Study: Green Walls Can Strip Pollution from Air, Some Plants Do It Better than Others

Biologists at the University of Surrey have investigated inter-species variation in particulate matter accumulation, wash-off, and retention on ten broadleaf plant species, with a focus on leaf characteristics.

Green walls are vertical systems which are getting particular attention as they can be installed without occupying additional space at street level.

They also provide other benefits, such as reduced dependency on existing soil conditions and additional ecosystem services.

Green wall is a term that encompasses all forms of vegetated wall surfaces.

Green facades and living walls are the two types of green walls, where green facades usually comprise climbing plants whereas living walls include planting materials and technology to support a wider variety of plants.

Air pollutant reduction by green walls depends on several factors such as plant species, barrier dimension, leaf area index, humidity, wind speed and direction at a place.

“By planting vertically on a green wall, communities can clean up their air without taking up too much street space,” said Mamatha Tomson, a postgraduate researcher at the University of Surrey.

“Our study suggests that this process depends not only on the shape of its leaves but on the micromorphological properties of their surfaces.”

“We think a good mixture of species will produce the most effective green walls — and look forward to carrying out further research to see if we’re right.”

In the study, Tomson and colleagues planted 10 species, including the evergreen candytuft (Iberis sempervirens), the ivy (Hedera helix) and the wild marjoram (Origanum vulgare), on a custom-built 1.4-m green wall.

The evergreen candytuft and ivy leaves were found to be especially good at trapping pollutant particles, large and small.

Meanwhile, rain was able to wash most of the pollution off the hairy leaves of lavender.

The candytuft and marjoram also performed well in washing off smaller particles of pollution.

“We hope that town planners and infrastructure experts can use our findings to think more carefully about what they plant,” said University of Surrey’s Professor Prashant Kumar.

“Having a green wall is a great way of removing pollution — but what you plant on it can make a big difference to how successful it will be.”

The results were published in the journal Science of the Total Environment.

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Mamatha Tomson et al. 2024. Exploring the interplay between particulate matter capture, wash-off, and leaf traits in green wall species. Science of the Total Environment 921: 170950; doi: 10.1016/j.scitotenv.2024.170950