As the connected problems of climate change and air pollution from traffic make living in cities more hazardous, the health benefits of greening our streets become ever more apparent.
On crowded city roads green barriers, in the form of dense hedges between traffic and people on the pavements, dramatically reduce the danger from fumes, but are difficult to squeeze in.
Green roofs and walls, which take up no ground space and were originally introduced sometimes for aesthetic reasons, are proving that they have great value. They insulate buildings and lead to energy savings, reduce temperatures outside by absorbing sunlight, and mitigate the urban heat island effect. They also contribute to storm water management by soaking up the rain, and reduce noise and increase urban biodiversity.
But the latest studies find green roofs and walls are also valuable for removing air pollution; they can reduce nitrogen dioxide by up to 35%, and make 50% cuts in the small particulates that damage lungs.
However, in some street “canyons”, where trees are surrounded by tall buildings, pollution can be concentrated as foul air gets trapped below the canopy. Local weather conditions, especially humidity and wind, also make a big difference to the success rate of green barriers.
Generally though the research shows that there are immediate benefits from having all sorts of plants growing in cities. Some evergreens with sticky or hairy leaves that capture pollutants, which can then wash off in the rain, are particularly effective all year.
But can they cope with the pollution? Finding the plants that can thrive in all of these stressful conditions could require special horticultural skills.
