Invasive Non-Native Species

Invasive species are animals or plants that have been brought to an area in which they do not naturally occur. In Calderdale, several invasive plant species are causing issues leading to increased flood risk.

Invasive Non-Native Species

Invasive species are animals or plants that have been brought to an area in which they do not naturally occur. In Calderdale, several invasive plant species are causing various types of damage, many of which lead to increased flood risk.

Large overgrowing Himalayan balsam.

The most problematic invasive species in Calderdale are:

  • Himalayan balsam
  • Japanese knotweed
  • Giant hogweed

To see what these plants look like and learn more about them, view the GB Non-Native Species Secretariat’s identification sheets.

Invasive species and flood risk

Managing invasive species is an important part of reducing flood risk. Himalayan balsam outcompetes native species leaving valley sides and riverbanks bare and exposed when it dies back in winter. This means rainwater flows faster over these areas, causing erosion which can lead to trees falling. Removing the balsam allows native species to revegetate these areas and slows the rainwater down on its journey to the river.

Managing invasive species

Work to combat invasive species along watercourses in Calderdale is led by Yorkshire Wildlife Trust and the Yorkshire Invasive Species Forum, which brings together partners from the Environment Agency, Yorkshire Water and the University of Leeds to ensure a strategic approach to the identification and treatment of invasives.

Japanese Knotweed pictured with its small white flowers, zig zag stems and lush green colour.
How you can help
Bash the balsam!

Summer is the peak season for ‘balsam bashing’. The Yorkshire Invasive Species Forum and Yorkshire Wildlife Trust have produced this guide:

Help report Japanese knotweed and giant hogweed

Please do not attempt to remove Japanese knotweed or giant hogweed yourself - they require professional treatment. Instead, you can download the ‘iRecord’ App for free to report sightings. This handy video shows you how.

If you are unable to use ‘iRecord’ you can also email the Yorkshire Wildlife Trust at and send your report to them.


Join a volunteering day to help to clear invasives and create natural flood management measures. Take a look at our News Page for upcoming volunteer opportunities.

Raise awareness

It’s easy to get involved! You can download various invasive species graphics below:

  1. Help stop the spread graphic
  2. iRecord graphic
  3. Keep your eye out for these INNS graphic
  4. Know the risk graphic
  5. How you can help graphic
Himalayan Balsam pictured with its pink-purple flowers, fleshy stem and characteristic leaves.
Be invasives aware  

Invasive species can spread easily through clothing, shoes and equipment. Here are some top tips to help stop the spread:

In your garden/on your land

Be Plant Wise is a campaign designed to raise awareness among gardeners, pond owners and retailers of the damage caused by invasive aquatic plants. It also aims to encourage the public to dispose of these plants correctly. The Horticultural Trade Association, Ornamental Aquatic Trade Association, Royal Horticultural Society, and wild plant conservation charity, Plantlife, all support the campaign.

When out and about

Whenever you’re out in the countryside, remember to Check, Clean, Dry to help stop the spread of invasive plants and animals in British water.

  • Check your equipment and clothing for live organisms - particularly in areas that are damp or hard to inspect.
  • 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 don't transfer water elsewhere.

Although available and useful for all water users, specialist training and resources are available for the following groups:

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