Greenwalls or greenwash?
We all want to see more living material in our cities. But unlike green roofs, which are universally acclaimed, there is controversy surrounding the value and cost of green walls. We look at the benefits they offer, and at the downside.
Jill White runs her landscape architecture practice in Devon, specialising in community projects and accessibility issues. She is a member of the LI Editorial Advisory Board.
I have to say I am disappointed by green, or living, walls. I am aware that they are able to bring new areas of biodiversity to erstwhile sterile environments, and I also know that green walls can reduce pollution and provide planting in spaces where this could not otherwise be possible. But I fear that design practitioners in both architecture and landscape architecture can easily fall into the trap of feeling they must specify them in order to appear contemporary and cutting edge. There is a danger of thinking that the next project simply has to feature a green wall and finding ways of making one fit, rather than offering clients a cheaper and better solution through other landscape design solutions, such as using wall climbing systems; planted walls or gabions, for example.
Green walls depend on a massive range of inputs, all of which have ongoing carbon implications and are far from green. They need life-support systems which use electricity for pumped irrigation systems; manufacture of large-scale plastic plant pocket structures; frequent input of chemical feeds; regular use of pesticides and selective herbicides, not to mention all the CO2 created by the landscape operatives who have to travel to site to maintain these features on a frequent basis.
Green walls are expensive to install and maintain and, if any element of this cosseting fails, the result is dead patches and ugly gapping. This makes for a good business model for installers of green walls, as there will be endless and ongoing future repeat business. In some cases this may be worth the expense and effort, but in too many cases clients may not even have been aware of the lower-input alternatives available. Use of clever climber planting could have produced similar biodiversity and visual benefits, with a minute fraction of the ongoing financial and carbon inputs and specialist maintenance requirements.
Are clients who insist on having this latest fad always aware of all the future implications of owning green walls? Perhaps they have no clue that with their scheme it might have been entirely feasible to plant a range of extremely well suited climbers straight into the ground – especially if it was for
a new building and the planting bed designed in at the start? This is where clients should be able to see the advantage of using a landscape architect, who can steer them away from expensive mistakes and advise on low-impact solutions that are sustainable in the long run.
We have recently seen a good example of professionals in our field seizing this challenge, with Nigel Dunnett’s garden at Chelsea this year (University of Sheffield and the Landscape Agency for the Bank of Canada). His green wall for a rooftop garden used drought-tolerant species in recycled clay pipes, creating a low-maintenance, low-water-use green wall with a high aesthetic and hibernacula value. This is where we should be heading, using these features in a much more intelligent way; sustainable in the future and continuing to look great with minimal expense and CO2 inputs and having the same (or better) biodiversity and anti-pollution benefits. Let’s see fewer off-the-peg, greenwash solutions to awkward planting situations and more clever, interesting, beguiling and truly green answers to our clients’ problems. This is a great opportunity to demonstrate to them why they really need a landscape architect on board with every building scheme.
Gary Grant is an ecologist who works as an independent consultant and as a director of Green Roof Consultancy.
One of the major advantages of vegetating buildings, whether through the use of green roofs or living walls, is that the building is shielded from the elements. Ultra-violet light damages the exterior fabric and infra-red is absorbed into the structure, only to be re-radiated at night (contributing to the urban heat island effect). With a living wall, the building is shielded from the sun, but there is also the cooling effect of evapo-transpiration boosted by irrigation.
Bernhard Scharf and his colleagues at the University of Natural Resources and Life Sciences in Vienna have recently completed a research project, looking at the building physics of a living wall on one of the City’s municipal buildings. Their results confirm similar findings by March Schmidt at the Technical University in Berlin, which demonstrate the extent of summer cooling (for example http://bit.ly/12mwJmD).
What has been surprising is that the living wall in Vienna has kept the building warmer in winter, with unprotected facades up to 7oC colder than the vegetated equivalent. Even in Austria, where buildings are well insulated, this translates into significant energy savings in both summer and winter and helps to create a more amenable micro-climate in the immediate neighbourhood.
A frequent criticism is that living walls consume too much water. But the new wave of products use emitters on irrigation lines to deliver precisely calibrated quantities of water to the growing medium or mineral wool within each module. Some living walls now use less than 1 litre of water per square metre per day.
