Hedges are not just for the countryside. They have a very important place in cities too.
An Urban Hedge Living Laboratory
We want the Great West Hedge to be a Living Laboratory, where the local community can work in partnership with public bodies and organisations in helping develop and prove in a real life environment the benefits of hedges for reducing urban pollution as shown by research (see below).
How do hedges help improve air quality?
Urban vegetation typically removes only a low percentage of emissions by a process called deposition, which is when pollution sticks to the surface of a leaf and is removed from the air.
However, urban vegetation can greatly reduce the amount of emissions people are exposed to. It does this by changing the distance they must travel from the source to reach people, and the extent to which they are diluted with cleaner air along the way– this process is known as dispersion.
The above diagram is taken from the excellent introduction First Steps in Urban Air Quality. Second Edition. A Trees and Design Action Group (TDAG) Guidance Document by Ferranti, MacKenzie, Levine, Ashworth and Hewitt, 2019.
What may it look like?
The wide grass verge along the length of the A4 provides a great opportunity for planting a hedge, probably along the pavement side. Ivy fencing, or similar, could be used to give it a quick start planted with suitable hedging plants known to be most effective in pollution mitigation.
An artist impression of a hedge. The exact size, and layout offering the best balance of safety for road workers and protection from pollution are still to be worked out.
It has been suggested that a programme of tree planting be included with the creation of the hedge. These could be an investment in longer term mitigation, supporting the hedge.
Have you got any local examples?
Alongside the A4 at British Grove there is a ivy panelling which was installed in 2014 as a single fence. Subsequently after the ivy had grown further, an additional layer of fence was added to bring it up to head height. In some areas artificial ivy provides an infill while the natural ivy grows through it.
Where can I read more?
- A great starting point with background, references and specific advice on approaches.
- This April 2019 guide for the Mayor of London summarises the current best practice for how green infrastructure can reduce public exposure to air pollution in the urban environment. It has been produced in consultation with the Birmingham Institute of Forest Research (University of Birmingham), the Global Centre for Clean Air Research (GCARE) (University of Surrey) and Transport for London.
- "Hedges can provide effective barriers between cars and pedestrians to protect people close to the side of open roads ... A hedge or green wall bordering open space, ... , has the additional potential to create a sheltered region of air immediately behind it. By forcing the main flow of polluted air over and around this space, the people within it, close to the hedge, are protected. The levels of pollutants here may be as much as halved, but the benefit tails off with increasing distance from the hedge."
First Steps in Urban Air Quality. Second Edition. A Trees and Design Action Group (TDAG) Guidance Document by Ferranti, MacKenzie, Levine, Ashworth and Hewitt, 2019.
- A very useful quick introduction.
- Urban planners should plant hedges, or a combination of trees with hedges -- rather than just relying on roadside trees -- if they are to most effectively reduce pollution exposure from cars in near-road environments.
- The researchers found that roadsides that only had hedges were the most effective at reducing pollution exposure, cutting black carbon by up to 63 percent. Ultrafine and sub-micron particles followed this reduction trend, with fine particles (less than 2.5 micrometres in diameter) showing the least reduction among all the measured pollutants.
- The maximum reduction in concentrations was observed when the winds were parallel to the road due to a sweeping effect, followed by winds across the road. The elemental composition of particles indicated an appreciable reduction in harmful heavy metals originating from traffic behind the vegetation.
- The hedges only -- and a combination of hedges and trees -- emerged as the most effective green infrastructure in improving air quality behind them under different wind directions.
- Roadsides with only trees showed no positive influence on pollution reduction at breathing height (usually between 1.5 and 1.7m), as the tree canopy was too high to provide a barrier/filtering effect for road-level tailpipe emissions.
- A comprehensive and critical review of the scientific evidence available. They found the main value of green infrastructure for urban air quality does not lie in its ability to remove pollutants, but in its ability to control their distribution.
- Especially, section 4 “Effect of green infrastructure on air quality in open roads” and Table S6, “Classification of studies investigated the impact of trees on air pollution in open road environment.” which discusses the research by Tiwary et al., (2008) at the Sutton Bonington campus, university of Nottingham.
- Especially section 9 "Urban Air Quality Improvement" which summarises a variety of research
Absorption of Airborne Particulates and Pollutants by Ivy (Hedera helix L) in Oxford, UK. STERNBERG, T., VILES, H. & EDWARDS, M. 2011.
- Examined ivy Hedera helix and how it may interact with airborne particulates in urban environments. Using Scanning Electron Microscopy ivy leaves collected along on traffic corridors were examined to determine dust absorption rates. Results showed that ivy acts as a ‘particle sink’ and was effective in capturing fine (1-2.5 µm) and ultrafine (<1 µm) particles and pollutants from man-made sources.