We are becoming an urban race. According to the United Nations, by 2050 70% of us (6.3bn people) will live in cities.

But our cities aren’t currently healthy places to live. Air pollution is responsible for around 3.2m premature deaths every year. That’s 5% of global premature deaths. This figure is expected to rise to 4.8m by 2050.

Air pollution is made up of dust, soot, heating and exhaust fumes, and a host of other noxious by-products of urban life that swirl around our streets in the form of tiny, poisonous particulates. We already know that air pollution causes heart and lung disease, cancer, asthma and bronchitis. New research is suggesting that it may also be responsible for some cases of Alzheimer’s disease, as well as depression and a decline in brainpower.

Plants are our best defence against air pollution. Trees, shrubs, hedges and other greenery trap particulates in their leaves, cleaning the air that we breathe. But we simply aren’t using plants enough, or properly.

As a garden designer, I’m increasingly asked by schools and private clients to devise planting schemes to limit air pollution. The questions come thick and fast. ‘Do we just need to plant lots of trees? All round the house?’ ‘How about green walls?’ ‘Do the neighbours need to do the same?’ It’s a confusing picture, partly because the research into how urban greening can be used to tackle air pollution is in its infancy and nobody has yet produced a detailed ‘how-to’ guide.

To try and find an answer, and for a piece in the Sunday Times, I’ve spoken to researchers across Europe who are conducting the latest trials in this field. We’re a few years away from clear guidance, but already a general picture is starting to emerge.

Here’s what you need to know.

1. Green Defences

Lets break down our green defences into eight areas – Trees, hedges, shrubs, plants, lawn, green walls, climbers and green roofs.

While some elements are more efficient than others, all the researchers agree that a planting plan that uses many of them in combination is best.

2. Placement

The findings of Professor Prashant Kumar, Professor and Chair in Air Quality and Health at the University off Surrey, (echoed amongst all the experts I spoke to) is that green defences should be placed closest to the source of pollution. In urban areas this means placing hedges or other green barriers as near to roads as possible.

(a) Hedges – In an urban front garden, a 1.5-2m high hedge bordering the pavement is ideal, says Kumar, reducing pollution by over 60%. You could place shrubs behind the hedge. They’ll help, but remember the roadside hedge is key.

(b) Trees – Trees are the next layer of your green defence. They can be very effective barriers to pollutants. Professor Barbara Maher, co-Director at Lancaster University’s Centre for Environmental Magnetism & Palaeomagnetism, found that placing a row of young silver birches in front of houses on a busy street reduced air pollution by around 60% in those houses.

But trees are tricky. Placed wrong, they can clog up airflow in city streets, 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 to create a green tunnel. All the pollution then has nowhere to go.

‘If I could do one thing, I’d pollard all the big trees in city streets,’ says Maher.

If there are a lot of large trees on your street, using them in your front garden is probably not a good idea.

If you have relatively few street trees, widely spaced, then placing trees with an upright form and narrow crowns between hedge and house can be effective, says Gallagher. But it’s key to ensure there is good airflow in the garden.

Don’t completely encircle your property with trees, warns Dr Alessio Fini, Professor of Arboriculture at the Department of Agricultural and Environmental Sciences in the University of Milan, as that will risk stagnant air.

(c) Lawns and grasses – A clipped lawn has no value in tackling air pollution. Plants, such as tall grasses help a little.

(d) Vertical performers – Climbers and green walls have the benefit of being a potentially dense vegetation layer, but one that won’t interfere much with airflow. They will certainly help catch pollutants, up to 35% in some studies. Green roofs will equally cleanse the air, but given their location won’t directly benefit the environment inside your house that much.

(e) Plants and shrubs – In rear gardens, far away from traffic, you can plant more densely. But do still allow air to circulate.

(f) Water – fountains, mist and general moisture in the air from planting literally washes away particulates.

3. Which Species?

You want to choose species that will grab particulates and hang onto them until they can be washed away by rainfall.

That means plants and trees with high leaf density. The more leaves, the more they catch. But be careful of using very dense plants as hedging (for instance yew or leylandii) if your garden experiences high wind. These can become almost solid barriers and particulates will hit them and crash over into your garden, almost like a breaking wave.

Also be aware that some effective species, like birch, emit pollen. Others, especially oak, produce volatile organic compounds when stressed, which can be dangerous to health.

Plants with hairy or sticky leaves are best. And as air pollution is a year-round problem (and often worst in winter), it makes sense that your first line of defence – the roadside hedge – is evergreen.

Right now, all the research teams are trying to work out the best species to use, both now and in the future. There is no definitive list, but here are some hedging, shrub and tree species that perform well in trials:

Hedging – Elaeagnus x ebbingei/angustifolia, Taxus baccata, Ligustrum japonicum, Laurus nobilis, Hedera helix, Cotoneaster franchetti

Shrubs – Spiraea japonica, Sambucus nigra, Syringa meyeri ‘Palibin’, Sorbaria sorbifolia, Pinus mugo,

Trees – Gingkgo biloba, Betula pendula, Gleditsia triacanthos, Sorbus aria, Crataegus monogyna

4. Good Stock, Well Planted

Cities can represent extreme environments for plants. They are generally hotter than the countryside, more prone to flooding, more exposed, and more polluted. Roots have to battle with drains and concrete. The average life expectancy of new trees in cities is as little as ten years, a tenth of that in the countryside.

So, it’s absolutely vital, when planting up a city garden, to create adequate planting pits and use only the best possible stock from reputable nurseries.

With so many new pests and diseases affecting plants, and our changing climate adding increased stresses, it’s also important to have a broad mix of species in any scheme.

Urban greening, done right is a ‘win-win solution’, says Professor 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 and wellbeing they bring. Dover also makes the important point that we mustn’t get too focused on air pollution, but also consider the other values of urban planting such as shade, insulation, food and habitat for wildlife.

Air pollution is a rising health crisis in our cities. It’s my hope that we will soon enter a period in which will see government, councils, researchers and garden designers pulling together to clean city air.

Every city house and school and office has different air pollution challenges. There’s no catch-all solution. For a professional job, I review the site, the surrounding area and – if in doubt – run it past a scientist before creating a scheme.

I’d be delighted to help, of course. It’s incredibly satisfying designing a scheme that both looks good and protects the client and their family. But, if you’re doing it yourself, I’d start by planting a hedge…