When is a ditch, not just a ditch
Why is a bioswale needed?
Dealing with the UK weather in the last a few years has been challenging for everyone. It has become common knowledge that our current storm water systems are struggling with the ever increasing pressure being applied by heavier rainfall and greater usage.
Enduring 2022’s long, hot summer, the cold winter over 2022/23 and 2023 having on average 11% more rain (1.29m total), it’s no wonder our gardens are struggling to adapt.
We are seeing extended periods of drought that are getting hotter and longer, baking the ground meaning that when it rains it normally rains harder and for longer. This can cause the water to struggle to soak into the ground and instead to run off into the waterways and sewers.
Sudden showers of torrential rain and extreme floods are affecting us in many ways such as housing damage, soil erosion and pollution of waterways.
As gardeners we have the ability to help mitigate water runoff through the use of green infrastructure and gardening in such a way that helps to capture, manage and filter the runoff from the more extreme weather events.
What is a bioswale?
Generally a bioswale will be a shallow broad channel with gently sloping slides planted up with vegetation that can slow down and help prevent flooding. It is made up of four layers to help mitigate runoff and filter the water before it enters our waterways.
Vegetation layer - Important to help slow down the water, provides all year round interest, creates a space for greater biodiversity, provides natural filtration to help remove some pollutants.
Mulch - Acts as weed suppressant, retains moisture, improves soil health and acts as another filtration layer.
Soil mix - normally composed of sand, topsoil and compost. Helps to act as a layer capturing pollutants as it percolates through the soil mix.
Gravel/ stone layer - A mix of gravel and stones help to further filter the water and a perforated pipe can be installed to manage the flow of water during particularly heavy downpours.
What plants can I use in a bioswale?
A key feature for plants in a bioswale is their resilience to handle both periods of wet and dry conditions as the moisture conditions can vary greatly. As you move further towards the center of the bioswale, planting that can handle slightly wetter conditions for longer can be used. Further up the bank, planting that has more tolerance for drier soil can be planted.
5 plants for a bioswale
Deschampsia cespitosa ‘Goldtau' - A lovely airy grass that likes to have its roots slightly moist, great to have further into the depression of the bioswale.
Eupatorium cannabinum - A tall perennial that can handle very moist soil, so perfect for the middle of a Bioswale. Also a magnet for pollinating insects.
Veronicastrum virginicum ‘Fascination' - An upright perennial with elegant flower spires. Best planted further up the slope for best results.
Dryopteris erythrosora - Fantastic evergreen structure can handle some sun if the roots are kept moist - best further into the bioswale.
Salix britzensis - Produces bright, coral red young stems and has brilliant winter colour. Famously thirsty trees and great at absorbing excess water.
Can I have a bioswale in my garden?
Wherever water runoff can be mitigated you can have a bioswale (as long as you follow some basic planning guidelines). Up until recently they have been used in public planting to help ease the amount of water entering our waterways whilst at the time providing some beautiful public planting for us to all enjoy.
Drainage in garden settings has always been important but more so in light of our changing climate. They offer a fantastic, discrete solution to manage the runoff in gardens from hard landscaping or from garden structures such as offices, sheds and garages.
In our Working With Nature garden at the RHS Hampton Court Palace Garden Festival in 2023 we created a garden to demonstrate how bioswales can be integrated into a garden whilst at the same time providing a place for beautiful planting. During the show, we also noticed a large variety of pollinating insects and because of the topography of the swale, it created a micro-climate of cooler air, where we observed insects taking refuge from the baking heat. Wildlife such as bees and butterflies also used the small puddles of water in the crevices to drink from between the sunshine and showers.
What is the maintenance like?
Bioswales are generally low maintenance but they do require some care. Regular inspections to ensure that the channels are free of debris and sediment build up can save the need for extensive site regrading at a later stage. Check that the channels are appropriately vegetated and that there is a balanced diversity of planting preventing woody plants from taking over.
Completing these regular checkups it will allow the bioswale to perform effectively, filtering the runoff and providing a beautiful addition to any garden.
A recap of the benefits
By having a bioswale in the garden you introduce an area that has many positive qualities such as -
Erosion control
Acting as a filter through the soil and helps remove pollution before it enters our waterways
Recharging and supporting the water table by managing the flow of water
Slowing down water from runoff
Adding biodiversity and an aesthetically pleasing design for water management
If you are interested in creating a sustainable garden using similar techniques, feel free to contact us - contact@butlerandparker.com | www.butlerandparker.com