Review on the roles and effects of growing media on plant performance in green roofs in world climates

Accepted Manuscript Title: Review on the Roles and Effects of Growing Media on Plant Performance in Green Roofs in World Climates Authors: Fatemeh Kazemi, Ruzica Mohorko PII: DOI: Reference:

S1618-8667(16)30060-7 http://dx.doi.org/doi:10.1016/j.ufug.2017.02.006 UFUG 25853

To appear in: Received date: Revised date: Accepted date:

9-2-2016 9-2-2017 9-2-2017

Please cite this article as: Kazemi, Fatemeh, Mohorko, Ruzica, Review on the Roles and Effects of Growing Media on Plant Performance in Green Roofs in World Climates.Urban Forestry and Urban Greening http://dx.doi.org/10.1016/j.ufug.2017.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights    

Available research on green roof substrates across different world climates was studied. Impacts of substrate properties on plant performance in green roofs were analysed. Research and guidelines should consider climate specifications in green roof substrates. Comprehensive media-plant performance studies especially in dry climates are needed.

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Review on the Roles and Effects of Growing Media on Plant Performance in Green Roofs in World Climates Fatemeh Kazemi a*, Ruzica Mohorkob1

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Department of Horticulture and Landscape, Faculty of Agriculture, Ferdowsi University of Mashhad, GPO Box: 9179-448-987, Azadi Square, Mashhad, Iran; e-mail: [email protected] b

School of Natural and Built Environments, University of South Australia, GPO Box 2471 Adelaide, South Australia 5001 Australia; e-mail: [email protected] *Corresponding author: [email protected] ; +98 51 38805756

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Present Address: Mackay Regional Council, 73 Gordon Street, Mackay QLD 4740, Australia, +61 402430480

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Review on the Roles and Effects of Growing Media on Plant Performance in Green Roofs in World Climates Abstract Growing media (substrate) is a fundamental part of a green roof, providing water, nutrients and support to plants. However, little research has reviewed how it affects plant performances in different climatic regions. This study aims to analyse published research on green roof growing medium across world’s climate zones. Findings are structured according to Köppen–Geiger climate classification, aiming to investigate the prevalence of research conducted in different climate zones. Results from full-scale studies and laboratory or greenhouse experiments were reviewed. The later were included as they provide systematic knowledge on the effect of individual factors on system performances although cannot provide climate specific information. Studies discussed effects of major substrate components and depths on plant survival and establishment using standard test procedures. Results showed that most research in the subject were in temperate (group C climate classification), continental (group D) and dry climates (group B), respectively. Considerable number of investigations was conducted in controlled laboratory or greenhouse environments. Based on the results, future green roof research and guidelines should consider climate specifications of the region in designing growing medium, depths and attribute of green roof substrates in order to ensure enhanced plant performance. Especially, for more fragile but less investigated dry climate, considerations should be made to tackle heat fluctuations and drought stress by enhancing water holding capacity and thermal isolation of the substrate. To move forward, sustainable building solutions as a part of future urban forms, climate-adaptive green roof systems should be included into future research, practice and guidelines.

Keywords: Cross-climate study; green roof; Growing media; substrate; urban green space

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1. Introduction Current urban development strategies lead to increasing pressures on natural landscapes through increasing stormwater runoff, waste disposal, land clearing and pollution (Ewing and Association, 2008). This problem is often intensified by the impacts of climate change. New urban development strategies suggest that by implementing green infrastructures such as rain gardens, green roofs, green walls and bioretention systems (Dunnett and Kingsbury, 2004; Department of Planning and Local Government, 2010) impacts of urban development on the environment can be mitigated. In recent decades, green roofs have gained popularity incidence throughout the world (Williams, et al., 2010b). Significant evidence show that green roofs in urban areas can enhance environmental performances and ecosystem services such as stormwater management (Berndtsson, 2010), urban heat island mitigation (Susca et al., 2011) , increased urban plant and wildlife habitats (Brenneisen, 2006), pollination of cultivated and non-cultivated plants in cities, air and water cleansing effectiveness or acting as indicators for it (Gorbachevskaya, 2010), educational and cultural benefits such as making people connected with nature and importance for human and environmental health in cities (Oberndorfer et al., 2007). However, green roofs are still not readily designed, understood or supported as tools to relieve the pressures induced by built environments in different climate zones across the world.

