Invasive Plants Are Causing Havoc On Our UK Waterways!
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Tue Jul 28, 2015 at 11:52am

There are many non-native invasive plants that have made their way into the UK's inland waterways system over the years. Including seemingly harmless weeds released by the general public when clearing out their garden ponds.

Non-native invasive plants have arrived in the UK in a variety of ways; from their use in the horticulture, agriculture and aquaculture industries to coming in on or attached to ships. People have also played a part in introducing non-native invasive plants into the waterways, after buying plants intended for garden ponds and aquariums only later to disposing of them into the system unaware of the harm they create to the waterways.

The Canal & River Trust has in the past highlighted the worst non-native species most likely to damage its canals, rivers and towpaths and have a detrimental effect on British wildlife. The general public need to consider  the impact it will have on the environment when buying plants and should be responsible about disposing of them to make sure that invasive plants don't get into the waterway system.

Many plants are now banned from sale in the UK due to their destructive nature. It has been reported that the Canal & River Trust along with volunteers are investing a large amount of time and money to the sum of around one million pounds a year to protect the canals & rivers.Through hard work and commitment they are constantly identifying, monitoring and controlling damaging plants, in a bid to rid the waterways of this problem.

How you can help The Canal and River Trust reduce the introduction and spread of these plants;

  • Only buy native plants for your ponds and gardens, to reduce the threat of spreading the problem.
  • Boaters and fishermen can help prevent the spread of invasive plants by cleaning equipment in warm soapy water and allowing it to dry fully, before using it in another area of the canal system.
  • Dispose of invasive non-native plants from a garden or ornamental pond in an appropriate and responsible way.
  • Do not put them in a river, canal or any other waterway, throw over the garden fence, or fly tip. Many plants can be disposed of in the domestic waste/recycling bins.
  • Only purchase correctly labelled plants, it is worth knowing that “problem plants” can be sold under different names. If you come across a retailer selling plants that are mis-labeled then let Trading Standards know.

Problems caused by these invasive plants include;

  • They are often resistant to traditional control techniques.
  • They can be poisonous.
  • They compete with native species for space, water and sunlight.
  • Their rapid growth can choke rivers and canals.
  • They can be caught on boat rudders, lock gates and canal fixtures.

Across the UK's 2,200 mile waterway network, there are numerous plants that cause problems for the canals, towpaths, bridges, cuttings, embankments and waterway banks.

Here are the top 8 most offending non native invasive plants:

Japanese Knotweed AKA Fallopia Japonica

Japanese knotweed was brought to Europe from Japan in the mid-1900. Japanese knotweed starts growing from early spring and can reach 1.5m by May and 3m by June, before dying back between September and November. It has been said that it can grow through solid concrete, walls and Tarmac!


It is extremely hard to eliminate without specialist help because it spreads through underground rhizomes rather than seeds.

Australian Swamp Stonecrop AKA Crassula helmsii or New Zealand pygmyweed

It used to be sold in garden centres as an ‘oxygenating plant’; this rapid growing plant can quickly smother native vegetation. It has been spreading across the country since the 1970s; this yellow and green stemmed plant can re-grow from tiny fragments as small as 1cm and lives in a variety of habitats.


It has three growth forms – terrestrial, emergent and submerged (up to 3 metre depth), and will grow in still or slow moving waters. It grows and produces a dense mat that covers the water depleting the oxygen levels in the water. It is also able to withstand all weather conditions giving it a competitive edge over our native species. It has been banned from sale since April 2014.

Giant Hogweed AKA Heracleum mantegazzianum

Growing as much as 20ft tall with dinner table sized leaves Giant Hogweed is an impressive plant that was once planted in gardens. However, it is highly invasive and has spread throughout the whole of Great Britain. Primarily favouring river banks, but also other areas such as parks, cemeteries and wasteland. Its huge leaves can shade out other plants and increases the risk of bank erosion.


Warning – it contains toxic sap that can cause severe burns! When the sap touches the skin and is in the presence of sunlight it causes the skin to redden followed by severe burns and blisters. This species is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore; it is also an offence to plant or otherwise cause to grow these species in the wild. Giant Hogweed has been in the news quite a lot recently for causing harm to people on several occasions.

Himalayan Balsam AKA Impatiens glandulifera

This is a relative of the Busy Lizzie family growing well overhead height and is spread by shooting its seeds up to 7m (22ft) away or by people passing on the seed to friends and family. Each plant can produce 800 seeds! Once established in the catchment of a river the seeds, which can remain viable for two years, are transported further afield by water. It is a very destructive plant which is causing damage to the canal and river banks as it causes erosion.


Just like other non native plants it has a negative effect on the growth of native plants.This plant is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales therefore; it is also an offense to plant or otherwise cause to grow these species in the wild.

Water Fern AKA Fairy Fern, Azolla filiculoides

Water fern is a small free-floating water plant that forms dense mats. It was introduced for ornamental use in ponds, but its introduction into the wild has meant it has spread rapidly throughout England in the last 50 years. The dense mats can also appear to be a solid surface so animals and humans can be fooled into thinking it’s a solid surface and fall into the water.


It can be spread by small pieces floating down the waterways or by producing minute spores which are carried in the air. It has been banned from sale since April 2014.

Parrot’s feather AKA Myriophyllum aquaticum

Commonly has been used in garden ponds once planted it has the potential to readily escape cultivation into the wild dominating lakes, canals, reservoirs and ditches where it has overwhelm native plants.


Tiny bits of the plant have moved through waterways, on boats and located themselves aggressively in many areas. It is mainly found in southern England, but is spreading, possibly assisted by our warmer winters. It is now a banned from sale since April 2014, due to its destructive nature.

Water Primrose AKA Ludwigia grandiflora, Ludwigia peploides and Ludwigia uruguayensis

Originally introduced as an ornamental and water garden plant. The water primrose bears bright yellow flowers but can cause havoc if released in the wild. Its rampant growth out-competes native plants and can clog waterways, with its dense vegetation contributing to flooding.


It is now a banned from sale since April 2014, due to its destructive nature.

Floating Pennywort AKA Hydrocotyle ranunculoides

This was a very popular and widely available aquatic plant forming dense mats of rounded leaves which floats on the water’s surface. This should not be confused with ‘marsh pennywort’, the common name for Hydrocotyle vulgaris which is a native British plant. It damages the waterways by taking over native plants and using oxygen that the fish and insects need.


It is most commonly found in south-east England, but is now been spotted in the west, the midlands and in Wales, so it is rapidly spreading across the country. It is now a banned from sale since April 2014, due to its destructive nature.

Have you come accross any of these non-native invasive plants? If you do, report your sightings to the Environment Agency via their plant tracker app and notify your local authority immediately.

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