Blog #01 Living walls 10.02.15

New Lives: New Vertical Landscapes
Terry McBurney

 


 

Living walls are ornamental planting systems used primarily to create a visual spectacle at prestigious locations such as hotels and shopping malls. Evidence is accumulating which suggests they could be adapted to provide new sustainable forms of urban order that are absorbent of social and ecological processes.  Meeting this aspiration would require investment in development of technologically effective and economically efficient processes. The results could lead to improved public and environmental health and new opportunities for artistic expression and for development of a new ‘green’ service industry.  Potentially, the benefits to society would be immense.

INTRODUCTION

Living walls provide a root substrate, water and nutrients required to sustain plant growth on inaccessible faces of buildings or other structures. First popularised by Patric Blanc , early types of living wall consisted of absorbent textile with built-in pockets for root substrate draped over the façade of a building and irrigated by percolating water through the textile.  Newer solutions consist of pre-assembled modular plastic or metal containers which allow more controlled distribution of irrigation as well as greater scope for artistic plant arrangements. Living roofs are technically simpler structures than living walls and generally lack irrigation.

Living walls serve as visitor attractions that project an environmentally conscious image and stimulate retail sales.  Living roofs are less visible and have a more functional use in enhancing thermal efficiency of buildings and providing other environmental services such as storm water storage and habitats for wildlife. Both living walls and roofs offer a possible alternative to conventional green spaces currently managed by public authorities who, increasingly, are constrained by the requirements of economic austerity. 

Access to green spaces is regarded as one of the most important life-quality indicators for urban populations but urban population growth and densification can reduce the amount of urban green space to an extent that it might warrant action to mitigate the loss. In the UK, nine out of thirteen cities surveyed showed losses of urban green spaces, which was associated with policies to limit urban sprawl. Possible ways to mitigate these losses are of interest to professionals “who work, manage or design the urban environment and for whom public well-being is a key concern”.

Much public research on the benefits of living walls has placed emphasis on attenuation of traffic noise, adsorption of air-borne pollution, creation of wildlife habitats, improvement of buildings thermal insulation and retention of storm water.  Conversely, the requirement for irrigation often results in unwanted environmental emissions from drainage and worsening of any local water supply shortage. However, a key advantage of living walls are that they are generally found in highly exposed locations at street level, which can be harnessed for psycho-social and other health benefits. 

Economic importance

Living roofs have demonstrated sustained market growth of around 20% per annum for almost a decade, largely due to regulatory stimulus that has encouraged their use on new building projects in both the public and commercial sectors.  Market data for living walls is lacking but similar growth rates are expected in future even if few government incentives exist for their procurement.  Many cities have an established supply chain consisting of commissioning architects, landscape designers, plant-raisers, materials suppliers, system installers and maintenance contractors, some of which have become specialised businesses or divisions of large landscape companies. The cost for a living wall is around £750 /m2 to erect and £25/m2/year to maintain, which is roughly equivalent to land values in the Greater London Area excluding central London. Corresponding costs for planting and maintaining areas of grass turf (£30/m2 and £1.5/ m2/year respectively) are considerably lower but it is generally impractical to convert urban land to grass turf. Conversely, most cities have ample vertical areas for plantation with living walls and, conceivably, could be considerably reduced in price with a settled system design and a major programme of procurement.

Funding models

Alternative forms of support may be appropriate such as private investment, crowd source funding, hypothecated taxes, stakeholder investment from utility companies, road construction and maintenance budgets, national and European level support for green infrastructure and non-budgetary support through building regulations and environmental accreditation schemes. However, potential conflicts may exist between these some of these stakeholders, for instance where private investment might wish to discourage loitering or activities unrelated to a corporate purpose

Eco-sociological benefits

Proximity to green spaces

Health benefits normally associated with proximity to green spaces should, in principle, also apply to living walls. They include reduction of long-term noise annoyances and stress-related psychosocial symptoms, reduced mortality from circulatory diseases and increased survival rates of urban senior citizens. The main beneficiaries appeared to be the elderly, the youth, and secondary educated people, partly related to outdoor activities and healthy modes of travel. Stress relief from even short-term visits to nature has been objectively demonstrated with reduced salivary cortisol and mere exposure to views of nature improved health and well-being by providing restoration from stress and mental fatigue or improved feelings of neighbourhood safety and has also been associated with decreases in aggression and crime rates.

