Vertical Greening Systems and Sustainable Cities

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=cjut20 Journal of Urban Technology ISSN: 1063-0732 (Print) 1466-1853 (Online) Journal homepage: http://www.tandfonline.com/loi/cjut20 Vertical Greening Systems and Sustainable Cities Luis Pérez-Urrestarazu, Rafael Fernández-Cañero, Antonio Franco-Salas & Gregorio Egea To cite this article: Luis Pérez-Urrestarazu, Rafael Fernández-Cañero, Antonio Franco-Salas & Gregorio Egea (2015) Vertical Greening Systems and Sustainable Cities, Journal of Urban Technology, 22:4, 65-85, DOI: 10.1080/10630732.2015.1073900 To link to this article: https://doi.org/10.1080/10630732.2015.1073900 Published online: 28 Jan 2016. Submit your article to this journal Article views: 1131 View Crossmark data Citing articles: 12 View citing articles Vertical Greening Systems and Sustainable Cities Luis Pe ´rez-Urrestarazu , Rafael Ferna ´ndez-Can ˜ero , Antonio Franco-Salas , and Gregorio Egea ABSTRACT Urban development is causing environmental and social concerns that are compromising human welfare and the sustainability of cities. New urban greening concepts are being developed to mitigate these problems in a sustainable and natural way. Vertical greening systems can be defined as structures that allow vegetation to spread over a building facade or interior wall. These systems are becoming popular though they are still evolving and more knowledge on some of their particular impacts is required. In the last five years, the number of studies published in the scientific literature on this topic, especially involving living walls, has significantly increased. This scientific interest has corresponded with an increased and parallel attention by the general public. This work offers a broad description of the different systems and a comprehensive review of the particular benefits of these green infrastructures. Knowledge gaps and shortcomings have also been identified and discussed. KEYWORDS living walls; green facades; vertical garden; green wall; sustainable construction; urban greening Introduction Not long ago, cities were surrounded by broad extensions of rural areas. Nowadays, rural populations are declining while urban populations are continuously increasing, leading to a rapid expansion of cities. This massive urban development is altering the land surface by concentrating materials which effectively retain heat and create impervious surfaces, thus affecting urban local climate and urban hydrology. Moreover, tall buildings provide multiple surfaces for the absorption of solar radiation that is subsequently reradiated as heat, thus enhancing the efficiency with which urban areas are warmed up. This fact, linked to the waste heat generated by energy use, leads to temperature increases in cities known as urban heat island (UHI) effect (Environmental Protection Agency, 2008), a precursor of other problems ranging from human health issues to higher energy consumption due to the overuse of air conditioning systems (Bass and Baskaran, 2001). Furthermore, urban areas are known to be major concentrators and emitters of multiple contaminants resulting from human activities within the built environment (Diamond and Hodge, 2007). As a result, carbon dioxide (CO 2 ) and harmful toxins such as Volatile Organic Compounds (VOCs) frequently reach damaging Correspondence Address: Luis Pe ´rez-Urrestarazu, School of Agricultural Engineering, Universidad de Sevilla, ETSIA, Ctra. Utrera, km 1. 41013 Seville, Spain. E-mail: [email protected] Journal of Urban Technology, 2015 Vol. 22, No. 4, 65–85, http://dx.doi.org/10.1080/10630732.2015.1073900 #2016 The Society of Urban Technology atmospheric levels in some metropolitan areas (OMS, 2011). Some VOCs are known carcinogens and have been associated with sick building syndrome (Kostainen, 1995). Besides these issues, other environmental and ecological problems arise because of unsustainable urban development (e.g., limited green spaces, biodiversity loss, alteration of landscapes, etc.) (Frank, 2000). As a consequence, new building strategies and methods are needed to minimize the deleterious environmental effects of urban infrastructures while improving their social and economic value (Li et al., 2007). Urban greening initiatives are a way of achieving ecological goals while reducing the undesirable effects of urban growth (Gomez et al., 1998; Westphal, 2003; Currie and Bass, 2005). However, the development and expansion of cities has traditionally been guided by the criterion of maximizing the constructed area to increase profitability. This leads to situations in which enjoying open green areasis becoming a difficult task for dwellers of many cities. The use of the outer surfaces of buildings offers the possibility of increasing the vegetation presence in densely-built urban areas that have a minimum of traditional horizontal green spaces. As a result, planting on roofs and walls turns out to be an innovative and rapidly developing field towards sustainable and environmental construction (Bass and Baskaran, 2001). In addition, given that the potential area for vertical greening is significantly higher than the surface area available horizontally, the capacity to amend negative environmental issues seems to be greater with vertical greening systems (VGSs) than with green roofs (Emilsson et al., 2007). Therefore, the current interest in VGSs as means of improving urban environments is soaring (Ko ¨hler, 2008). The objective of this paper is to provide a state-of-the-art view on VGSs as well as a critical discussion of the potential of these novel green infrastructures to meet the goal of increasing sustainability of densely built urban areas. Vertical Greening Systems: An Overview Vertical Greening Systems (VGS) are also known as green-wall technologies, vertical gardens, or bio walls. They consist of vertical structures that spread vegetation that may or may not be attached to a building facade or to an interior wall. Attending to the level of complexity, there are several green-wall typologies that range from the simplest configuration to the most complex and high-tech design. Based on the type of vegetation and support structures used, these systems can be divided into two major groups: green facades and living walls (Kontoleon and Eumorfopoulou, 2010; Manso and Castro Gomes, 2015). In green facade systems, or green screens, the vegetation cover is formed by climbing plants or cascading groundcover (See Figure 1). Specially designed structures can be used to force the plant development through the building’s wall, which can serve as support for the climbing vegetation. Normally, green facades are rooted at the base in the ground or in plant boxes, but intermediate planters, fixed to the wall at a certain height or even on rooftops as a falling green cascade, can also be used. Due to the lower diversity and density of plants, green facades normally require less intensive maintenance and protection than living walls (Ottele ´et al., 2010). Living walls are generally more complex infrastructures that involve a supporting structure with different attachment methods. A waterproof backing is required to isolate the living wall from the building in order to avoid problems 66 Journal of Urban Technology associated with dampness. An irrigation network is also necessary while fertigation, monitoring, and lighting systems are optional. Cloth (or felt) and panels (or boxes) are the most commonly used vegetation attachment methods to the supporting structure (See Figure 1). Within the panels system there exists the possibility of planting in situ once the structure is attached to the wall or using pre-vegetated panels that are prepared before planting in specially designed structures. In each of these cases, the growing medium is allocated inside the pocket (if cloth system is used) or the panel. The systems that use either organic or inorganic growing media along with mineral nutrients to grow plants outside the soil are also known as soilless or hydroponic systems. In some particular configurations, plants are not rooted on growing media, but the roots are directly attached to the cloth, becoming a singular hydroponic system that depends on the irrigation reliability. Overall, the materials used to build vertical structures are highly variable and may support a great variety of plant species (Loh, 2008). Regarding placement and installation, VGSs are usually located outdoors and fixed to the exterior wall of the building. Nonetheless, they can also be built in the interior of the building envelope (termed as indoor living walls) though, in that case, some considerations regarding indoor humidity, lighting, and plant species must be taken into account (Ferna ´ndez-Can ˜ero et al., 2012). Even though living walls were initially inspired by the epiphytic plant growth in the tropical forests, over time, it was found that a wide range of plant types could adapt and grow suitably on a living wall. Plants used on living Figure 1: Vertical greening systems: categories and examples Vertical Greening Systems and Sustainable Cities 67 walls can be epiphytic (growing on other plants for mechanical support, deriving moisture and nutrients from the air, rain, and sometimes from debris accumulating around it) or lithophytic (growing on rocks or stony soil and deriving nourishment from the atmosphere). Ferns, succulents, shrubs, and herbaceous and climbing plants can also be used. Over and above ornamental species, there are many successful experiences of planting edible herbs and vegetables to create vertical gardens of lettuce, mints, thyme, strawberries, etc. (Weinmaster, 2009; Utami and Jayadi, 2012). Species selection should not be based solely on esthetic considerations, but should be based on multiple factors such as the system of cultivation, microclimatic conditions, and sun exposure. For example, in an indoor living wall, plants are subjected to very different conditions from those found in an outdoor living wall, therefore the plant species used are usually different. Thus, in any indoor