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Invasive Non-Native Plant Species in Ponds

Meg investigates the problem of invasive species in ponds, what to look out for and how we can help.

What are Invasive Alien Species?

Invasive Alien Species (IAS) are defined as organisms which have been introduced and established in areas outside their native geographic range either accidentally or intentionally by humans, and which have adverse impacts on the economy, environment, or human health.

What is the issue?

The rapid increase in invasive species is a leading cause of global biodiversity loss and species extinctions. Invasive species are considered the most common threat to amphibians, reptiles, and mammals on the IUCN Red List. Invasive species can alter ecosystem structures and functions, and they present a global threat to food security and human health.

Aquatic invasive species negatively impact both native plants and invertebrates. Stiers et al. (2011) found that Floating Pennywort and Parrot’s Feather significantly reduce native plant species richness and cover. Growing rapidly, they inhabit large areas and form dense carpets over the pond, blocking out sunlight and oxygen exchange, consequently outcompeting native plants. Submerged vegetation are the most vulnerable plants to invasive species, which usurp large amounts of space in the water column. Rare species such as Common Bladderwort are vulnerable to invasion and have been found absent in invaded ponds.

Additionally, invasive species are very costly to manage; in the UK alone, they cost over £400 million in losses per year through eradication and restoration costs.

Climate change may increase the spread of invasive species; therefore, it is likely to become a more significant problem in the future.

Which invasive non-native aquatic plants do we look out for?

These are the main species which we look out for whilst undertaking Ice Age Pond surveys:

New Zealand Pigmyweed (Crassula helmsii)

  • Succulent perennial plant native to NZ and Australia.
  • Can establish different growth forms depending on the environment (submerged, emergent, or semi-terrestrial).
  • Main species we are concerned about, as it spreads very easily between ponds and generates dense stands which can cover an entire pond, reducing biodiversity and decreasing water quality.
  • Very difficult and expensive to control.
  • Can grow from tiny fragments.
  • No form of natural biological control in the UK.
  • Reduces breeding success of protected species Great Crested Newts due to the absence of plant species preferred by newts for egg laying, and potentially because the leaves of New Zealand Pygmyweed are small and inflexible, lacking an optimum environment for egg development (Langdon et al., 2009). 
Close up of green weed in a pond

New Zealand Pigmyweed – emergent form © Clare Dean (CABI, 2021)

Floating Pennywort (Hydrocotyle ranunculoides)

  • Perennial plant native to the Americas.
  • Exhibits many advantageous characteristics: grows rapidly, is able to adapt to low temperatures and variable nutrient conditions, and it is highly resistant to herbivory and chemical control.
  • Forms dense floating mats on the surface of the water, reducing light and oxygen availability.
  • Able to grow from small fragments, allowing it to rapidly recolonise downstream areas once it has been cut.
Close up of green, round leaves with indented edges

Floating Pennywort © Centre for Aquatic Plant Management (CABI, 2021)

Parrot’s Feather (Myriophyllum aquaticum)

  • Aquatic plant characterised by feathery leaves, native to South America.
  • Submerged aquatic plant but can form stems which emerge from the water.
  • Reduces oxygen levels.
  • Commonly grown in garden ponds, spreading rapidly from stem fragments.
  • Inhabits large areas, swamping out native vegetation.

Canadian Pondweed (Elodea canadensis)

  • Submerged plant native to North America.
  • Leaves arranged in whirls of 3.
  • Tolerates a wide range of ecological conditions.
  • Disturbance increases dispersal, meaning it is difficult to control.
Close up of green weed in water

Canadian Pondweed © Robert Vidéki (CABI, 2021)

Nuttall’s Waterweed (Elodea nuttallii)

  • Perennial submerged plant native to North America.
  • Grows rapidly, reproduces through fragments, and can be dispersed by waterfowl and water currents.
  • Displaces Canadian Pondweed (displayed above)
  • Distinguished from Canadian Pondweed by its narrow, twisted and curved back leaves.
Stems of green waterweed laid on a white surface

Nuttall’s Waterweed © Robert Vidéki (CABI, 2021)