Severe environmental conditions normally found on rooftops impose challenges on long-term survival of vegetation (Boivin et al., 2001; kl; et al., 2007) through daily and seasonal temperature fluctuations (Eumorfopoulou and Kontoleon, 2009; Ouldboukhitine et al., 2012). Limited water availability, exposure to wind, solar radiation and sometimes flash flooding (Schwarz, 2005) also create harsh environments for plants in green roofs (Nagase and Dunnett, 2010). Such pressures are more prominent in some world climate regions where extreme temperatures, precipitation patterns, or uneven distribution of rainfalls normally occurs. Such climate condition may require more specific green roof design to address local climates. There are arguments that current green roof technologies are not regionally appropriately designed (Lanbrinos, 2015). Generally, if green roof systems are to be cost effective, they should be designed to depend primarily on rainfalls as their main source of water supply (F.L.L., 2008). However, the amount and distribution of rainfall and temperature extremes will dictate the need for irrigation. Usually high levels of solar radiation and low medium moisture and depth compared to the ground level planting increase possibility of drought stress in green roof plants. Therefore, with the aim to obtain low or no-irrigation green roofs, in most climate regions it is important to develop strategies in design, construction, plant selection, and maintenance to enhance green roof drought tolerance (ASTM international, 2014).

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As engineered structures, green roofs are complex systems consisting of various components (F.L.L., 2008). However, growing medium (substrate) and plants are commonly seen as fundamental elements. Growing media are often shallow in depth, well drained, lightweight and poor in nutrients (Getter and Rowe, 2008; Hauth and Lipton, 2003) which are needed to support and anchor plants (Dunnett and Kingsbury, 2004). The provision of sufficient nutrients, water, and oxygen encourage healthy growth of plants (Dvorak and Volder, 2010). Sound knowledge of physical and chemical characteristics of growing media, combined with in-depth understanding of the native ecosystem, deep mechanistic understanding of water-substrate-plant relationships and plant eco-physiology are essential for the sustainability of green roofs across different climates (Dvorak and Volder, 2010; Williams et al., 2010b). Water relationships between substrates and plants have effects on plants photosynthesis and evapotranspiration and are important for having healthy plants in green roof systems. Substrate’s chemical and physical properties influence such relationships. Water holding capacity (WHC), permeability or water infiltration rate, density, pore volume and air filled porosity, nutrient holding capacity, pH, electrical conductivity (EC) are some of the substrate properties which affect plant performances (Young et al., 2015). Emerging green roof designs should define substrate components, depths and attributes and plant selection with particular consideration to environmental and climatic conditions of the region. However, there is still little research conducted to investigate frequencies of investigations with regard to climates, their different characteristics, and impacts on growing media and plants in green roof systems across the world. This study aimed to analyse available research on green roof growing media across different climate zones with respect to their effect on horticultural performances of plants (growth, establishment, and survival). The objective was to provide an overview of the current knowledge regarding relationships between green roof growing media and plants to better understand how to design green roof growing media in each climate more effectively for desirable plant performances. Areas and climates, which require further research, were also identified. These findings can assist in developing future climatespecific guidelines and can help landscape professionals and decision makers to design and adopt regionally appropriate green roofs across the world.

2. Methodology 2.1. Criteria for selecting literature This literature review considered research work published in peer–reviewed journals. To undertake a comprehensive investigation of emerging trends, studies, and data, conference proceedings, government reports, theses, dissertations, and books were also included as second priority references. Some authoritative references and leading research in green roofs are published in German language and these were also retrieved. The main source of peer-reviewed papers was German journal Dach + 5

Grün, Neue Landschaft and conference proceedings (i.e. Tagungsband). In addition, one author’s knowledge of Persian enabled access to research of Middle Eastern literature in this language, however, this research was not relevant. Sourcing of relevant literature was conducted using academic citation indexing and search service (ISI Web of Science and Elsevier), freely accessible internet search machines (Google Scholar) and websites as well as cross–disciplinary research of bibliographic databases. Keywords used to identify literature were the green roof, growing medium, substrate, recycling, depth and climate. There was no restriction on literature publishing period.

2.2 Climate zone classification This study was structured to investigate different climate zones, with the aim to examine the prevalence of previous research on the effect of climate on growing media and plant interaction in green roofs. Köppen–Geiger climate classification was selected as one of the most accepted climate classification systems. It is based on the concept of native vegetation being the best indicator of climate (Trewartha, 1967; Bailey, 1983). Choice of an appropriate substrate will influence plant viability when subject to specific climate conditions. Distributions of rainfall, temperature, and evapotranspiration rates are also important factors in selecting growing media.

