Planting for Cleaner Air

Is your premises being polluted by nearby traffic?  

 

 

In our towns and cities, traffic is the main source of harmful air pollutants such as nitrogen dioxide gas and particulate matter. The most effective way to improve air quality is to remove the polluting vehicles and increase active and sustainable travel behaviour. However, urban planting may compliment such behaviour change initiatives as an additional way of reducing air pollution levels in your grounds. The right green infrastructure in the right place can reliably reduce exposure to air pollution1.

Here is a brief introduction to ways in which planting green infrastructure may improve air quality.

 

Vegetation can help reduce the impact of air pollution on our health and environment.
At a regional and national
scale, 
trees and plants have been found to improve air quality by absorbing gaseous pollutants through respiration and capturing particulate matter as deposits on their leaves2. At a street level, plants can control the flow and dispersion of pollutants and act as barriers or 'green screens' to separate people from air pollution being emitted from traffic. Green screens or hedges have been found to as much as halve the level of pollutants immediately behind the barrier3

Vegetation and green infrastructure offer important additional benefits including providing habitats to enhance biodiversity, shade and cooling, less flooding by the capture of storm water, reduced noise pollution and attractive surroundings for outdoor activities. 

 

What type of green infrastructure should you install to improve your air quality?

 

 

Care needs to be taken when deciding which type of green infrastructure would work best in your location, and consideration given to maintenance and other infrastructure issues. The closer the planting is to the source of the pollution the more effective it will be at capturing pollutants before they disperse into the air. 

 Questions to consider:

  • Where are the nearest sources of traffic-related air pollution, such as busy roads and areas where drivers idle their engines such as dropping off and crossing points?
  • Where are people on your site spending most of their time outdoors and exposed to increased air pollution? 
  • How will you evaluate if your new green infrastructure has made a difference? Could you measure the air quality before and after your planting project?

If you are interested in carrying out an air quality investigation, email jean.macgrory1@hants.gov.uk to find out if you are eligible to participate in the My Journey Clean Air Project. 

 

What type of design will be most effective?

Your designs should feature plants suitable for the location and be easily managed and maintained over the long term.

The most effective planting for dispersing air pollution will be different for street canyons than for open roads, and according to the size of the area to be protected4.

Find out if there are any local community groups, such as allotment or gardening groups, who might be happy to share their gardening expertise and support your efforts to improve the air quality.

The cost of different planting schemes will vary significantly depending on the age and size of plants purchased, and the type and volume of materials required. Reducing costs will invariably result in having to wait longer for the plants to become established and effective in their 'greening' properties.  

Consult with all stakeholders to ensure that they are happy with the designs to avoid future problems.

 

Trees and hedges:

Trees are a vital weapon in the fight against the climate emergency as they 
remove atmospheric carbon through photosynthesis. Planting trees will help the UK meet its national targets to be carbon net zero (remove as much carbon as we produce) by 2050 by increasing its woodland cover from the current 13%5. Trees capture large quantities of pollutants and provide a wide range of physical benefits, as previously mentioned6. Planting trees of varying sizes and shapes increases the deposition of particulates as more air turbulence is created2.

Trees, shrubs and hedges planted beside roads can provide effective barriers to protect people from air pollution. Effective green screens or barriers should extend from the ground to a height of at least 2m and be as dense and thick as possible4. Trees planted close to classroom windows can reduce air pollution and provide cooling shade that can save energy from decreased use of air conditioners in summer months.  Evergreen species will provide year-round protection from pollution while deciduous trees will only give protection when in leaf.  Nest boxes may be installed to increase biodiversity.

 

Green screens:

In sites where it isn't possible to plant directly in the ground, raised beds or planters, 
with trellis or mesh panels, may be used to create a green screen of climbing plants.

Information on how to build a raised bed may be found here.

Portable planters may be used to change traffic flows, relocate parking and to improve and sign-post alternative walking routes.

If soft ground is available, climbers may be planted directly into the soil beside existing fences to create a more natural green screen. Other plants may also be planted at the foot of the climbers, and/or bug hotels installed to increase the biodiversity of the scheme. Planting green screens provides opportunities for incorporating food crops into the design to encourage healthy eating. Click here for information about producing ‘vertical vegetables’.