Air quality and carbon economy

Urban air pollution is one of the most serious causes of respiratory disease and avoidable death in Europe with an estimated 400,000 deaths occurring during 2011.  The problem is chiefly due to microscopic particles and volatile chemicals emitted by combustion of carbon fuels used in domestic heating and in transportation, which is exacerbated by the topography and population density of city environments. Urban densification is a planning response to population growth that can benefit air quality by reducing journey lengths and encouraging commuters to walk instead of using transportation; but urban densification is also detrimental to public well-being as it concentrates air pollution sources and increases pressure to build on vital urban green spaces.  Vegetative cover is known to capture atmospheric pollutants which are broken down to harmless components in the leaf canopy, roots, root substrates and rhizosphere micro-organisms.  Vegetation on living walls has been shown to reduce level pollution at street level, at least for particulates (PM10) and nitrous oxides, when wind-speeds are low. However, living roofs act across the skyline boundary layer with little consequence at street level.

Biodiversity

Green roofs can greatly extend the otherwise limited opportunities for establishment of ecological restoration areas in the built environment in order to improve urban biodiversity, particularly when it involves citizen participation. Limited evidence suggests that green roofs can provide valuable wildlife habitat to mitigate losses due to redevelopment of abandoned sites and that species often arrived accidentally but could be deliberately encouraged to colonize. Living walls can also be surprisingly diverse and have potential to be ecologically engineered to encourage a greater diversity and range of species. The requirement is for a better understanding of the interactions between its ecosystem elements, especially the relationships among growing media, soil biota, and vegetation, and the interactions between community structure and ecosystem functioning.

Place attachment

Another positive role of living walls is likely to be place attachment, which is the concept that positive human bonds can be formed to social and physical settings that support identity and provide other psychological benefits. They provide opportunities for artistic expression and it is notable that designs often draw inspiration explicitly from the impressionist art movement and related concepts such as the crisis of modernity, the loss of connection to rural life and the associated unity between individuals that define and constitute a community. A key concept in the emerging field of ‘socially restorative urbanism’ is the boundary between the built fabric and the adjacent open space, which is precisely where living walls are located. These boundaries, termed ‘transitional edges’, serve an important function in urban design by providing ‘porosity’ between the public and private space.  Specific roles for living walls might be to soften the form of a large dominant block and segment different ‘territories’.  Although living walls require professional inputs and resources they may foster local empowerment through expression of local community preferences in their design and choice of site and perhaps also in correcting earlier errors in urban planning. 

Technological constraints and opportunities

The serviceable life of living walls may be required to match that of the building they cover, possibly up to 60 years; though typical rates of replanting imply the design life for individual plants is 10 years.  Catastrophic loss of living walls can occur from irrigation failure, as notably occurred at Paradise Passage, London and is exacerbated by the low root container capacity of most living walls.  In the longer term organic root substrates, such as peat or coir, tend to decompose and develop sub-optimal hydraulic properties. More durable alternatives are therefore used, including processed minerals, waste from brick manufacture, or hydroponic cultivation systems based on hydrophilic mineral wool and polymer foams.  These substrates generally have low buffering capacity for nutrients, which therefore must be added in controlled quantities to the irrigation water while avoiding build-up of harmful salts.

Gradual deterioration of plants in a living wall can be due to a failure of module design, maintenance or plant species selection.  A major problem is maintaining adequate root moisture through irrigation, which is generally based on gravity water flow through vertical tiers of plant compartments, which causes considerable variability of moisture between plant compartments. Consequently, irrigation tends to be applied in excess so as to ensure adequate moisture is maintained in the upper tiers, resulting in wasteful drainage and waterlogging in the lower tiers.   The range of suitable species may be restricted to those which can withstand such prolonged water-logged conditions.

