Attributes

As a green infrastructure component, green walls have the potential to deliver many ecosystem services. They have experienced a revived interest since the creation of the Living Wall Systems approximately 20-25 years and are now quite familiar in urban areas.
Green walls, as with any other green infrastructure, create a semi-natural habitat when used in an urban environment. Irrespective of their size, structure or vegetation composition, they provide visual amenity. When free-standing, by structuring open areas they provide intimacy. Against a wall, they have the ability to hide ugly features or prevent damage such as graffiti.
Along with other infrastructure, Green Walls have the potential to address the following issues:

They can reduce temperatures in urban areas and act as insulation on buildings.

They can act as a noise barrier when installed along roads.

They can reduce air pollution.

They can mitigate water run-off as well as reduce storm-water flows as part of a sustainable urban drainage system. Green walls on a building can be watered by the run-off from the building's roof.

They can increase urban biodiversity.

Microclimate Mitigation and Urban Heat Island reduction

Several studies have proved the potential of green walls to mitigate the weather through passive heat island reduction and thermal regulation of buildings. The mitigation is due to four mechanisms: (i) the ability of plants to intercept solar radiation through their shading, (ii) the effect of evapotranspiration by plants that can extract heat from the surrounding air, (iii) the thermal insulation provided by the vegetation (and by the substrate and structure in the case of living wall systems) and by the air gap between the vegetated structure and the building, and (iv) the alteration of the wind effect on the building.

The ability of plants to improve a building’s microclimate has been well investigated, focusing at first on green façades and hedges, and, more recently, on living wall systems. In the 1980’s, one of the first studies looked at the use of wisteria, vine and ivy as solar control and established the inverse correlation between solar transmittance and ivy growth conditions; thus highlighting the potential use of green walls for thermal insulation.

Urban hedges and green screens, through their freestanding status, were studied for their thermal mitigation effect on the microclimate of the street canyon. As other green walls, they have significant functions of shading, lowering temperature, increasing humidity and modifying wind. Their efficiency will depend on the type of plant species and on the rooting media; e.g. mulch, acting as insulation, re-radiates more energy than other media like turf. Hedge presence (shrub cover or tree and shrub cover) was shown to reduce temperature in a built environment by at least 1°c, comparable to the modification of ambient temperature by living walls and green façades (from 1 to 4°c,), thus reducing the urban heat islands (UHIs).

 Due to the complexity of thermo-dynamic transmission process, studies usually focused on the development of computer models exploring the effect of a vertical greening system on a building and in a street canyon. They showed that the best efficiency of green walls for thermal mitigation will depend on locality, climatic elements and wall aspect but that the leaf density (expressed by the Leaf Area Index), which affects the amount of shade produced, and the evapotranspiration from the plantation are also important.

Experimental and simulation studies were mainly done in the Mediterranean area or in tropical climates; the main aim was usually to establish how green walls could assist in cooling building in summer rather than reducing heat loss in winter. Thus, green walls were showed to provide a significant cooling effect on the building surface, reducing the peak temperatures in summer. But in addition to the shading that reduces solar gains to the building and reduces heat flow into the building through passive cooling, green walls were also shown to be able to decrease heat flow losses and hence improve the energy efficiency of buildings.

Noise attenuation

Noise annoyance, especially along road corridors, is a major issue in urbanized areas and noise barriers have become ubiquitous features along busy roads. Their efficiency and integration into their surrounding appears to be enhanced by the presence of vegetation, as plants (especially leaves and stems) scatter high frequency sound waves, which have been shown to have a significant effect on human health.
Along with green roofs, green walls have been investigated for their acoustic effect, although to a smaller extent. Urban hedges were shown to have a significant effect in lowering noise, especially when composed of both shrubs and trees due to the multi-layered structure.

Air quality improvement

In urban areas, green walls, as well as other vegetated elements, have been investigated for their potential role in reducing air pollution, through particulate filtering. The dust-filtering ability of a plant is directly correlated with the foliar surface characteristics, the size, the hair density on the leaves and the quantity of lead waxes. With a large collecting surface area, green walls can play a great role in improving air pollution, especially as they promote vertical transport by enhancing turbulence. By assessing particulate abatement capacity (PAC), it has been found that hedgerows can provide an efficient barrier against road dust and can reduce particulate matter by 30% to 40%. Hedges can remove concentrations of total suspended particulate (TSP) and PM10 by up to 40-50%. The efficiency of vegetation to mitigate particulate pollution appears to depend on the density of planting, the plant canopy density, porosity and size, and the leaf morphology. As such, shrubs and hedges appear to be more efficient than trees (especially conifers) for dust-retention.
In addition to the particulate size, the chemical composition of the trapped particulates is of interest, especially in terms of heavy metals, as it has significant effect on human health. Having a green façade is an easy way to improve air quality as climbers like Parthenocissus tricuspidata (Japanese Creeper) are passive accumulators of heavy metal aerosol pollutants.

Stormwater management

Green roofs have been extensively studied for their ability to manage urban stormwater. Following the trend, similar studies have been made on green walls.

Urban Biodiversity

Green walls have the potential to contribute to the improvement of urban animal biodiversity by creating habitat, food sources (e.g. for wintering birds), corridors, nesting sites, etc. However, little work has been done on this topic. Previous work published in German (see Köhler, Barth, Brandwein, & Gast, 1993 and reference therein), showed that green façades (either with ivy or grapevines) can be colonized by 19 different taxa of invertebrates from Araneae and Diptera to Diplopoda and Siphonaptera. Although important work has been done on the animal biodiversity of green façades in Germany and of green roofs (Grant & Lane, 2006; Kadas, 2011; Madre, Vergnes, Machon, & Clergeau, 2013), findings are either difficult to access for non-German speakers, or not transferable to green walls. Recently,a 2-year study on green façades showed the value of green walls for urban birds. More birds were recorded directly on the walls or in their close surroundings, than in the exact same environment without vegetation on the wall (Chiquet, Dover, & Mitchell, 2013). Another work has studied the snail populations of green façades, showing the influence of seasonality and foliage on the relative abundance of species (Chiquet, Dover, & Mitchell, n.d.).

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Contact

Christine Dover
IESR
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Staffordshire University
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e: c.j.dover@staffs.ac.uk