Curly Waterweed (Lagarosiphon major)

  • Submerged plant native to Southern Africa.
  • Commonly grown in garden ponds.
  • Denser than other Pondweeds: mats can grow up to 2-3m thick, reducing water quality and biodiversity.
  • Persistent species and difficult to eradicate as it grows back from the roots.
  • Displaces Nuttall’s Waterweed (displayed above).
Green columnular spikes of green weed under water

Curly Waterweed © Rohan Wells (Invasive Species Specialist Group, 2006)

Water Fern (Azolla filiculoides)

  • Small free-floating fern native to the Americas.
  • Clusters of tiny leaves.
  • Reduces light availability and creates an anaerobic environment. 
  • Associated with high-nutrient ponds.
  • Not a significant issue in Herefordshire according to project ecologists.
Close-up of green weed on dark water surface

Water Fern © Richard Shaw (CABI, 2021)

Least Duckweed (Lemna minuta)

  • Native to the Americas.
  • Small free-floating plant, with leaves less than 3mm long.
  • Creates dense mats on the water surface, reducing light penetration and oxygen availability, and displacing submergent plants.
  • Prefers more shaded conditions than native Duckweeds.
Close up of tiny weed leaves in water

Least Duckweed

Which Invasives have been found in Herefordshire?

Unfortunately, New Zealand Pygmyweed is widespread in Herefordshire. The worst infestation has been observed at a pond on Bircher Common. Moreover, the species was also found at another site on the edge of an aquifer-fed pool, which presents a significant management problem, as the pool can swell up to one hectare.

The Bircher Common pond is also home to other problematic invasive species, including non-native Canadian Pondweed and Water Fern.

Floating Pennywort has been reported as present in a fishing pool at Warlow Pool in the Wye Valley, although this has not been investigated by Herefordshire Wildlife Trust.

Pond covered in bright green weed with some rush-type grass in the centre and trees in the background

Crassula helmsii (New Zealand Pygmyweed) carpeting a pond at Bircher Common © Will Watson

Controlling invasive non-native aquatic plants

As you can imagine, controlling the spread of invasive species is a major priority. Legislation such as the Wildlife & Countryside Act in the UK, and the EU Invasive Alien Species Regulation, bans the sale and growth of certain aquatic invasive plants. One of the EU Biodiversity Strategy’s targets aims to control or eradicate priority species and prevent the introduction and establishment of new invasive species (EC, 2011).

The most effective way to address IAS is by preventing their spread, which can be done through early detection, monitoring and eradication. The main measures used to control, or preferably eradicate, invasive species, include manual/physical (e.g., cutting, pulling, digging, smothering), chemical, and biological means (e.g., weed control membrane and herbicides).

In Herefordshire, there are plans to eliminate the infestation of New Zealand Pygmyweed at one of the sites by first removing most of it with a digger, followed by physical removal of the remaining plants by volunteers. The infestation of New Zealand Pygmyweed at the other site may only require the efforts of volunteers during the winter to suppress it before it becomes more problematic to remove.

What can we do?

Logo with text in front of green circle and plant illustration

There are some simpler ways in which you can look after the environment and protect native species in your garden at home. Be Plantwise by following these tips encouraged by DEFRA:

Text on top of green circle with hand lens illustration
  1. Know what you grow
      • Research plants before buying. Do not use aquarium plants in your pond.
      • Prioritise native species and remember that it is an offence to share or sell some pond plants.
  • Do not take plants from the wild.
Text on top of green circle with red hand illustration
  1. Stop the spread
  • Keep your plants in your garden.
  • Ensure your pond is not next to a natural waterbody or drain.
      • Wastewater from your pond or aquarium should only be used to water plants in your garden, away from waterbodies.
      • Practice biosecurity measures: regularly sterilise pond equipment and garden tools and clean the treads of your footwear after visiting a pond, especially when known invasive species are present. Dry all equipment and clothing afterwards.
Text on top of green circle with brown recycling bin illustration
  1. Compost with care
  • Responsibly dispose of unwanted plant material and water.
  • Ensure nothing removed from your pond ends up in the wild.
      • Unwanted aquatic plants should be used as mulch in dry areas of your garden.
      • Some invasive species require specific disposal methods which must be advised and approved by the Environment Agency (EA, 2010).
Man stood in front of garden pond with white tray on floor in front and houses/ garden in background

Demonstration of biosecurity measures: cleaning boot treads and pond nets © Will Watson

References

CABI (2021) Invasive Species Compendium. Available at: https://www.cabi.org/isc/  (Accessed: 31 August 2021).

