New technologies of robust soil moisture sensors that are linked to the internet have considerable potential utility in city landscapes for ensuring high quality ornamental gardens that are efficiently managed and environmentally sustainable.
The greatest impact is likely to be accelerating investment in ‘living wall’ projects that could eventually transform city landscapes and help establish living walls as a major green-tech industry.
Living walls are vertical gardens constructed from modular panels of ornamental plants and are increasingly common in city landscapes. They are mainly required for prestigious locations such as hotels, boutiques and public spaces where they create spectacular visual impacts evocative of the hanging gardens of Babylon.
They are similar in many respects to the less conspicuous ‘living roofs’ which are expanding by tens of millions m2 per year, assisted by planning regulations that promote environmentally sustainable buildings.
Living walls mainly benefit the environment by providing habitats for wildlife which can make an important contribution to biodiversity. They also improve the thermal efficiency of buildings by reducing summer heat loads and provide cooling effects and sound abatement on city streets for potentially important health benefits.
Crucially, however, living walls are not currently regarded as environmentally sustainable due to their high maintenance requirements and inefficient use of irrigation water.
The rapidly expanding market for living walls is indicated by the increasing examples of projects that are posted on the internet, with values ranging from £5k to several £100k.
Projects are often commissioned by tender and served by a range of subcontractors including plantsmen, landscape designers, installers/maintenance companies and by suppliers of planting modules, irrigation equipment and root substrates.
The main UK industry event, ‘Ecobuild’, recently featured five competing designs of modular planting systems and similar competition also exists on the European continent, North America, the Far East and Australia.
Living walls are commonly assembled from small modular arrays of plastic or metal containers filled with organic or mineral substrate that provide limited moisture capacity and support for roots of ornamental plant species.
Other designs serving the same functions are made from porous fabrics or rigid panels of porous material in which planting pockets have been formed.
There is generally no direct input from rainfall and irrigation is required to replace considerable amounts of water taken up through roots and lost by transpiration from the leaves.
Irrigation is applied to the top of the module and infiltrates or overflows through each row of pockets to the bottom rows where it tends to accumulate.
Moisture gradients may also arise if the module design permits a gravitational suction head between the rows.
Suitable plant species tend to be limited to those that tolerate boggy conditions and unsightly areas of patchy growth can often occur. Excess irrigation is eventually wasted along with nutrients through run-off into the drainage system from where it pollutes the environment.
Recycling untreated overflow through the irrigation system risks spreading plant pathogens and is not generally practiced.
Constant vigilance is required to ensure the restricted root systems receive timely and efficient amounts of irrigation and to ensure that systems are maintained and growth remains tidy.
The city environment presents challenging problems partly because transpiration varies with wind speed, which increases with the height of the module above ground with confounding effects also occurring due to air turbulence, heat reflected from pavements and shading from adjacent buildings.
Most irrigation control systems turn water on and off automatically using a timer, but these are unable to adapt to the changing needs of plants as they grow, or as evaporative conditions vary with position on the wall or fluctuate over time.
Contractors may adjust irrigation timer settings during inspection visits, but these are minimised in order to save running costs. Failures of mechanical or electrical components may go undetected between visits and have resulted in plant death and even abandonment of the investment, most notoriously in the case of Paradise Passage, London1. Irrigation needs to be adaptive to the changing needs of the plants and this is ideally achieved with soil moisture sensors used in conjunction with a set-point value that triggers an irrigation event and is optimised for the module design and plant species grown.
A sensor-based system can also be connected to the internet providing possibilities for early warnings of irrigation failures and also remote adjustment of the settings for irrigation amount, timing and trigger set-point to manage the growth environment.
A variety of soil moisture sensors are available2 but many are unreliable or too expensive for use in living walls. The majority of sensor types infer moisture from measurements of soil electrical properties which may be confounded by the effects of fluctuating temperature and dissolved nutrient salts that commonly occur in living wall environments.
Measurement sensitivity in the range required for irrigation is partly determined by the texture and moisture characteristics of the substrates which are variously based on soils, organic composts and solid or loose mineral mixtures. High frequency irrigation also requires a rapid sensor response.
Promisingly, miniaturised sensors based on a thermal dissipation principle can cover the whole range, either configured as a naked probe or embedded in a porous ceramic matrix.
Electronic units for implementing an automatic irrigation control system provide sensor excitation, signal capture, data processing and wireless data transmission via GPRS for remote display on a website, and also a relay output to switch an irrigation valve for a predetermined time when readings reach a pre-programmed moisture threshold value.
A simple approach uses an external pulse timer to trigger irrigation events which are then simply enabled or disabled by the sensor relay output. More complex algorithms for multi-zone irrigation control might be implemented with additional logic controllers ranging from a simple bespoke circuit to an industrial programmable device with multifunctional capabilities.
Future systems should allow flexible configuration to support multiple sensors and irrigation valve switching circuits, provide self-configuring wireless networking as well as web connectivity via GPRS or WLAN.
Sensor information can simply be displayed in graphic and tabular form on the web to confirm that irrigation is functioning normally, or to diagnose problems.
Warnings can also be transmitted to maintenance staff as a mobile text or email message.
To avoid overwhelming amounts of data from numerous large projects it should be possible to limit the display to irrigation triggering events and other operation statuses of the living wall. Software controls could eventually even include automatic fault detection agents and decision-support algorithms currently being developed for applications in agriculture3.