living wall project, regardless of the geographical location, it is common to find species of subtropical and tropical origin, e.g., Anthurium scherzerianum,Codiaeum variegatum,Chlorophytum comosum,Ficus pumila,Nephrolepis exaltata,Spathiphyllum wallisii,orSyngonium podophyllum. On the other hand, species used in outdoor living walls vary greatly depending on location, as well as sitespecific microclimate (sun and wind exposure, height, etc.). In this way, in a south-facing living wall located in the Mediterranean region, species such as Allium spp., Asparagus sprengeri,Dianthus barbatus,Felicia amelloides,Lavandula dentata,Russelia equisetiformis, and Tulbaghia violacea are often found. Traditionally, the green wall has acted as a “passive” bio filter, but new approaches and technologies are moving towards the integration of living walls (both indoors and outdoors) within the building’s air conditioning and ventilation systems. The result is called “active living wall,” in which an air current is forced to pass through the green wall and collected afterwards so that the recycled fresh air can be supplied to the building’s interior as the air has been cooled, filtered, and humidified by the plants and growing media. Benefits Researchers are becoming increasingly aware of the potentiality and positive effects that urban greening initiatives play on numerous aspects of the urban environment. Over the last 25 years, many efforts have been made by the scientific community to shed light on the multiple ecological, environmental, and social benefits of green roof systems (Getter and Rowe, 2006; Takebayashi and Moriyama, 2007; Yang et al., 2008; Getter et al., 2009). This interest has extended over recent years to VGSs to provide responses and further knowledge on their particularities as compared to other urban green infrastructures. What follows is a comprehensive review of the particular benefits of VGSs, focusing on their ecological, environmental, and social implications. Ecological Implications Urban areas are known to exhibit, in most cases, lower biodiversity than the preurban ecosystem (Pauchard et al., 2006). The need to preserve biodiversity for the continuance of our species and the fact that the land devoted to protected areas is insufficient to prevent biodiversity loss (Laurance et al., 2012) has led to the development of new ecological concepts and ways to preserve Earth’s biodiversity. In 68 Journal of Urban Technology this context, reconciliation ecology (Rosenzweig, 2003) aims to modify and diversify anthropogenic habitats to support a greater range of species without compromising land use. VGSs, as well as green roofs, represent a good example of the practice of reconciliation ecology, as these infrastructures create habitats for plants and animals on land that is being directly used by humans as living space (Francis and Lorimer, 2011). They enhance urban biodiversity and thus the urban environment (Whitford et al., 2001) by allowing spontaneous vegetation to colonize these systems (Dunnet et al., 2008). As they can mimic natural vertical habitats such as cliffs or vegetated waterfalls (Madre et al., 2015), they provide habitats (Brenneisen, 2006) for birds and insects responsible for numerous functions and ecosystem services, such as pollination or biological control (Madre et al., 2015). They may also be used to grow species that can address specific functions that are missing in the urban environment, mostly oriented to plants’ ability to remove air pollutants (Francis and Lorimer, 2011). Species such as Adiantum raddianum,Chrysantheium morifolium Dieffenbachia spp.,Dracaena godseffiana,Epipremnum aureum,Hedera helix,Marraya sp., Nephrolepis exaltata,Philodendron sp., Rhododendron obtusum,Sansevieria trifasciata,Spathiphyllum maunahoa, and Vriesea splendens have that capacity (Loh, 2008; Weinmaster, 2009). But, despite the number of existing studies performed on the ecological implications of green roofs, observations focused on living walls are still anecdotal. It is, therefore, desirable that new and specific wall-oriented trials are conducted to disentangle the real ecological potential of living walls as well as the optimal design details that allow enhancing urban biodiversity. Environmental Implications The use of plants to improve the quality of their surrounding environment is becoming a key design consideration in modern building developments that seek not only an esthetic impact but also a means of providing environmental services, such as natural cooling of air, water filtration, and mitigation of several environmental problems (Ushada et al., 2007; Birkeland, 2009; Ushada and Murase, 2009). In the case of indoor living walls, a building’s interior environment may benefit from the reported positive effects of vegetation on air quality (Wolverton and Wolverton, 1993; Raza and Shylaja, 1995; Todd, 2005), which are not only related to the plants’ ability to adhere particles from air but also to their efficiency (along with microorganisms found in the substrate) in taking