Can green walls help tackle pollution health risks?

It is one of the world's most prolific killers. Air pollution causes more than seven-million deaths globally every year, according to the World Health Organization.

A CityTree installed in Glasgow. Image: Green City Solutions
A CityTree installed in Glasgow. Image: Green City Solutions

That amounts to one-in-eight of total global deaths, is more than double previous estimates and confirms air pollution is now the world's largest single environmental health risk.

"Reducing air pollution could save millions of lives," says Denes Honus, who hopes to chip away at that figure in Glasgow. Last month his Berlin-based company installed two large green walls. Each "CityTree" is 4m tall, 3m wide and 2m deep, and is said to have the environmental benefit of up to 275 urban trees. Glasgow is the first UK city to take CityTrees, each one costing about £20,000.

If CityTrees are new to the UK, green walls are not. What is new is the thinking on what plants in green walls are best for tackling pollution. Green City Solutions uses moss cultures with a larger leaf area than any other plant and can capture more pollutants. Honus insists each CityTree removes 12.2kg of dust and other matter, nitrogen dioxide and 240 metric tonnes of CO2 annually.

"The function of the CityTree is based on biotechnology — a special moss culture that has the ability to attract air pollution from its surroundings," he explains. "It then converts it into its own biomass. Thus the moss literally eats air pollution. Supported by internet-of-things technology, the air pollution reduction is traceable, while the CityTree is free of regular maintenance."

Honus adds: "Every CityTree reduces local air pollution in a proximity of 50m by up to 30%. Compared to 275 normal trees, a CityTree is 95% more cost effective and requires 99% less space on the ground. The mobile installations have also popped up in Oslo, Paris, Brussels and Hong Kong. Our ultimate goal is to incorporate technology from the CityTree into existing buildings."

Urban tests

Udeshika Weerakkody, a Sri Lankan PhD researcher based at Staffordshire University, is also looking at green walls and hedges, and has been testing different urban situations — in Stoke-on-Trent for roads and Birmingham for rail pollution. The project focuses on the environmental values — thermal insulation and climate mitigation as well as particulate pollution — of green walls.

The university's Green Wall Centre has been working with UK companies such as ANS, Biotecture, Treebox and Mobilane, and its director, Professor John Dover, has written a book on the benefits of green infrastructure. For her most recent project work, Weerakkody has also tapped up the expertise of Cascade Gardens and Czech Republic company Nemec, which have trialled systems.

In Stoke-on-Trent, for example, Weerakkody has used 20 different species, from pyrus to geranium, beside a busy road. At Birmingham New Street station, meanwhile, she has been looking at how effective plants such as Buxus are at controlling rail pollution. She is experimenting with green walls of single and multiple species as well as hydroponic and soil-based walls.

"In Birmingham the smaller-leaved species capture more particulates because the leaves create more turbulence — better for taking dust out of the atmosphere. It's too early to be sure, but we have found complex leaf clusters — small, hairy or waxy — are better at tackling pollution. But it's not just about right species. It's about the right design, so we are looking at the form and structure of green walls."

Unusual results

Also looking at pollution control is Biotecture director Richard Sabin, who has worked with Imperial College and come up with unusual results. In tests, green wall staples such as hairy plants and ivies did not perform as well as others. Sabin looked at "near surface roughness" of leaves and found dense, mat-like foliage tends to deflect and bounce particulates back into the environment.

"But plants like ferns and grass, with more complex structures, generally do a better job at controlling particulates," he says. "Plants that formed a mat did not perform as well, so ivy, which has some benefits, is by no means a panacea. The green wall industry is small and needs help from academia and Government bodies like the Building Research Establishment to improve understanding."

But while the sector needs more blue-sky thinking, Biotecture has recently applied its green-sky thinking for a four-storey green wall along Southampton Row in London. A major planning condition was to cut air pollution and Sabin's team used Pachysandra terminalis, Euonymus fortunei 'Dart's Blanket', Vinca minor 'Bowles's Purple' and Heuchera 'Green Spice'.

Mobilane and Hedera Screens recently installed a 72sq m modular system called LivePanel at the family-owned Village Bakery in Wrexham. The system is based around exchangeable plant "cassettes" with cups that hold a range of evergreen perennials and ferns, ducting and water reservoir. The plants absorb water via a capillary system while the foliage controls air pollution.

Mobilane national business manager Paul Garlick says: "Modular systems have revolutionised the use of plants, allowing the basic concept of the living wall to be applied to a range of uses and situations. Recent advances in the technology of modern living walls enable rapid and easy installation, while delivering low maintenance and a large degree of design flexibility."

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