4). A combination of mechanical and chemical control may be the most effective method of getting rid of this invasive species (2).

Although no appropriate methods of biological control are currently known for use in the UK (4), research is being conducted on a weevil (Listronotus elongatus) which is known to feed exclusively on the floating pennywort in Argentina. By feeding on the plant, the weevil sets off a cascade of events which may help to control the floating pennywort, and so its use in the UK is currently being investigated (2) (4).

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Find out more

Find out more about the floating pennywort:

Find out more about the control of invasive species in the UK:

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

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Glossary

Emergent
Aquatic plants whose stems and leaves extend beyond the water’s surface.
Inflorescence
The reproductive shoot of a plant, which bears a group or cluster of flowers.
Perennial
A plant that normally lives for more than two years. After an initial period, the plant usually produces flowers once a year.
Photosynthesis
Metabolic process characteristic of plants in which carbon dioxide is broken down, using energy from sunlight absorbed by the green pigment chlorophyll. Organic compounds are made and oxygen is given off as a by-product.
Vegetative reproduction
Type of asexual reproduction (reproduction that does not involve the formation of sex cells) in which a new plant grows from part of another plant, rather than from seeds or spores. The resulting individual is genetically identical to the original plant.
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References

  1. Species 2000 and ITIS Catalogue of Life (September, 2013)
    http://www.catalogueoflife.org/
  2. NOBANIS: Invasive Species Fact Sheet - Hydrocotyle ranunculoides (September, 2013)
    http://www.nobanis.org/files/factsheets/Hydrocotyle_ranunculoides.pdf
  3. DiTomaso, J.M. and Healy, E.A. (2007) Weeds of California and Other Western States. Volume 1. University of California Publications: Agriculture and Natural Resources, Richmond, California.
  4. Centre for Ecology and Hydrology: Information Sheet - Control of Floating Pennywort (Hydrocotyle ranunculoides) (September, 2013)
    http://www.ceh.ac.uk/sci_programmes/documents/hydrocotyleranunculoides.pdf
  5. GB Non-native Species Secretariat (NNSS): Factsheet - Floating pennywort (September, 2013)
    https://secure.fera.defra.gov.uk/nonnativespecies/factsheet/factsheet.cfm?speciesId=1766
  6. GB Non-native Species Secretariat (NNSS): Invasive Species Action Plans (September, 2013)
    https://secure.fera.defra.gov.uk/nonnativespecies/index.cfm?sectionid=92
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Floating pennywort blocking ditch  
Floating pennywort blocking ditch

© Bob Gibbons / www.flpa-images.co.uk

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