Return to Westfield
BY ALEXANDRA STEED and WARREN OSBORNE
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
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
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.
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
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
Alexandra Steed is senior director, landscape architecture and Warren Osborne is a director at AECOM.