Invasive Species and Flooding

Guest blog by Rosie Holdsworth at National Trust

Flooding might not necessarily be the first thing that springs to mind when we think about Invasive Non Native Species (INNS). The impact of non native species such as Himalayan Balsam, Rhododendron and Japanese Knotweed on biodiversity in native woodlands and wildflowers is widely understood. However INNS can also contribute to increased flood risk, putting them firmly on Slow the Flow’s radar. 

Non-native plant species tend to dominate a habitat until they completely out-compete all other species in an area, forming a total monoculture. We know that Balsam and Knotweed spread very easily along river networks; they thrive in damp conditions and their seeds or roots are easily spread downstream. When these plants die off and rot down in the winter, they leave large areas of bare soil; completely devoid of any vegetation cover or root network. 

This bare ground allows rapid runoff of water during rainstorms as there is no vegetation or ground cover to intercept, slow down and soak up the water. This means rainwater reaches streams and rivers more quickly – overwhelming them and causing flooding. 

Bare ground is also very vulnerable to erosion during storm events. Water moves quickly over unvegetated land and with no roots to hold soils together, erosion happens very quickly. Not only does this damage river banks and slopes, but the eroded material becomes silt in river systems which can further exacerbate flooding problems by reducing capacity for water.

Given the damage that INNS can do, it’s really important that land managers identify and tackle them on their land. Organisations such as the Yorkshire Invasive Species Forum are coordinating control efforts of INNS such as Japanese knotweed in Calderdale. They are also trialling new restoration techniques to stabilise riverbanks and increase biodiversity following control of INNS. To coordinate these activities effectively, a good understanding of the distribution of these species is needed. YISF are encouraging the use of the iRecord app to record INNS within Calderdale, adding to an open access database of INNS in the region. For helpful identification guides visit (

Fortunately, Himalayan Balsam is relatively easy to control: At Hardcastle Crags, the National Trust and Slow the Flow are building on the success of their natural flood management projects by undertaking invasive species control through “Balsam bashing” this summer. Balsam is so prevalent in Calderdale that we’ll never eradicate it completely, but regular balsam bashing through the summer helps to control the spread and allows other ground flora to gain a foothold. Calderdale Council and the Environment Agency have also been working on trials of biological control of Balsam using a rust fungus, which it’s hoped will help limit Balsam’s spread and begin to turn the tables in our favour.

Various conservation organisations working in Calderdale will be raising awareness and organising balsam bashing events through the summer, so keep an eye on Slow the Flow socials for updates and opportunities to get involved. 

Join us for balsam bashing at Hardcastle Crags every first and third Sunday of the month, please reserve a space here.