It is now common practice to collect rainwater to irrigate living walls and there are even firms using grey water (water collected from showers, baths and sinks), including for example the Babylon living wall company in Tarragona, Spain http://bit.ly/19qz2rH. Another way of avoiding the consumption of potable water is to divert downpipes into living walls, which have integrated storage tanks, a technique being pioneered by the Green Roof Consultancy working with TreeBox. The first rain-garden living walls were installed in May 2013 by Team London Bridge as part of the work being funded by the Drain London initiative. This, in effect, is a living wall that is a vertical rain garden or sustainable drainage feature.
Where water is pumped into a living wall, a small amount of energy is required, typically equivalent to the electricity used to boil a kettle each day. This can be taken from photovoltaics erected elsewhere on a building and is in any case an investment because of the energy saved. The amounts of energy whether embodied or required to operate a living wall, are insignificant when compared with the construction and operation of the building on which they sit.
The dense vegetation that can be established on living walls has been shown to clean the air by intercepting soot and absorbing gaseous pollutants. Most of the plants that have been shown to perform well in this regard by Kyle Shackleton and his colleagues at Imperial College for example – see http://bit.ly/18xuEaH – can be incorporated into living walls, and Biotecture’s living wall outside of Edgeware Road Station has been funded by Transport for London specifically to help improve air quality – see http://bit.ly/11vPYUE. Finally, living walls can be planted with native species or species with a documented value for wildlife (for example like the RHS list of plants for pollinators) providing habitat in places which might otherwise be barren and helping to ensure that the built environment contributes to the restoration of biodiversity, which supports the ecosystems on which we all depend for our existence. Every little helps
Andrew Thornhill is a director at Churchman landscape architects.
Ever since the years of botanical research by Patrick Blanc began to grace the walls of our cities, there has been debate about the value of the vertical garden. Comparing highly artificial systems such as the hydroponic variants with climbers planted in the ground doesn’t add much to the debate, as they each have their merits. Given the rapid advance of green wall systems and the potential for further development, including integration with dynamic building facades, I suspect the comparisons will look increasingly irrelevant.
In the context of the dense city, where conditions often do not allow roots access to ground water or offer suitable facades to cope with the invasive nature of aerial roots, living walls can offer an alternative. They have great potential for increasing biodiversity, which can be achieved without any loss of amenity space, a valuable commodity of inner cities.
In accessible locations, green walls can serve the edible landscape agenda very well, as the regular attendance to productive gardens allows the level of monitoring that green walls typically require. We designed an edible green wall using a soil-based system to an elevated courtyard for City University, allowing fresh herbs to be picked for the adjacent café.
Green walls also reduce particulate pollution, with hairy leaved species typically capturing significantly more than smooth foliage species such as Ivy.
Urban heat gain will arguably become more important in the planning of our cities. So while I do not advocate everyone installs a green wall in combination with other green infrastructure, they can provide significant cooling benefit to the environment. Irrigated systems provide greater benefit during extended periods of high temperatures as they maintain their cooling effect when most needed, an effect that is often overlooked with thinner substrate living roofs. Water consumption is an important consideration but should be balanced against the associated benefits that living walls can bring to the city environment.
Most of these systems are relatively thirsty, with the thinner mat-based soil-free systems consuming the highest amount of water. This is often exacerbated by the exposed locations they are used in; the wind causes greater evaporation and transpiration and can lead to soil loss over time. But with careful positioning, use of water recycling and thicker systems this can be much reduced. There is also growing research into drought tolerant systems; Roisin O’Riordan’s student thesis, recognised at the LI awards last year, demonstrates the potential for certain species to offer drought-tolerant variants established on high porosity walls. Simple robust alternatives that might be described as extensive systems are likely to be developed over the coming years to complement the largely intensive types currently available.
The challenge to the profession is how to best use the various systems. We have successfully installed a number of different systems, both soil and hydroponic based, and only recently have had any difficulty with the establishment phase. This arose from a combination of split contractual responsibilities and extreme environmental conditions, factors that can impact on any landscape. These systems are artificial environments and as such there is less room for error, but they do not require significantly more input than a well-managed garden would do.
Not all forms of horticulture can or should be managed with a strimmer or hedge cutter; some need a bit more effort; I thank Patrick Blanc for so dramatically adding to the richness of our city walls and look forward to the evolution of the green wall in our city fabric.