Walls covered in moss are popping up in major cities, along with promises that they can help reduce air pollution – but can a few square metres of plant matter really tackle the smog?
Berlin-based firm Green City Solutions believes so. Its moss walls, called CityTrees, are roughly 4 square metres in size. It says they can filter up to 80 per cent of pollution particles out of the air, including the tiny ones linked to respiratory and cardiovascular diseases.
The walls collect rainwater, which is pumped through an irrigation system to water the plants. Solar panels power fans to increase airflow. As a result, the firm says its product filters 3500 cubic metres of air an hour, which is the equivalent of the breathing air of 7000 people.
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Around 50 CityTrees have been installed in European cities in bus stops and busy streets where people are exposed to harmful particles emitted from passing traffic – one of the biggest sources of air pollution. The European Commission is interested in the idea and so is funding a dozen moss walls in Berlin over the next year. Each CityTree costs around US$60,000.
Alison Haynes at the University of Wollongong in Australia and her colleagues recently looked at how effective moss and trees are at absorbing pollution. They found that moss was up to four times better at trapping particles than the Australian native tree, Pittosporum undulatum.
“Mosses are like a ragged carpet, so there are lots of little spots where little particles can get caught and trapped,” says Haynes. Because moss has no roots, it gets minerals through its leaves, absorbing them from the air, meaning it also traps pollution particles, such as heavy metals, in its tissue.
This doesn’t mean moss walls will necessarily protect people from pollution at busy bus stops, says Zoran Ristovski at the Queensland University of Technology in Australia. In a small room, a moss wall only needs to filter a fraction of new air each hour, but tens or hundreds of times the volume of air is pushed past by buses, he says. He says the moss walls are unlikely to make any difference.
This view was backed up by a study of CityTrees by researchers at the Netherlands Organisation for Applied Scientific Research. This found that eight walls installed in a busy street in Amsterdam failed to reduce the concentration of particulate matter and nitrogen dioxide. The report concluded that doubling the number of moss walls would do little to improve their effectiveness.
Different solutions will be needed depending on an area’s layout, says Ruby Michael at Griffith University in Australia. Sometimes planting trees in urban canyons where streets are flanked by tall buildings can backfire, she says, because they reduce airflow – and so moss walls may be a better alternative.
Cities are unlikely to rush to replace their trees just yet. “It’s important to remember that street trees provide a whole host of other benefits including refuge and habitat for urban wildlife, shade and cooling for people on the street, reduction of urban heat islands, which is important in our warming urban environments,” says Michael.
