8 May 2014

Photo: Himalayan Balsam (photo by Alison Smedley)

Not all non-native species are detrimental to the environment they are introduced into. Non-native species that do have a negative affect on the environment are known as invasive non-native species.

These invasive species are a widely recognised problem because of the threat these different plants and animals can pose to the native environment, the economy and the public’s health. The problem in the UK is being addressed at a national and European level with bodies such as the GB Non-native Species Secretariat created in the UK to work to control invasive species and the European Commission developing policies to tackle the problem on a wider basis.

Along the waterways, invasive plant species pose a threat as they can cause navigation and water control problems as well as a reduction in habitat availability and the quality of the water. There are many invasive species that are found along waterways and four that are often encountered on either navigable waterways or those under restoration are Himalayan Balsam, Japanese Knotweed, Floating Pennywort and Giant Hogweed.

Himalayan Balsam (Impatiens glandulifera)

Volunteers with large Himalayan Balsam plant at Consall (photo by Stuart Collins)

Photo: Volunteers with large Himalayan Balsam plant at Consall (photo by Stuart Collins)


  • Native to the Himalayas.
  • Introduced to the UK as a garden plant in the mid-nineteenth century.
  • First recorded in the wild in the UK in 1855.
  • Now widespread and common across the UK primarily on the banks of waterways and other damp locations.

Identification Guide:

  • Stems are hollow and greeny red early in the year, turning a pinky red in the spring.
  • Leaves are green, slender and up to 15cm long.
  • Flowers appear between June and October, are slipper shaped and range in colour from a purplish pink to a very pale pink.
  • The plants can grow to over 3m in height.


  • Can take over whole stretches of waterway banks crowding out native species.
  • Dies back in the winter leaving banks bare and succeptible to erosion, which can cause navigational problems and disrupt fish spawning beds.
  • Spreads very quickly through explosive release of up to 800 seeds per plant.


  • Himalayan Balsam can be easily removed by simply pulling plants from the soil making it an ideal volunteer activity.
  • Plants can be disposed of by burning or composting if not in flower.
  • Plants should be removed before seed pods develop to avoid further dispersal of seeds.
  • Seeds can remain dormant in the soil for up to three years so sites should be revisited annually to make sure the plant is completely removed.
  • Himalayan Balsam should be reported via the Recording Native Species Counts scheme.

IWA campaigns against the spread of Himalayan Balsam. On a yearly basis, a variety of local branches organise work parties where volunteers visit known Himalayan Balsam hot-spots and pull the plants up before they go to seed. The aim is to visit sites annually until the plant ceases to grow at that site. In 2013, 116 volunteers took part IWA Himalayan Balsam work parties. For more information visit the IWA campaign pages.

Japanese Knotweed (Fallopia japonica)

Fallopia japonica, Japanese Knotweed (photo by GBNNSS, Crown Copyright 2011)

Photo: Fallopia japonica, Japanese Knotweed (photo by GBNNSS, Crown Copyright 2011)


  • Native to Japan, Taiwan and northern China.
  • Introduced to the UK as an ornamental plant in the nineteenth century.
  • Now widespread and common across the UK with extensive infestations in the south-west of England, south Wales and Greater London.
  • Mostly found along waterway banks and in urban areas.

Identification Guide:

  • Main stems of a developed plant has a purple speckled appearance with regular nodes (like bamboo).
  • Side stems zig-zag with a shield shaped, flat based, green leaf.
  • In the summer spays of small white flowers appear on long stems.
  • There is a rhizome crown at the base of the plant.
  • The plant grows to around 2m in height.


  • Grows into dense thickets and crowds out native species.
  • Contributes to bank erosion and increases likelihood of flooding.
  • Can cause structural damage as stems grow through materials such as asphalt.
  • Does not produce seed in the UK but spreads by dispersal of root pieces, this can easily happen via waterways.
  • It is not the most environmentally damaging of invasive species but causes economic problems as it is expensive to remove.


  • Japanese Knotweed can be controlled by cutting but this is a laborious task, which must be carried out accurately and repeated every two to four weeks throughout the growing season over a number of years.
  • The use of herbicides is the most affective form of control but this should only be undertaken by qualified individuals.