up air pollutants (Miyawaki, 1998). In the latter process, known as biofiltration, carbon dioxide (CO 2 ) and harmful toxins are retained by both plants and substrate as the air is drawn through the living wall. In an experiment performed with these biofilters, indoor air concentrations of three noxious VOCs (toluene, ethylbenzene, and oxylene) were significantly reduced when air was forced to pass through the vegetation (Darlington et al., 1998). Indoor living walls can, therefore, be used to supplement traditional ventilation techniques to maintain indoor air quality (Darlington et al., 2001). The biofiltration effect can be enhanced when the air is mechanically forced to go through the vegetation and substrate using a permeable structure leading to an active living wall. However, the design process of these systems is somewhat more complex, as it requires prior assessment and more scientific knowledge on the behavior of some of their principal components, such as the substrate used for plant development (Franco-Salas et al., 2012). Vertical Greening Systems and Sustainable Cities 69 Besides air pollution mitigation, air temperature and humidity are other key variables of indoor air quality, very much related to human comfort, which may be altered by indoor living walls (Fang et al., 1998; Darlington et al., 2000). In warm climates, temperature reductions up to 4–68C have been observed in the surroundings of an indoor living wall (Ferna ´ndez-Can ˜ero et al., 2012). The evapotranspiration process of plants and substrate requires energy that is taken from the indoor air, thus decreasing air temperature and increasing humidity (ASHRAE, 2009). This physical process generates the so-called “evaporative cooling,” which depends, among others factors, on the vegetation type and exposure. During the evaporative cooling of air, indoor air humidity is also increased. Given that indoor air is usually too dry, particularly when internal heating or cooling systems are being used, the humidity increment of indoor air provided by the living wall may be beneficial. Nonetheless, under certain conditions (e.g., humid locations), ventilation may be required to avoid health problems associated with excessive humidity (Husman, 1996; Ren et al., 2002). When installed on the exterior walls of buildings, living walls have been reported to have an important effect on the thermal performance of buildings and on the urban environment. As reviewed in Pe ´rez et al. (2011) and more recently in Hunter et al. (2014), exterior VGSs may modulate indoor temperature through a combination of four fundamental mechanisms: interception of solar radiation by vegetation; thermal insulation provided by the vegetation and substrate; evaporative cooling that results from evapotranspiration; and modification of the wind pattern in the building envelope. The successful combination of all these processes has demonstrated the potential for exterior living walls to be used as passive energy-savings systems (Pe ´rez et al., 2011). Susorova et al. (2013) proved that a plant layer added to the facade can improve its effective thermal resistance. Mazzali et al. (2013) registered a difference of up to 208Cin the external temperature of a wall depending on the existence of a living wall covering it. Also the incoming (positive) heat flux through a bare wall was found to be higher compared to the living wall. Therefore, with the appropriate selection and placement of vegetation, an important reduction of cooling energy demands can be achieved (Meier, 2010), as the thermal gap between environment and comfort temperature decreases. Recently, Wong et al. (2009,2010a) pointed out the potential thermal benefits of VGSs in reducing the surface temperature of buildings facades in a tropical climate, leading to a reduction in cooling load and energy costs. The adoption of plant-covered wall designs in a Mediterranean climate also appears to be a suitable strategy to reduce energy consumption of buildings during the cooling period (Eumorfopoulou and Kontoleon, 2009; Kontoleon and Eumorfopoulou, 2010). In a modeling work conducted for the city of Toronto (Ontario, Canada), the shading and cooling effects of VGSs reduced the energy needed for cooling by approximately 20 percent (Bass and Baskaran, 2001), though potential cooling energy savings of up to 60 percent during warm summer days have also been described (Parker, 1987). Despite all these positive outcomes reported regarding energy savings in buildings, other authors such as Archer (2011) state that the overall thermal benefits of living walls, as compared to a well-insulated building wall, can be small. Therefore, while the energy-saving benefits of living walls retrofitted on poorly-oriented and insulated buildings seems generally accepted, a full financial assessment of the potential costs and benefits should be accomplished before implementing these systems in well-insulated modern buildings (Castleton et al., 2010). 