Hedges for urban planning

January 2019: Planting dense hedges as street level air filters cuts back black carbon, decreases heavy metals originating from traffic, and reduces the level of ultrafine and sub-micron particles in the air. Urban planners that include a combination of trees and hedges might have better results in improving health and well-being for urban residents.

Information

A recent UK study establishes that hedges are more effective as road-side green infrastructure than trees. Planting dense hedges as street level air filters has the potential to cut back black carbon by up to 63%, decreases heavy metals originating from traffic, and reduces the level of ultrafine and sub-micron particles in the air. Road-side hedges are passive control measures and showed particular efficiency as filters when winds were blowing parallel to the hedges. The study ranks hedges as standalone green infrastructure as most efficient, followed by a combination of trees and hedges, while trees as standalone infrastructure showed no positive influence on pollution reduction at breathing height.

Implications and opportunities

Dense hedges acting as strategically placed air filters can implicate future urban planning for road-side green infrastructure. Urban planners that include a combination of trees and hedges might have better results in improving health and well-being for urban residents. Further, planting dense hedges would emphasise urban greening and strengthen urban ecosystems. Urban planners could replace or complement existing green infrastructures to improve air quality and support the formulation of appropriate guidelines for future green infrastructure designs. The study coincides with the UK Government’s publication of its Clean Air Strategy that aims at the reduction of air pollution and the level of non-communicable diseases specifically attributed to air pollution in the UK. The government’s strategy does not include any specific measures on roadside air pollution.

Limitations

The study focuses on the efficiency of hedges to improve urban air quality at street-level. While hedges are the most effective passive control measures at this level, trees remain effective air filters for urban areas at canopy height and should form part of wider measures to improve urban air quality.

Sources

Abhijith, K. V., & Kumar, P. (2019). Field investigations for evaluating green infrastructure effects on air quality in open-road conditions. Atmospheric Environment. doi:10.1016/j.atmosenv.2018.12.036 

The London Economic. (2018) Hedges Along the Side of the Roads are the Best for Reducing Air Pollution. Retrieved from https://www.thelondoneconomic.com/news/hedges-along-the-side-of-roads-are-the-best-for-reducing-air-pollution/06/01/

BBC. (2019). Wood Burners and Open Fire Places Face Restrictions in New Clean Air Plan. Retrieved from https://www.bbc.co.uk/news/science-environment-46823440