 

Modular living walls: 

Where there are space and sub-structure constraints, living or green walls may be installed as barriers between pedestrians and vehicles.

The picture shown here is Southampton City Council's living wall at Millbrook Roundabout in Southampton.

Living walls usually consist of a panel of pockets or containers that may be attached to an existing structure or building. They require irrigation systems to water from the top down through the growing medium. They are expensive and need higher maintenance. When green walls are installed against buildings it is not yet clear how effective they are in removing pollutants from air4. Plants grown on buildings may help save energy by cooling the structure in warm seasons, reduce noise (by reducing echoing on hard surfaces) and protect building structures from damage by weathering.

 

What type of plants are best for improving air quality?

Factors to consider when selecting your plants:

  • Are they hardy and able to withstand air pollution and all-weather conditions?
  • Are they drought tolerant?
  • Are they safe – do they have sharp thorns, or will they aggravate pollen-related allergies?
  • Will the plants damage buildings or structures?
  • Will they be visually attractive?
  • How fast do they grow? Fast growing is best for green screens and hedges, but slow growing is ideal for modular schemes.
  • Are the plants suitable for the specific local ecology and conditions?

Plants that will effectively disperse or deposit particulate matter have hairy leaves, large leaves or grooves on their leaves7. Some examples include:

           

 Yarrow                     Wallflowers                  Common Ivy                    Firethorn

 

The 'Urban Air Quality Tree Score (UTAQs) developed by the University of Lancaster8, ranks trees according to their properties for reducing air pollution. Some of the trees that have the highest UTAQs include:

                  

     Alder                       Black Pine                        Hawthorn                      Field maple

                 

 Grants for trees and hedges:

The Tree Council Branching Out Fund has funds available to assist schools and community groups proposing to undertake well-planned tree and hedge planting projects this winter (November 2020 - March 2021), preferably during National Tree Week (28 November - 6 December 2020). They will fund 100% of the cost (excluding VAT) of strong applications from £300 to an increased limit of £1500.  https://treecouncil.org.uk/take-action/grants-for-trees/

The Woodland Trust are giving away hundreds of thousands of trees to schools and communities. They have two delivery periods per year, one in March and the other in November and are currently taking applications for trees to be delivered in March 2021. https://www.woodlandtrust.org.uk/plant-trees/schools-and-communities/

 

Educational opportunities associated with planting for cleaner air.

Outdoor learning and play activities in green spaces have been linked to cognitive development9 and improved physical and mental health10. Current Covid-19 restrictions may make educational visits more problematic. Maximising the use of school and college grounds will be an invaluable way to teach pupils outside of their classrooms and enhance their education and development. 


Designing and planting schemes to improve air quality offers an opportunity to raise awareness of environmental issues and how they may be tackled. Through these projects, students may develop new practical and gardening skills, and learn about nature and native plant and animal species.

Outdoor classrooms may be created using trees, shrubs and climbers that will improve air quality and create shade and shelter so that they may be used all year round.

A planting project may be used to support accreditation for the Eco Schools green flag award, the John Muir Award or as part of the Hampshire County Council Trailblazer outdoor learning scheme.

See our educational air quality resources on the My Journey Hampshire website.

 

References and sources of useful information:

1Role of trees and other green infrastructure in urban air quality. E Ferranti et 2019                              

2Air Quality. Green Infrastructure. A Toolkit for Schools. Groundwork

3Impact of green screens on concentrations of particulate matter and oxides of nitrogen in near road environments. Environmental Research Group, King's College London February 2015

4Using Green Infrastructure to Protect People from Pollution. London Mayor's Report April 2019.

5 Woodland Trust, We plant trees.

6Top 22 benefits of trees. The Tree People.

7 Phytosensor Toolkit. Museum of London and Citizen Sense Research Group. 2017

8Trees and sustainable air quality. University of Lancaster. 

9Green spaces and cognitive development in primary schoolchildren, Dadvant P, Sunyer J et al, June 2015 PNAS;

10Improving access to greenspace. A new review of 2020. Public Health England