2.3 Classification of literature findings and method of analysis The literature directly related to the relationship of green roof growing media with horticultural performances of plants were found and classified into two tables. Table 1 summarises research studies related to the subject on green roofs according to their respective climate region. They were sorted based on climatic regions according to the location of the research site. Table 2 shows a summary of experiments conducted in highly controlled environments, such as laboratories or greenhouses. These are included in the manuscript because they do provide useful knowledge on growing media-plant relationships in green roofs but were treated separately because their findings were not necessarily climate-based. The review in text sections was not limited to publications relevant to green roof growing media and their plant performances. In fact, any green roof literature that was found to enhance the subject and discussion of the review may have been included in the manuscript. Common growing media traits such as depths, components or major attributes deemed to be changed in different climates and defined in major green roof guidelines such as F.L.L. (2008) and ASTM international (2014) were used as major subheadings to categorize the literature in the text. If applicable, in each section, firstly the results retrieved from the research in tables 1 and 2 were reported. This was followed by an analytical discussion of the literature based on guidelines or other relevant green roof literature from around the world. Cross-climate comparisons were made on the 6

literature findings where applicable. Conclusions and recommendations for future research and selection of appropriate growing medium in green roofs across the world were finally made.

3. Results and discussion 3.1. Characters of major classified climates and frequency of green roof research Only five studies listed in Table 1 were conducted in dry semi-arid climate (Group B of Koppen climate classification) with no evidence of research from arid climate. Nineteen studies were conducted in temperate climates (Group C) and nine were in cool continental climates (Group D). Also, ten studies were found to investigate the effect of growing media on plants in green roofs in controlled environments (Table 2). Temperate climates are generally defined as environments with moderate rainfall spread across the year or portion of the year with sporadic drought, mild to warm summers and cool to cold winters (Simons, 2015). Cool continental climates are generally characterized by hot to mild summers but wet climates all the year. Conversely, hot dry climates (arid or semi-arid) are characterised by cool to cold winters and warm to hot summers with limited seasonal rain events. Based on table 1, it seems that efforts on green roof research and perhaps construction in hot arid or semi-arid climates compared to other climate regions, especially temperate climate, were much less. In such climate, water stress and high temperatures are the main challenging climate factors in green roof designs. These climate conditions affect the ecology of green roofs by heat stresses, droughts, and periodic saturation. In addition to climatic constraints, Williams et al. (2010b) explained and Simmons (2015) echoed that lack of good understanding and ample experience in industry, lack of locally based research and inappropriate mimics of green roof design (choosing right substrate composition, depth and plant selection) from European experiences and guidelines (ex. F.L.L., 2008) directly to hot climates is the reasons for less progress of green roofs in hot regions. Transferring this technology directly to warmer or cooler regions presents a challenge to climate suitability adopted with flash flooding, prolonged droughts, high day and night temperature fluctuations and soil temperatures (Alexandri and Jones, 2008, Simon, 2008).

3.2. Climate influences on plant selection in green roofs and strategies for their success Generally, rapid coverage and high lifespan are important characteristics of plants to be considered in plant selection for green roofs. Species with the ability to self-sustain, reseed or spread vegetatively and grow aggressively (but are not invasive) are the most appropriate species for green roofs. In most conditions species such as sedums will not become invasive and can grow well in shallow and dry substrates where most other species cannot survive (ASTM international, 2014, Durhman et al., 2004) 7