Ideally, the timing and amount of irrigation applied to each plant should match the plant transpiration demand, which is partly determined by the local meteorological conditions such as solar radiation and wind-speed. However, use of standard methods for determining irrigation demand from meteorological information using the Penman-Monteith equation is generally precluded by the confounding effects of partial shade, reflected heat from pavements and variations in wind speed or turbulence and leads to invalid assumptions.  So-called ‘smart’ electronic irrigation controllers that have become popular in the US landscape industry are similarly prone to this limitation.

An improved method of irrigation management is required to save management time and to increase the reliability of irrigation, save costs and lessen the risks of ownership. A promising approach is to continuously monitor substrate moisture status with electronic sensors and to use these in conjunction with an appropriate computer algorithm to control irrigation automatically. Such electronic monitoring and control of irrigation can earn accreditation under industry sustainability criteria such as the Building Research Establishment Environmental Assessment Method (Section WAT 04: water efficient equipment, BRREAM, 2011). Unfortunately these guidelines lack information on appropriate use of sensors, which requires specialist knowledge. 

In particular, a method is needed to determine the appropriate number and placement of sensors required to ensure representative measurements so as to avoid either inaccuracies or prohibitive costs for instrumentation, which would otherwise involve considerable trial and error. A promising approach is to use measurements from just a few sensors to calibrate a computer model (such as Hydrus 2D/3D) that simulates the moisture flow and use it predictively to develop and test an irrigation control algorithm. Moisture measurements from sensors can be integrated with irrigation controllers and accessed remotely via the internet and either used to inform periodic manual adjustment of irrigation settings or fed into a control loop for a fully automated control system.

An alternative approach is to develop a plant module design that distributes irrigation intrinsically according to plant demand.  Such modules can incorporate an integral water reservoir and capillary flow pathway for hydrating the substrate in which the flow naturally adjusts according to the water content of the substrate, tending towards optimum root moisture conditions if hydraulic characteristics of the substrate and hydraulic head are correctly selected. Module designs based on this approach have been realised in at least one commercial system developed by Mobilane Ltd, Stoke-on-Trent, UK and others are in development. Another promising approach is to use controlled plant water deficits to reduce plant growth, pruning requirement and hence maintenance costs.   Partial root dehydration is a method in which the roots system of each plant is divided between two compartments which are alternately subjected to cycles of drought and irrigation. The method could save water as well as producing a more attractive and compact plant canopy, potentially eliminating or reducing the need to prune.

Aerial micro-environments could also be passively controlled, which would provide opportunities for designers to create interesting and spectacular living walls using species adapted to different habitats. Modular planting systems could also be developed specifically for use as bio-filters to improve air quality, which would be effective for all sources and types of air pollution. A particularly dangerous form of respiratory pollutant is atmospheric smog consisting of finer particulates (PM2.5) generated by reaction between fuel combustion products and agriculturally derived ammonia. Conceivably this problem could be tackled at source by developing living roof bio-filters for vented emissions from poultry houses and other intensive meat production systems that are major sources of atmospheric ammonia.

Conclusions

Opportunities exist for advancing research and technological innovation in the design and application of living walls with a focus on potential contributions to urban sustainable living. The evidence indicates the need for a tripartite conceptual framework consisting of interdependent social, ecological and technological dimensions. Technological innovation is central to meeting the requirement for practical delivery of such sustainable living solutions through urban infrastructure. In particular, the need is for a means to manage irrigation in timely and economically efficient ways and to develop better guidelines for planners and commissioning organisations regarding urban restorative use of living walls and roofs.

Other key questions to be addressed by research include:

  1. To what extent do observed beneficial health effects of green spaces apply to living walls or address the underlying determinants of urban health?

  2. How could living wall modular design and configuration be optimised for abatement of air pollution?

  3.  What is the local optimum plant species composition to support biodiversity?

  4. What is the relationship between planted area and diversity of associated species?

  5. How does connectivity between different plants areas affect species biodiversity?

  6. How might government instruments be best used to encourage living wall construction?

  7. To what extent could economies of scale and a settled design of planting system reduce costs of procurement?

 

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