Invasive aquatic plants

Ponds can create a wonderful feature in your garden and are often a haven for wildlife. However, if not cared for properly, some pond plants can take over your pond. Getting rid of some of the worst invasive species can take some time and a number of growing seasons.

Controlling invasive aquatic plants

It is important to remove as much of the plant material as possible, while avoiding breaking it into many small pieces, which could make it spread further.

By regularly checking for and removing re-growth you should be able to keep these plants under control or completely get rid of them.

You should:

  • report – if you come across an invasive plant in your pond or elsewhere you can report its location using the CEDaR Online Recording tool, in order to help scientists to understand the distribution and spread of invasive species
  • act quickly if you find you have an invasive plant and want to remove it - waiting a few weeks or months could allow the plant to spread and become better established
  • remove every last bit - most invasive species can re-grow from tiny fragments, so try to remove every last bit and make sure to check for re-growth in the following season, removing new plants as necessary
  • time your work - problem plants can be pulled out at anytime, but big clear outs should be saved for autumn when they will cause the least disturbance to your pond and any associated wildlife
  • clean your kit - make sure to clean your footwear and equipment when removing unwanted plants as tiny fragments could be carried on them and re-grow
  • not spread plants with your waste water - any waste pond water should be emptied away from streams, rivers, ponds, lakes, lochs and ditches, or drains that flow into them
  • instead, use the excess water on the lawn or to water plants in your garden
  • be careful to avoid damp areas of your garden as some of these plants grow if the soil is too wet
  • protect pond life - before disposing of plants, leave them for a few hours beside the pond, so that invertebrates and animals can return to the pond

Disposing of pond plants

You should:

  • always dispose of plants and pond material by composting them or using your local green waste
  • never place them in a nearby pond, waterway or in the wild.  This could be illegal and the plants could damage the environment
  • not give unwanted plants to other people either – this will spread the problem and harm their ponds

Further techniques for dealing with particularly tricky species can be found at the Compost With Care section of the Invasive Species Ireland website:

Examples of invasive aquatic plants

Floating Pennywort

Image of floating pennywort

Its scientific name is Hydrocotyle ranunculoides and it may also be sold as water pennywort or simply, pennywort.

It is identified by its shiny, kidney-shaped leaves with crinkled edges. This weed was first brought into Northern Ireland as a plant for tropical aquariums and ponds. It has since escaped into the wild. 

Floating Pennywort is listed as an Invasive Alien Species of Union Concern in European legislation, and cannot be sold, exchanged, cultivated or released into the environment.

Curly Leaved Waterweed

Image of curly leaved waterweed

Its scientific name is Lagarosiphon major and it may be sold incorrectly as Bunched Elodea Crispa Oxygenating Plants.

It is identified by a long stem that is brittle and easily broken (aiding dispersal) and its leaves that are strongly recurved and are arranged in whorls or in a spiral arrangement. Surprisingly, most active growth occurs during the winter and it can also spread easily from broken fragments.

Curly Leaved Waterweed is listed as an Invasive Alien Species of Union Concern in European legislation, and cannot be sold, exchanged, cultivated or released into the environment.

New Zealand Pigmyweed, also known as Australian Swamp Stonecrop

Image of New Zealand Pigmyweed

Its scientific name is Crassula helmsii but it is also sold incorrectly as Crassula recurva, Tillaea recurva and Tillaea helmsii.

It's recognisable when growing at the water’s edge by its narrow, fleshy leaves. It grows both in and under the water, as well as on nearby land. It can regenerate from tiny fragments and therefore easily spreads to new areas. The plant grows on the muddy margins of ponds and it does not die back in winter. 

Water Fern, also known as Fairy Fern

Image of water fern infestation

Its scientific name is Azolla filiculoides.

This plant has tiny scale-like leaves no bigger than 2.5mm, forming small plants around 2.5cm long which can cluster to form a dense mat. It is green in summer but usually turns a distinctive red in autumn and winter. It is a small, free-floating water fern that grows and thrives in canals, ponds and sheltered aquatic habitats.  

Always take care when buying new plants for your pond and make sure that water fern isn’t already present on the plant.

Parrot’s Feather

Image of Parrot's feather

Its scientific name is Myriophyllum aquaticum and it may also be sold as Myriophyllum brasiliense, Myriophyllum proserpinacoides, Brazilian water-milfoil, or simply as ‘oxygenator’.

It has bright green leaves, sometimes with a blue-grey sheen, that have a characteristic feathery appearance. It is a perennial plant that grows submerged but it also produces emergent, feathery shoots. 

Parrot's Feather is listed as an Invasive Alien Species of Union Concern in European legislation, and cannot be sold, exchanged, cultivated or released into the environment.

Choosing the right plants for your pond

There are plenty of alternative plants you can use for your pond. Which plants are best will depend on what kind of pond you have and where you are in the country. Talk to your pond plant retailer and ask them for advice on which species are most suitable and won't become invasive or escape into the wild.

Here are some suggested native alternatives for your garden pond or aquarium:

Native marginal/emergent plants

  • Flowering rush (Butomus umbellatus)
  • Marsh marigold (Caltha palustris)
  • Yellow flag iris (Iris pseudacorus)
  • Bogbean (Menyanthes trifoliata)
  • Ragged robin (Lychnis flos-cuculi)
  • Water mint (Mentha aquatica)
  • Marsh cinquefoil (Potentilla palustris)
  • Brooklime (Veronica beccabunga)
  • Water lobelia (Lobelia dortmanna)
  • Arrowhead (Sagittaria sagittifolia)
  • Common water-plantain (Alisma plantago-aquatica)

Native oxygenating plants

  • Curled pondweed (Potamogeton crispus)
  • Perfoliate pondweed (Potamogeton perfoliatus)
  • Lesser pondweed (Potamogeton pusillus)
  • Spike Water milfoil (Myriophyllum spicatum)

Native floating plants

  • Yellow water lily (Nuphar lutea)
  • White water lily (Nymphaea alba)
  • Broad leaved pondweed (Potamogeton natans)

Importance of controlling invasive aquatic species

It is important to stop their spread as invasive non-native species are both an economic problem and a threat to the environment. The biodiversity impacts are severe and growing, from challenging the survival of the rarest species to damaging some of the most sensitive ecosystems. Once a species has been introduced, it can persist and get worse as the species spreads further. If it becomes established in the wild, it can smother native plants, clog waterways, disrupt the navigation of boats, interfere with recreational activities such as fishing, cause flooding and remove oxygen from the water, which can harm fish.

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