although controversial results have been achieved by different sedum species in different climate regions (Simmons et al., 2008; Livingston et al., 2004; Simmons, 2015). In temperate climates, maximum leaf/root temperature is not as high as it is in hot climates. In this climate, although sustained periods of low temperatures and drought may occur, these may not be as frequent as it is in hot arid and semi-arid climate (Simmons, 2015). Such climate is not as challenging for plants as hot climate conditions on the roof are. Using frost resistant growing media containing mineral components with a high porosity which is well defined in German guidelines (F.L.L., 2008) has been addressed for green roof systems in mostly temperate zones. Arid and semi-arid climates with strong low or seasonal precipitations and insufficient water to plants (no supplementary irrigation) are seen as more challenging (Ondono et al., 2016). Even if plants can survive under such conditions, they will have reduced plant biomasses or aesthetic performances (Nagase and Dunnett, 2010; Farrell et al., 2012). Such phenological or even physiological changes in plants are in line with adaptation strategies that plants develop to tackle the drought. Lambrinos (2015) well defined five water use strategies including water loss minimization, water loss adaptation, water stress avoidance, water loss tolerance and water loss sensitivity in green roof plants. Some strategies have been defined toward getting resiliently planted roof systems. One is to avoid selecting plants with narrow taxonomic groups or horticultural or environmental needs as climate conditions change within and between years. Having water use plasticity is a suitable character for plant selection (Lambrinos, 2015). Secondly, a mixture of plant growth forms might be a better solution to overcome extreme green roof conditions especially in hot climates (MacIvor et al., 2011, Wolf and Lundholm, 2008). Thirdly, using native species of each region in plant palettes in green roof designs can ensure better adaptability to climatic condition of the region and is an increasingly used strategy (Sutton et al., 2012, MacIvor and Lundholm, 2011, Razzaghmanehs, 2014). Fourth, combining succulent plants with the main planting can facilitate better plant establishment and survival of green roofs through ameliorating microclimate conditions for some projects in hot climates (Butler and Orian, 2011), although this effect might be adverse depending on the time of the year or the locality. For example, Young et al. (2015) did not found a facilitative effect on main plant’s performance in a green roof research in Sheffield. Butler and Orians (2011) found a negative effect on the growth of some perennial forbs in presence of sedum species in non-stressed conditions, while the effect was positive during hot summer times. Therefore, care should be taken and further investigations may be required when using this strategy. All in all, compared to ground level planting, harsh climatic conditions of the green roofs along with low nutrient and shallow substrates has a limited selection of plants for these living systems. Such small range of plants may not satisfy public preferences or perceptions. An alternative might be to adapt growing media to support a larger number of plant taxonomic groups.

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3.3. Characteristics of growing media with regard to plant performances in world climate 3.3.1. Growing media components Growing media compositions (components) have direct effects on plant performance. An ideal growing medium for plants can sustain plant life, require little input and does not break down easily (ASTM international, 2014). Table 1 shows a wide range of materials used to trial traits of growing medium with different design objectives in mind often including lightweight, good drainage, good water and nutrient-holding capacity, not prone to leaching and local availability of materials. Several studies evaluated different lightweight growing media deriving from naturally-occurring materials such as lava, pumice, and scoria (Retzlaff et al. 2008; Benvenuti and Bacci 2010; Schroll et al., 2010; Voyde et al., 2010; Morris et al., 2011; Panayiotis et al., 2011) to synthetically produced materials, such as expanded slate, clay or shale (Rowe and Monterusso 2006; Durhman et al., 2007; Hilten et al., 2008; Simmons et al., 2008; Thuring et al., 2010; Voyde et al, 2010; Bousselot et al., 2011; Gregoire and Clausen, 2011). In addition, sand and volcanic fines have also been used (Durhman et al., 2007; Simmons et al., 2008; Werthmann, 2008; Benvenuti and Bacci, 2010; Nardini et al., 2011). Mineral components such as pumice, lava, expanded shale, and slate are light weight but have less water holding capacity (ASTM international, 2014). Adversely, clay, sand, and topsoil have good physical attributes for plant growth but are heavy, especially in saturated conditions (ASTM international, 2014), and makes them prone to water logging. The mineral content of the substrate in green roof systems provides cation exchange capacity (CEC) and pH buffering. Cation exchange capacity of the substrate is important in providing micro-nutrients through the media. It must be initially tested and not be less than 25 meq /1b (6 meq/100g) (ASTM international, 2014). Studies across dry climates showed that for plant success medium depth (the deeper the better) is more influential than medium components (for e.g. see Ondono et al., 2016 in table 1). However, the addition of components such as water retention additives (Savi et al., 2014), and organic materials (Kancechi et al., 2014) to mineral components, are important drivers for plant prosperity. This is mainly due to the enhancement of substrate water holding capacity, which will be discussed in next sections. This statement was endorsed by a finding where a 100% mineral mix medium was the worse plant performer when compared to the similar medium mix with added organic materials (Morris et al., 2011, Table 1). Typically, minerals are blended with organic matter in different ratios depending on design objectives and vegetation types. Based on table 1 and 2 it seems that selection of the types of organic materials is mainly based on local availability rather than any climate suitability consideration, such as biodegradability rate. Some commercially available materials such as composted pine bark and coir fibre are widely used in green roof studies in different climate regions (e.g., Razzaghhmanesh et al., 9