70 Journal of Urban Technology Sometimes, employing deciduous climbers to offer seasonal regulation of shading may be interesting (Hoyano, 1988; Ip et al., 2010), especially in winter when energy consumption may increase because of the shadow produced by the vegetation on the building (Pe ´rez et al., 2011). The capacity of VGSs to limit the diurnal fluctuation of wall surface temperatures is also valuable in prolonging the lifespan of building facades and slowing down wear and tear as well as cost savings in maintenance and replacement of facade parts (Wong et al., 2010a). Given that plants absorb a considerable quantity of solar radiation for their growth and biological functions, such as respiration, transpiration, and photosynthesis (Kontoleon and Eumorfopoulou, 2010), VGSs can also play an important role in shaping the urban microclimate (Wong et al., 2010a). Indeed, the combined use of vertical greening structures and green roofs are increasingly being used as a new “bioclimatic” design concept of buildings in order to alleviate the urban heat island effect (Gomez et al., 1998; Ip et al., 2010; Sheweka and Mohamed, 2012)in cities by reducing the amount of reradiated heat (Bass and Baskaran, 2001). As an example, a temperature reduction of 8–98C has been observed in an urban canyon (tall buildings on both sides of the street) when walls and roofs were covered with vegetation in an experience conducted in Hong Kong, the wider the canyon is, the weaker the effect on temperature decrease (Alexandri and Jones, 2006). Similarly, Wong et al. (2010a) found promising results on the use of VGSs to cool the ambient temperature in building canyons, albeit their effects on ambient temperature were found to depend on specific systems. A major consequence of the effects of living walls on ambient temperature of street canyons is the fact that air intakes of airconditioning at a cooler ambient temperature translate into savings in energy cooling loads (Wong et al., 2010a). Living walls can be also used as noise barriers (Ottele ´et al., 2010), as the substrate where plants grow has a sound-absorbing effect and provides acoustic benefits derived from noise isolation (Azkorra et al., 2015); the broader the greenery coverage is, the higher the sound absorption. When the living wall is installed internally, it may also be useful in protecting speech privacy (Wong et al., 2010b). A well designed system can also have positive effects on urban hydrology, avoiding sudden discharges of storm water to the sewers when rainfall water is harvested on the roofs to irrigate the living wall, especially if it is combined with green roof technologies that act as water buffers (Carter and Jackson, 2007). Aesthetic and Social Implications The use of VGSs is also a highly impactful way of transforming the urban landscape (Wong et al., 2010a), incorporating advanced materials and technology to promote sustainable building functions (Ko ¨hler, 2008). Creative architectural solutions that implement greenery elements either for the restoration of old buildings or in the design of new ones would definitely improve the esthetical aspect of our cities (Elinc¸ et al., 2013). As an example, White and Gatersleben (2011) carried out a study to assess the esthetic impact and the level of perceived restoration that may be achieved by placing green roofs and facades onto homes. The results of the study showed that houses with building-integrated vegetation were more preferred and considered more beautiful, esthetically pleasing, and restorative than those without vegetation. Those conclusions were consistent with the findings by other researchers such as Kaplan et al. (1989) and Van den Berg et al. (2007), Vertical Greening Systems and Sustainable Cities 71 who also pointed out the importance of an adequate selection of plant species in the construction of a green roof or living wall from the esthetic point of view. There are some other “perceived benefits” of these greening initiatives. Sometimes, they provide a place for recreation and retreat where people can enjoy quiet moments amidst dense urbanity (Yuen and Nyuk Hien, 2005). As another example, the service sector tends to offer more environmentally conscious products and services (Rosenblum et al., 2000), so these systems provide an added ecological and esthetic value that is highly appreciated by current clients. In addition, there is an increasing level of social awareness of the benefits of promoting local green initiatives within urban communities for which these systems can have a favorable reception in densely built and populated cities. Indeed, the social component may be enhanced when these technologies are incorporated into the portfolio of “green” options within urban greening movements. The number of initiatives that have successfully implemented the use of new urban greening concepts within urban communities is growing, e.g., the success of the cultivation of vegetables on green roofs (Whittinghill and Rowe, 2012), but the use of VGS has not yet approached that of green roofs. Possibly, in order to introduce them into the social movement of