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AECOM has carried out extensive research, three years after the completion of a substantial living wall, to see how it has fared. The research highlights an inherent conflict between a wish for biodiversity and aesthetic appeal.

One of Europe’s longest green walls is located at the Westfield Shopping Centre in west London. Designed by EDAW /AECOM, this 170m long, 4.5 metre high, living wall feature has, for the last three years, provided a visually striking landmark that stands between a retail mall and local homes. In addition to providing an attractive, green feature from both sides, the original design intent for the wall was to improve local biodiversity.

The living wall incorporates a modular system constructed from plastic panels. The system is essentially a tray system turned on its side, with plants growing in a soil medium composed specifically for the plant species contained. The plants providing a total of 1275 m2 of primarily native woodland mix to the north side and sun-loving plants to the south side are watered from the top down by a weep hose irrigation-type system.

But this article isn’t meant to bore with the details of the design. Following a brief description of the wall, we will focus on whether or not the design has fulfilled its objectives, three years on. Living walls are still a pioneering technology, and much is to be learned through trial and error. This article considers the success of the Westfield London living wall in terms of the original design intent. It is based on site visits conducted over the last three years, interviews with people involved in its care and maintenance, and by observations made by ecologist Dr. Martina Girvana. It concludes with a summary of ‘lessons learned’ — what worked and what could be improved upon in future living wall projects.

About the wall

AECOM explored a number of living wall systems and selected a simple plastic modular system for two important reasons: plants could be grown offsite, thereby producing a mature effect from day one; and, panels could be easily repaired or replaced if necessary. The modular system was developed by Canadian firm ELT. Each panel is 500mm high by 500mm wide and 65mm deep, subdivided into 45 cells for soil containment. To secure the panels and provide fixing locations, a steel A-frame was constructed along the entire 400m length.

The plant palette was composed primarily of native species, a woodland mix on the north side, and sun-loving plants on the south side. However, at implementation, a decision was taken by the contractor to swap a number of natives for ornamentals due to availability. Originally a proportion of non-natives were included to provide splashes of seasonal colour, texture and form. Nine plant mixes were developed in swathes along the length of the wall to maximise year-round interest, with each panel inter-planted with up to five different species.

A top-down weep hose drip system irrigates the wall about 150 times a year, with a flow rate of 3 litres/m2/cycle. It is a largely automated system with sensors in the substrate that send text messages to a maintenance company if the wall becomes dry or if a fault is apparent. Liquid feed is applied through the irrigation system twice a year.

What we learnt

Since the completion date, over three years ago, the team has visited the living wall a number of times, collected comments from the client and staff of Westfield London, and conducted interviews with maintenance staff and suppliers. Importantly, we also had a vegetative survey and invertebrate appraisal completed by AECOM ecologist Dr. Martina Girvan. She included a comparison of the species assemblage described as conceptual stage versus implementation stage and the maintenance stage; what persists, what was added, and which species had colonised.

The surveys provide a snapshot only, recording incidental sightings rather than providing a systematic survey. AECOM site visits and Westfield staff reports confirm that the north-facing wall, visible from the retail mall, has remained green, with fortnightly maintenance, and is enjoyed by customers and staff alike.

Any irrigation issues have been solved quickly, and failing plants have been replaced, albeit by ornamentals rather than native species. Reports suggest that customers enjoy sitting by the living wall, and that it has become a great attraction at Westfield London.

The wall’s south side, facing the residential area, has suffered from access issues, less frequent maintenance and greater sun exposure. Irrigation by weep-hose is uneven, due to faults in the irrigation line, and the wall edges in particular have suffered. Some sections that have failed have not been replaced, and where they have been replaced, again plants tend to be ornamental rather than native. Thus the south-facing wall has not maintained a consistent level of attractiveness and greenness.

While the south-facing wall has received less care, invertebrate sightings suggest it has greater overall biodiversity value than the more ornamental north-facing wall. Drought-tolerant species, including ivy and sedums, dominate, but other species - ox-eye daisy, yarrow, various grasses - from a nearby wildflower area, have also colonised the south wall in areas of plant failure and decay. Irrigation failures caused a dramatic gradient effect, from drought-loving species at the top, to wetloving species at the bottom, providing a range of wildlife conditions.

Lessons learned

There is a conflict between creating a wall that is both attractive and green all-yearround, and one that is biodiverse. To achieve one aspiration, the other must be at least partly compromised. Technological advances in living wall construction and irrigation, combined with well-informed plant choice, make attractive green walls easily attainable these days. (For example, emitter pipes combined with capillary mats ensure more even water coverage than weep-hose systems, while deeper, tilted tray cells hold more moisture for individual plants.)

Achieving biodiverse living walls is more complicated. We have observed through this study that applying cultivated native plant species does not necessarily bring about biodiversity. Sufficient time and resource is required to research and select the plants. For example, fast-growing drought-tolerant plants should not be associated with less competitive species that will be overtaken in time.

Allowing colonization and succession by nearby native species is favourable and so too is seasonality, yet both will provide patches of brown and decay. Gary Grant, of Green Roof Consultancy Ltd., and original AECOM design team member says, ‘There is a problem establishing native species in natural associations to provide year-round uniformity of appearance’. Certainly, conditions that favour biodiversity can be at odds with what most people consider ‘attractive.’ Until the general population’s aesthetic changes, it is probably best to accept that one goal — either attractiveness or biodiversity — will take priority over the other.
Alexandra Steed is senior director, landscape architecture and Warren Osborne is a director at AECOM.

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