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Sustainability 67: Green Walls
An increasingly popular variation of the more well-known green roof concept is that of the green wall. In a green wall, vegetation is encouraged to grow on a vertical surface, rather than being restricted solely to the ground or to a horizontal or sloped plane.
Green walls show particular promise in urban areas, where ground areas or horizontal surfaces may be somewhat limited. Within more densely developed and populated areas, green walls can also offer valued visual relief from the otherwise hard or sterile environment.
In cities, green walls can also bring about reductions in the typical urban heat build-up of our modern age. The hard and dense surfaces of city roads, buildings and rooftops continually absorb the heat of solar radiation, holding that heat for some period of time, and then re-radiating it into the atmosphere. This creates an urban ‘heat island’ effect, where cities persistently remain much warmer than surrounding rural areas, leading to substantial energy use for conditioning of living and working spaces.
In contrast, the plants employed in green walls do not gain as much heat from solar radiation, and also convert much of it to photosynthesis and growth. Via transpiration of water, a layer of plants also tends to cool any substrate or backing materials, as well as cool and humidify any passing airflow. Green walls can also sometimes be employed as a filtering medium for reclaimed greywater.
There are several methods of creating a green wall, ranging from the do-it-yourself and small scale, to the sophisticated and large scale. At the simple end of the spectrum, the green wall makes use of a loose growing medium, similar to any gardener’s soil and nutrient mix. That loose medium may reside in a ground level planting bed that allows ivy or climbing roses to ascend a wall, for example. It may also be established along a number of shelves or ledges, or contained in troughs, to allow clambering plants to ascend higher still. As the medium is loose, with no provisions for water retention or nutrient replacement, such low-end green walls require substantial continuing maintenance, with replacement of major components every few years.
Mat growing mediums provide an incremental improvement. By incorporating growth medium, nutrients and even some water-retention properties in several thin layers of substrate, such mats sustain plants for a longer duration with less maintenance. Since plant roots tend to interweave the mats, such systems can also better span gaps and resist heavier winds or perhaps even seismic shocks. Though mat growing mediums lend themselves to both exterior and interior use, they may require a system of water recirculation to be truly viable.
Structural growth mediums provide the best and most sophisticated overall performance. Usually formed as incremental blocks, structural mediums incorporate all of the best features of mat mediums, plus the potential additional benefits of greater water control, pH-correction, and the ability to tolerate intentional building airflow through the green wall. In such a high-end installation, the green wall thus becomes what is known as a living wall, as it becomes part of the building’s overall breathing system.
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