urban agriculture, more affordable and simple-handling systems should be available for “non-expert users.” Discussion Research about Vertical Greening Systems In the last five years, the number of authors publishing studies involving VGSs has grown significantly. The scientific interest in this topic has corresponded to an increasing and parallel interest by the general public (See Figure 2). As can be seen from Table 1, the effect of living walls on the thermal performance of buildings has been by far the main topic studied involving VGSs. In fact, during 2014 Figure 2: Plot 1: Cumulative research articles published on Thomson Reuters Web of Knowledge over the last two decades and on the following topics: “living walls,” “green facades,” “green walls,” and “vertical greenery systems.” Plot 2: Relative search volume over time provided by Google Trends tool (Google Inc.) for the following search-terms: “living wall,” and “vertical garden.” In plot 2, number 100 represents the peak search volume. 72 Journal of Urban Technology an important role through developing new cultivars that increase, for instance, the plant capacity to sequestrate contaminants from the air. Other systems that are also becoming popular and that are likely to be part of the future concepts of green cities are so-called “vertical farms” (Despommier, 2009; Abel, 2010). These systems represent a variant of the VGSs herein described that are specifically developed and designed to grow crops that can partly supply urban food requirements. Under these circumstances, the monetary and environmental impact of producing and transporting food from distant places would be reduced. Furthermore, some studies have pointed out that a great increase in productivity with a considerably decrement of inputs could be achieved in urban vertical farms (Despommier, 2011). Although the use of urban vertical gardens to grow crops is still anecdotal, there are already some new projects designed for skyscrapers that include vertical farms (Welte, 2011). Other proposals are centered on converting abandoned urban properties into vertical food production centers. Conclusions Vertical greening systems have been used for many years, but nowadays they are becoming more important because of the need for making our cities more sustainable and the increasing environmental concern of their citizens. These systems are evolving, so the traditional compositions of plants and substrates have been combined with new technologies in order to improve their performance and enhance the benefits achieved with them. The success of VGSs in some cities has inspired many designers, engineers, and architects to introduce living walls in their new building projects. Furthermore, the number of researchers studying these systems and the information available about their positive effects and new applications is growing rapidly. These urban greening attempts can also help to alleviate climate change and improve living conditions in our growing cities. Therefore, VGSs have a great potential to contribute to provide a better environment for sustainable cities. Acknowledgments We would like to thank Iva ´n Gavin ˜o, Fernando Hidalgo, and Marta Ruiz, from the Spanish company “Terapia Urbana” for their insight and help with some technical questions. Disclosure Statement No potential conflict of interest was reported by the authors. Notes on Contributors Luis Pe ´rez-Urrestarazu is a professor of agro-forestry engineering in the School of Agricultural Engineering of the Universidad de Sevilla. Rafael Ferna ´ndez-Can ˜ero is an assistant professor in the Department of Agro- Forestry Sciences, School of Agricultural Engineering of the Universidad de Sevilla. Vertical Greening Systems and Sustainable Cities 79 Antonio Franco-Salas is an associate professor of agro-forestry engineering in the School of Agricultural Engineering of the Universidad de Sevilla. Gregorio Egea is an assistant professor of agro-forestry engineering in the School of Agricultural Engineering of the Universidad de Sevilla. ORCID Luis Pe ´rez-Urrestarazu http://orcid.org/0000-0002-3378-9285 Rafael Ferna ´ndez-Can ˜ero http://orcid.org/0000-0002-3407-9445 Antonio Franco-Salas http://orcid.org/0000-0001-6842-8685 Gregorio Egea http://orcid.org/0000-0001-6285-0981 80 Journal of Urban Technology Bibliography C. Abel, “The Vertical Garden City: Towards a New Urban Topology,” Council on Tall Buildings and Urban Habitat Journal 2 (2010) 20–30. E. Alexandri and P. Jones, “Temperature Decreases in an Urban Canyon Due to Green Walls and Green Roofs in Diverse Climates,” Building and Environment 43: 4 (2006) 480–493. S. Archer, “Green Walls: Thermal and Hydrological Costs and Benefits” E-Futures Mini Project 2 Summary Report (2011) http://e-futures.group.shef.ac.uk/publications/pdf/118_3.%20Stuart% 20Archer%20Summary.pdf. ASHRAE, ASHRAE Handbook of Fundamentals (Atlanta, GA EEUU, 2009). Z. Azkorra, G. Pe ´rez, J. Coma, L.F. Cabeza, S. Bures, J.E. A ´lvaro, A. Erkoreka, and M. 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