What Is This Strange Tower?

The View From Here — 29 June 2026

A ‘CityTree’ — Photo by Author

Introduction

In Putney, south-west London, there’s an odd construction on the side of the High Street known as ‘CityTree’ (branded as one word). It’s the world’s first biotech filter to measurably improve the city’s air quality.

This technology filters air by combining living moss and ventilation to remove harmful, ultra-fine particulate matter and fine dust from the surrounding air. At the same time, it produces oxygen.

It filters the breathing air of about 7,000 people an hour (3.5 million litres). It’s been proven to improve the surrounding air by up to 53 per cent within a distance of 1.5 metres (~5 feet). Intrigued, I checked out more.

Why moss?

The variety of moss used has lots of fine hairs, which gives the air purifier a large surface area to absorb the polluted air. The cultivated moss metabolises the fine particles, effectively eating those particles.

Due to the vast surface area of the mosses, they store and evaporate large amounts of moisture. This cools the ambient air by up to 4°C (39.2°F).

The result? Puportedly fresher air.

How it works

Close-up from ‘CityTree’ — Credit: Author

Polluted, warm air is sucked in by the integrated fans at the base. The air then flows through eight moss mats and is cleaned and cooled to flow out higher up.

Sensors ensure that the moss stays healthy and constantly measure the performance of this bio-engineered novelty.

Below are the metrics given on the side of the ‘CityTree’:

Metrics — Credit: Author

London

Unfortunately, London exceeds the World Health Organisation’s limit for PM2.5 particles (ultra-fine particulate matter). This poses health risks associated with inhaling polluted air containing these ultra-fine particles (and nitrogen dioxide).

While London’s air quality is improving, we still have a way to go.

The London Mayor, Sir Sadiq Khan, is taking this seriously, and he’s doing something about it on the 70th anniversary of the Clean Air Act in the UK. As of 18 June 2026, the London Mayor announced a further £6 million to tackle air pollution through the Mayor’s Air Quality Fund for innovative projects to reduce air pollution.

Although the ‘CityTree’ innovation isn’t mentioned in the latest move to improve London’s air quality, it still contributes a little in addressing this problem.

The World

This technology is made in Germany, and there are ‘CityTree’ installations in Oslo, Paris, Amsterdam and Berlin. So far, this innovation seems to be mostly installed in European cities, although there are now ‘CityTree’ installations in Hong Kong, too.

Finally…

This is the first time I’ve seen the ‘CityTree’. Now I’ll have to keep my eyes open as I walk around London!

Have you seen a ‘CityTree’ near you?

Thank you for reading

People, who’ve asked to be tagged: Aidan Gerard Patrick McCarthy Lu Skerdoo

For The View From Here: Dawn Ulmer