Feature, Outdoor

Two-minute explainer: Lighting for living walls

The living wall at West London's Westfield shopping centre. Picture: London_Lime via Flicr .
The vertical living wall at The Athenaeum Hotel on Park Lane in London.

Living walls are becoming a regular feature of architectural projects. Westfield Shopping Centre in West London boasts one of the longest green walls in the UK and the Athenaeum on Park Lane has one of the tallest. In Rochester in New York state there is a living wall in the atrium of the Golisano Institute for Sustainability, which although inside, relies entirely on natural illumination.

So what happens when we build living walls within buildings where there is no access to natural illumination?

Put simply, plants will not survive without an appropriate amount of light as poor illumination slows photosynthesis. Photosynthesis requires light levels of at least 1000 lux for plants to thrive, with levels of 2500 lux often being required.

A living wall on the outside of a building will experience levels above 30,000 lux. It is not practical to assume that kind of illumination can be achieved indoors, so localised lighting needs to be managed to ensure that the necessary illuminance level is achieved.

LEDs are already making their way into horticultural lighting, particularly when it comes to farming fruits and vegetables. In such situations, white light is not an issue and the LED spectrum is designed to optimise growth.

Living walls are a decorative feature and have to be treated in a different way. Coloured lighting is not appropriate and it is necessary to use a wider light spectrum to present the living wall in an attractive way.

We can typically expect to see 6500K LEDs being used to nourish the greenery of a living wall. If the wall was created to provide leaf cover, then a blue-enriched light is appropriate.

Where a living wall includes flowering plants it will be necessary to further enhance the lighting by introducing a red peak into the colour spectrum. Before LEDs, that job was often done by tungsten filament sources, with their high red content.

It is still early days for LEDs and living walls, but we should expect to see the ‘horticultural’ version of RGB systems becoming available that will offer ‘tunable’ spectra to facilitate plant health.

But if plants need light, they also need dark. Like every living organism, plants need their rest time. It is known as ‘photoperiodism’. This can be difficult in spaces that operate throughout the day and night, such as hotel entrances.

This can be managed by reducing illumination levels beyond the switch-off point for photosynthesis. The range of a photoperiod on a specific wall can be determined by the plant species.

These are early days for internal living walls and the experiences of the horticultural sector will be vital in providing an analysis of how plants can best be illuminated. The journey has started; it will be very interesting to see how things develop.