04 Mar THE SUNDAY TIMES – Feature
An in-depth look at how clever planting can improve air pollution in our cities.
As we pound the pavements in the heat of high summer, air pollution is more apparent than ever in our cities. It’s made up of dust, soot, heating and exhaust fumes, and a host of other noxious by-products of urban life, all swirling around our streets in the form of tiny, poisonous particulates.
We already know that pollution causes heart and lung disease, cancer, asthma and bronchitis, and new research suggests it may also be responsible for some cases of Alzheimer’s, depression and a decline in brainpower. On July 31, the government will publish its plan to improve air quality — but you can take action in your own garden. lants are our best defence against air pollution: trees, shrubs, hedges and other greenery trap particulates in their leaves before we breathe them in. But can we really plant our way to a healthier future?
Mark Laurence creates green walls across the world as head of Mark Laurence Design, a Chichester-based company that specialises in sustainable landscapes. He has long been a champion of the environmental benefits of planting, and wishes other garden designers were equally progressive. Yet he fears that for many of his peers — and their clients — “aesthetics come first and environmental impact last”.
So how do you design an urban garden that will combat air pollution?
Placement
Green defences should be placed closest to the source of pollution, according to Prashant Kumar, professor and chair in air quality and health at the University of Surrey. In urban areas, this means planting hedges or other green barriers as near roads as possible.
In a front garden, a 4ft- to 6ft-high hedge bordering the pavement is ideal, Kumar says, and will reduce pollution by more than 60%. You could also place shrubs behind it. They will help, but the roadside hedge is key.
Trees are the next layer, as they can be effective barriers to pollutants. Professor Barbara Maher, co-director at Lancaster University’s Centre for Environmental Magnetism and Palaeomagnetism, found that planting a row of young silver birches in front of houses on a busy street reduced air pollution by about 60% in those homes.
That said, much as we want them on our streets, trees can clog up airflow when wrongly placed, leading to a build-up of pollution. The worst case is when street trees rise up above house height and their canopies join over a road, creating a green tunnel. The pollution then has nowhere to go. “If I could do one thing, I would pollard all the big trees on city streets,” Maher says.If there are a lot of large trees on your street, using more in your front garden is probably not a good idea. If you have relatively few street trees, widely spaced, then planting trees with an upright form and narrow crowns between a hedge and the house can be effective.
Don’t encircle your property with trees, though, as that will risk stagnant air, warns Alessio Fini, professor of arboriculture at the Department of Agricultural and Environmental Sciences at the University of Milan. In rear gardens, away from traffic, you can plant more densely, but make sure air can still circulate freely.
As for other plants, a clipped lawn has no value in tackling air pollution. Tall plants such as grasses help a little. Green walls and climbers won’t help much with airflow, but they have the benefit of being a potentially dense vegetation layer that catches pollutants: up to 35%, according to some studies. Green roofs will cleanse the air, too, but, given their location, won’t directly benefit the environment inside your house.
What to plant?
Choose species that will grab particulates and hang on to them until they can be
washed away by rainfall. That means plants and trees with a high leaf density — the more leaves, the more they catch. Think twice, however, about using dense hedging such as yew or leylandii if your garden is exposed to high winds. These can become almost solid barriers, and particulates will hit them and crash over into your garden. Another thing to bear in mind is that some otherwise effective species, including birch, emit pollen, which can cause hay fever and breathing difficulties for those with asthma. Others, especially oak, produce volatile organic compounds when stressed, which can be dangerous to health.Plants with hairy leaves (such as silver birch, Betula pendula; whitebeam, Sorbus aria; or lamb’s ear, Stachys byzantina) or waxy leaves (ivy, Hedera helix; Chinese juniper, Juniperus chinensis; or cherry laurel, Prunus laurocerasus) are best. And, as air pollution is a year-round problem, it makes sense that your first line of defence — the roadside hedge — should be evergreen.
Urban greening, done well, is a win-win solution, says John Dover, emeritus professor of ecology at Staffordshire University. It’s not hard to imagine greener, cleaner cities of the future, and the benefits to public health. Dover also makes the important point that we shouldn’t be too focused on air pollution, but should consider the other values of urban planting, which include shade, insulation, food and habitat for wildlife.
Tijana Blanusa, principal horticultural scientist at the Royal Horticultural Society, reckons we can all do our bit to help and get planting. “Put something green in to start with,” she says. “Plonk it in!”