Floating Pennywort (Hydrocotyle ranunculoides)

Hydrocotyle Ranunculoides, Floating Pennywort (photo by GBNNSS Crown Copyright 2009)

Photo: Hydrocotyle Ranunculoides, Floating Pennywort (photo by GBNNSS Crown Copyright 2009)


  • Native to southern coastal states in the USA.
  • Introduced in the UK as an ornamental plant for ponds.
  • First recorded in the wild in the UK in 1990.
  • Now common in the south east of England and spreading across the British Isles.
  • Found emergent or floating on still or slow moving waterways.

Identification Guide:

  • Stalks are fleshy with fine hair-like roots.
  • Leaves are green, shiny and kidney shaped with crinkly edges and a width of around 7cm. Leaves can be emergent or floating.
  • Tiny white flowers may appear between July and August but are rare.


  • Can grow up to 20cm a day forming thick mats that quickly dominate waterways making navigation difficult and impeding water flow.
  • Can outcompete other species by blocking sunlight, deoxygenisation and preventing insects from reaching the water’s surface.
  • Spreads easily via root segments that break off and move through the waterways via currents or boats.


  • The plant is difficult to control as the smallest fragment can lead to the plant re-establishing.
  • Mechanical removal is best but the area should be netted off to prevent plant fragments from travelling downstream.
  • Once the main bulk has been removed it is important to go over the area and remove all smaller fragments to prevent spread.
  • Chemicals can be used but only by those with suitable training.
  • Floating Pennywort should be reported via the Recording Native Species Counts scheme.

Giant Hogweed (Heracleum mantegazzianum)

Heracleum mantegazzianum, Giant Hogweed (photo by RPS group Plc Creative Commons 2.0)

Photo: Heracleum mantegazzianum, Giant Hogweed (photo by RPS group Plc)


  • Native to the Caucasus Mountains.
  • Introduced to the UK as an ornamental plant in 1820.
  • First recorded wild in the UK in 1828.
  • Now widespread and common across much of the UK with extensive infestations in Scotland and the north of England, often found by large waterways.

Identification Guide:

  • Produces flowering blotchy purple stems that are up to 5m in height and 5-10cm in diameter with sharp bristles.
  • Produces white or pinkinsh umbrella shaped flowerheads with a diameter of up to 80cm.
  • Leaves are sharply divided/serated with bristles on the underside and up to 3m in size.
  • Once fully grown Giant Hogweed is easily identified due to its size but can be confused with Hogweed while still growing.


  • Giant Hogweed can grow in thick stands crowding out native species.
  • The plant produces a phytotoxic sap, even the smallest amount of contact with skin can cause heightened photosensitivity and severe blistering. Heightened photosensitivity can last for several years.


  • Control is difficult because of the plant’s sap. Cutting the plant should only be attempted by professionals who are wearing full protective clothing. The stem must be cut below ground.
  • Chemical control can be carried out between April and May by those with the relevant qualifications. The only chemical known to work against Giant Hogweed that can be used by waterways is Glyphosate.
  • Plant remains should always be disposed of carefully as the toxic affects of the sap can last for several days.
  • It is thought that seeds can lie dormant in the soil for between seven and fifteen years so areas will have to be retreated annually.

What can you do if you spot an invasive species?

  • If you think you have spotted an invasive plant species on a waterway report it to the relevant navigation authority.
  • Take part in the Check, Clean, Dry campaign by following these three simple steps when you leave the water:
    • Check your equipment (such as kayaks and paddles) and clothing for live organisms.
    • Clean and wash all equipment, footwear and clothing thoroughly. If you do come across any organisms, leave them at the water body where you found them.
    • Dry all equipment and clothing – some species can live for many days in moist conditions. Make sure you do not transfer water elsewhere.
  • Take part in an IWA Himalayan Balsam work party to help remove the invasive species from the waterways.

More information on the Check, Clean, Dry campaign is available at the GB Non-native Species Secretariat website.

More information on invasive species can be found at the GB Non-native Species Secretariat website.