Invasive Non-Native Species (INNS) and flood risk in Calderdale

Action number 81 in the Calderdale Flood Action Plan is concerned with managing invasive plant species - introduced into this country no doubt with the best of intentions but now causing various types of damage, many of which lead to increased flood risk.
Principal amongst invasive species in Calderdale are the Himalayan balsam that you will see spreading everywhere on river banks, railway embankments and hillside slopes, and also the less frequently found Japanese knotweed and giant hogweed. To see what these plants look like, and just why they are harmful, view the GB Non-Native Species Secretariat’s identification sheets.

Managing invasive species

Work to combat these invasive species 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 INNS.
Practical work on the ground is undertaken by Yorkshire Wildlife Trust, Calder Future (the district’s river partnership), the Calder Valley Clean Up team and the River Stewardship Company.
The reason why invasive species control is an important part of reducing flood risk is because of their effect on native biodiversity. Himalayan balsam, for example, out-competes native species in ecologically sensitive areas, particularly river banks. Where it grows in dense stands along river banks it can impede flow at times of high rainfall, increasing the likelihood of flooding. Die back of extensive stands over winter can leave river banks bare and exposed to erosion. 
Developing a strategy and action plan to begin to manage these invasives by removing them from the fringes of watercourses, starting progressive eradication at the top of the catchment so that seeds do not wash downstream, is one of the natural flood management techniques being supported by the Calderdale Flood Action Plan.
Volunteer work parties are combining this work with the removal of debris that inevitably end up in the river, whether fallen trees or the odd sofa! As you’ll understand this is a long-term challenge.
INNS present significant issues for water companies not only from flood risk but also increased sedimentation, health and safety and biodiversity loss. Yorkshire Water supports the INNS programme with a view to helping reduce the propagule pressure and treatment costs on its assets whilst benefitting the wider river system and its biodiversity.
Himalayan balsam
Summer is the peak season for ‘balsam bashing’ (Himalayan balsam needs to be cleared before seeding in August) and during this time hundreds of young volunteers from the National Citizen Service work away at sites from Sowerby Bridge to Todmorden as part of their four-week assignment.
They do a fantastic job, showing a real interest in the task when they understand that they are contributing to protecting the Calder Valley from flooding, which in turn is just one of the consequences of climate change - issues that really concern them.
Recent work has focussed on sites that tend to get overlooked (some on private land, some in less frequented areas, some where there may not be so much awareness). Many new balsam sites have been noted throughout Calderdale and the species is spreading to sites away from watercourses, including fields, roadsides and gardens (i.e. if previously approx. 10% of sites were away from watercourses, it's now around 15%).
The railway corridor is an ongoing problem as it cannot be accessed, and in places it's overrun with invasives. Weather conditions are another common issue – for example, the minor flooding we saw on 16 March 2019 is likely to have dispersed seeds, while a cool and wet spring followed by a warm and dry summer can favour the balsam, giving it more of a head start over other species.
Giant hogweed and Japanese knotweed
As part of the wider non-native invasive treatment programme within the Calder Valley, the River Stewardship Company (RSC) is working on behalf of the Environment Agency (EA) to undertake a treatment programme to control giant hogweed and Japanese knotweed where it impacts on its assets.
To facilitate strategic catchment decisions regarding INNS treatment, the EA has commissioned the RSC to survey and treat INNS across the main rivers in Yorkshire. An extensive survey programme has been underway for two years to identify and record the instances of the INNS. The project team uses this information to make strategic decisions regarding the management of these species over the medium or long term.
Calder Greening
Calder Greening is a series of projects to improve green spaces, encourage tourism and business growth, and complement the Flood Alleviation Schemes with naturally managed flood risk across the Calder Valley.
Part of this work, led by the Yorkshire Wildlife Trust and Yorkshire Invasive Species Forum, involves monitoring, surveying and mapping the distribution of INNS (giant hogweed and Himalayan balsam) on the River Calder, undertaking joined-up headwaters-down treatment of INNS, engaging the public and spreading the message about INNS.
Part funded by the European Structural and Investment Fund (ESIF) as part of a £1.3m package, Calder Greening is being delivered by Calderdale Council in partnership with the Yorkshire Wildlife Trust and the Environment Agency. More information can be found at

Get involved

  • If you want to report the location of an invasive species you can do so via the INNSMapper tool or email Do not attempt to remove these species yourself - they require professional treatment.
  • Volunteering events where you can clear invasives or remove river debris can be found on the Green Calderdale volunteering calendar and the Yorkshire Invasive Species Forum (YISF)’s calendar.
  • You can promote your organisation or community group’s invasive species events by adding them to the Yorkshire Invasive Species Forum (YISF)’s calendar to share and promote them with a wider audience. Please contact and you will be provided with access and instructions on how to add events quickly and easily.


How you can help prevent the spread of INNS

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 and 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.
The Yorkshire Invasive Species Forum offers free advice on the best method for treating INNS on your land and can provide treatment itself, completed by Yorkshire Wildlife Trust.
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:

More useful links

iCASP - Invasive Non-Native Species. Developing evidence-based resources and strategies to slow the spread of aquatic INNS through river catchments in Yorkshire.