The location and number of sensors need to be optimised for the different module designs and possibly for each installation, which involves considerable amount of trial and error. The process can be accelerated by taking a few measurements and using these to calibrate a computer model that simulates the flow of water in the module compartments. However, module designs that have a highly non-uniform moisture distribution may nevertheless prove to have a prohibitively expensive instrumentation requirement.
Improved module designs are required that provide a more uniform moisture distribution, making each plant compartment more or less representative for the irrigation zone to be controlled and enabling a simplified sensor configuration that minimises the instrumentation cost. Such modules exist as computer simulation models and as physical prototypes that are currently undergoing calibration and verification trials.
New concepts of exploiting plant physiological reactions to water stress that determine plant growth and development can be exploited to substantially reduce both water use and plant pruning costs.
This involves restricting water supply to parts of the root system to stimulate production and translocation of plant stress hormones that inhibit growth and transpiration in the plant canopy.
Such precise control of moisture conditions can only be achieved by continuous monitoring with soil moisture sensors.
Optimum conditions need to be established for a wide range of ornamental plant species and fail-safe mechanisms need to be incorporated to ensure reliable operation under all conditions. Independent accreditation will also be required to confirm the compliance with sustainability criteria.
The developments will enable a broader range of plant species and hence a larger ‘palette’ of colours and forms for designers to use in creating even more stunning ‘living walls’.
Remote access to measurements and control settings via the internet further reduces the need for inspection visits and provide extra reassurance in protecting the valuable investments for planners and corporate customers.
The result could be a proliferation of living wall projects, leading to profoundly improved quality of life for city inhabitants. The expected improvements are most noticeably aesthetic but also include amelioration of noise and temperatures, indoor air quality and environmental sustainability.
Western Europeans suffer exposure to excessive noise, which is second only to air pollution as an environmental cause of ill health4. Plant canopies absorb more than half the energy of sound at middle and high frequencies, which is very effective in noise control for urban environments predominantly derived from road traffic5.
Action plans to manage the noise exposure now have to be drawn up under EU Directive 2002/49 due to be fully implemented by July 2013, and living walls could contribute to ensuring compliance.
Cities are generally warmer than surrounding countryside due to the urban heat island effect. This effect is reduced by intercepting heat which would otherwise be largely absorbed and re-radiated from building surfaces by converting it into latent heat of evaporation using vegetation cover. By this means living walls can reduce urban temperatures by as much as 8.4° C in warm, humid climates. Shielding the surface from ultra-violet light could also be an important consideration for some modern building materials.
Living walls contribute to air quality improvement by absorbing toxins, particularly volatile organic compounds such as benzene and n-hexane. Absorption occurs both in plant tissues and in the root substrates and this effect is exploited in some interior living walls, known as bio-walls, by actively circulating the room air through the plant canopy and roots.
Loss of biodiversity is one of the greatest anthropogenic threats on the planet and is partly due to habitat loss and the effects of climate change. Plants on buildings provide a food source for invertebrates which support a food chain of other invertebrates, bats and birds. They also provide a wildlife refuge and a breeding habitat for invertebrates and birds. One UK priority species to benefit is the house sparrow.
By providing shade and a certain amount of insulation, living walls can significantly reduce the external temperature of a building so that peak-cooling load transfer on warm day may be reduced by as much as 28%. Air movement is also reduced affecting heat gain/loss. In cool climates the energy costs are reduced by 23%.
Thus, living walls provide a valuable means of adapting cities to climate change by mitigating the effects of rising temperatures and providing refuges for wildlife. They also address the causes of climate change by improving the energy performance of buildings, which reduces CO2 emissions.
Importantly, they also considerably enhance human living conditions through noise abatement, air cleansing and breathtaking aesthetic impact.
Resolving problems of high maintenance costs and inefficient use of water will make living walls more acceptable to city planners and encourage similar regulatory support as that currently provided for living roofs.
This article has outlined solutions based on emerging technologies of robust soil moisture sensors in an automated irrigation control system that is also linked to the internet to enable remote inspection and management.
It has also identified technology improvements in the planting systems in order to minimise the instrumentation requirement and to enable new concepts to be implemented for reducing maintenance pruning, and further minimising water requirement by utilising plant reactions to water stress, with a further bonus from expanding the range of suitable ornamental plant species for creative designers. ?
1 http://www.architectsjournal.co.uk/news/daily-news/the-paradise-park-fallout- are-living-walls-worth-it/5208251.article 2 Terry McBurney (2010) Soil moisture sensors and applications.
AWE International June 2010, 20-27
3 http://www.water-bee.eu/ 4 http://www.euro.who.int/_data/assets/pdf_file/0008/136466/e94888.pdf 5 http://www.greenroofresearch.co.uk/acoustic/EuroRegio-262.pdf
Dr Terry McBurney is an environmental and research consultant and founder of McBurney Scientific Limited, who are experts in monitoring soil and plant water status. He achieved a PhD and an international reputation for plant water research at Horticulture Research International (now University of Warwick), and following two years of working in Papua New Guinea in a development context he served as farm consultant and research project manager at ADAS consultancy for land based industries.
McBurney Scientific Limited Malvern Hills Science Park Malvern, Worcestershire WR14 3SZ, UK
T: +44 1684585286 E: [email protected] W: www.soil-moisture.co.uk Title photo courtesy of Frosts Landscape Construction Ltd.
Published: 10th Jun 2011 in AWE International