CBD (2010) Aichi Biodiversity Targets. Available at: https://www.cbd.int/sp/targets/ (Accessed: 31 August 2021)

EC (2011) Combat invasive alien species – Target 5. Available at: https://ec.europa.eu/environment/nature/biodiversity/strategy/target5/index_en.htm (Accessed: 31 August 2021).

Environment Agency (2010) Managing invasive non-native plants in or near fresh water. Available at: https://www.gov.im/media/62585/ea_invasive_plants.pdf (Accessed: 31 August 2021).

GB Non-native species secretariat (2021) Biosecurity & Prevention: Be Plant Wise. Available at: http://www.nonnativespecies.org/beplantwise/ (Accessed 31 August 2021).

Heywood, V.H., and Sharrock, S. (2013) European code of conduct for botanic gardens on invasive alien species. Council of Europe, Strasbourg, Botanic Gardens Conservation International, Richmond. https://designatedsites.naturalengland.org.uk/SiteSearch.aspx (Accessed: 31 August 2021).

Hulme, P. (2011) ‘Addressing the threat to biodiversity from botanic gardens’, Trends in Ecology & Evolution, 24 (4), pp. 168-174.

Invasive Species Specialist Group (2006) Lagarosiphon major. Available at: http://www.iucngisd.org/gisd/species.php?sc=403 (Accessed 31 August 2021).

IUCN (2021) Issues Brief: Invasive alien species and climate change. Available at:  https://www.iucn.org/resources/issues-briefs/invasive-alien-species-and-climate-change (Accessed: 31 August 2021).

IUCN (2021) Our work: Invasive Species. Available at: https://www.iucn.org/theme/species/our-work/invasive-species (Accessed: 31 August 2021).

Langdon, S.J., Marrs, R.H., Hosie, C.A., McAllister, H.A., Norris, K.M., et al. (2009) ‘Crassula helmsii in U.K. Ponds: Effects on Plant Biodiversity and Implications for Newt Conservation’, Weed Technology, 18 (1), pp. 1349-1352.

Legislation.gov.uk (1981) Wildlife and countryside act 1981. Available at:  https://www.legislation.gov.uk/ukpga/1981/69/section/14 (Accessed: 31 August 2021).

Martin, P.A., Shackelford, G.E., Bullock, J.M., Gallardo, B., Aldridge, D.C., and Sutherland, W.J. (2020) ‘Management of UK priority invasive alien plants: a systematic review protocol’, Environmental Evidence, 9 (1).

Newman, J.R., and Dawson, F.H. (1999) ‘Ecology, distribution and chemical control of Hydrocotyle ranunculoides in the U.K.’, Hydrobiologia, 415, pp. 295-298.

Royal Horticultural Society (2021) Invasive non-native plants. Royal Horticultural Society. Available at: https://www.rhs.org.uk/advice/garden-health/weeds/Invasive-non-native-species (Accessed: 31 August 2021).

Stiers, I., Crohain, N., Josens, G., and Triest, L. (2011) ‘Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds’, Biological Invasions, 13, pp. 2715-2726.

Welsh Government (2020) Invasive aquatic plants: a guide for pond and aquarium owners. Available at: https://gov.wales/sites/default/files/publications/2020-09/invasive-aquatic-plants-guide-pond-aquarium-owners.pdf (Accessed: 31 August 2021).

Williams, F., Eschen, R., Harris, A., Djeddour, D., Pratt, C., Shaw, R.S., Varia, S., Lamontagne-Godwin, J., Thomas, S.E., and Murphy, S.T. (2010) The Economic Cost of Invasive Non-Native Species on Great Britain. Wallingford: CABI Publishing.

 

 

 

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