2014 a,b; Morris et al., 2011) while others such as composted turkey litter is used in limited number of studies in regions such as Michigan (see Durhman et al., 2007, Rowe et al., 2012 and Vanwoert et al., 2005a in Tables 1, 2). The amount of organic material also affects plant growth, survival, and many other attributes in the substrate (ASTM international, 2014). A number of studies have investigated the effect of variation in organic matter in growing media (Emilsson 2008; Molineux et al, 2009; Olszewski et al., 2010; Nagase and Dunnett 2011). Guidelines recommend a 3-15% by volume of this material in temperate regions (ASTM international, 2014). However, some studies recommended much higher (over 60%) range for high yield performance in a cool continental climate of Michigan (Eksi et al, 2015). Generally, organic material is added to provide nutrients to plants, to improve water-holding capacity and to increase cation exchange capacity of the growing media. However, despite having several advantages, the amount of organic matter should be kept low (Nagase and Dunnett, 2011). This might be because of the lack of their stability (Emilsson, 2008) and its decomposition over time, resulting in compaction and growing media settling, hindering healthy root growth (Rowe and Monterusso, 2006) and making plants more susceptible to drought (Nagase and Dunnett, 2011). Other findings show that elements of decomposing organic material can leach into the water and contribute to poor water quality and coloration in green roof runoff (Rowe and Monterusso, 2006). Investigating the current studies showed no cross-climate study has been undertaken on the effect of diverse organic material types in different amounts in green roofs. Natural topsoil should not generally be used as the growing medium for green roofs due to its clay and mineral components that make the substrate susceptible to clogging, waterlogging and compaction. It is also a potential source of unwanted plant material (ASTM international, 2014). However, to increase water-holding capacity, Papafotiou et al. (2013) used soil as an additive to substrate blend (a mixture of soil, perlite, and compost) in the Mediterranean climate of Greece. Best et al. (2015) also confirmed concerns that fine particles do increase roof load and unpredictable biological activity when using natural soil but also expressed that using natural soil can provide ecological benefit for green roofs by providing a quickly available plant habitat. Using recycled materials as additional components in substrates is also common. In a study by Carson et al. (2012) recycled substrates were created using drywall waste, concrete, roof shingles, glass and wood cuttings. Another study from the UK tested the potential of four recycled materials: crushed red brick, clay and sewage sludge pellets, paper ash pellets, and carbonated limestone pellets, all blended with 15% and 25% of conifer-bark compost (Molineux, et al., 2009). Investigating Table 1 implies that selection of recycled material type is either depending on local availability of the materials or specified as component in commercial substrates (e.g. see Razzaghmanesh et al., 2014a,b, Graceson et al., 2014 and Bates et al., 2015b, Morris et al., 2011). Although some materials such as crushed brick has been used in different climate zones (dry climate of Adelaide by Razzaghmanesh et al. (2014a,b), 10

temperate climate of UK by Graceson et al. (2014) and Bates et al. (2015b)), there is no evidence of cross climate comparison of these and similar studies or reports of the methods. Further, no study was designed systematically to examine the effects of recycled materials on plant performance. Only Razzaghmanesh et al. (2014a) reported growth and survival of Carpobrotus rossii and four other Australian native plants in a mixed media containing a blend of scoria, crushed brick, coir fibre and compost in semi-arid climate of Adelaide. While using recycled or demolition materials such as crushed brick or terra cotta can increase green star rating of the building by assisting the environment and decreasing waste output, great care should be taken when using these materials to prevent mixing toxic materials to the substrate (ASTM international, 2014). Several investigations focused on improvement of different substrate characteristics by including soil amendments such as biochar, activated carbon, and super-absorbent polymers. Water retention additives were evaluated in several studies (Olszewski et al. 2010; Farrell et al., 2013a; Savi et al., 2014; Sutton, 2008). Olszewski et al. (2010) in a laboratory and greenhouse study in Pennsylvania evaluated the effects of amended hydrogel and compost, in different percentages, in a shallow slatebased growing medium and their impact on initial plant growth. Results showed both hydrogel and compost had positive effects on initial plant establishment. This research also suggested that the same substrate with hydrogel would be suitable for application in climates with high evaporative water losses and areas prone to wind erosion, as well as in shallow depth systems that are more prone to soil evaporation. However, results also showed that water retention of hydrogel-based media was reduced with repeated fertilization, which is perhaps due to chemical reactions (Olszewski et al., 2010). Panayiotis et al. (2003) also in a field study in the Mediterranean climate of Greece indicated salt accumulation in the tested growing medium, which was attributed to another superabsorbent polymer, namely urea-formaldehyde resin foam. This indicated that future research should investigate other types of superabsorbent polymers and their short and long-term performance under different environmental conditions. Farrell et al. (2013a) tested silicate granules and hydrogels for their influence on initial plant growth in two substrates, based on scoria and crushed roof-tiles, respectively. Findings from this greenhouse-based experiment showed that both hydrogel and silicates improved plant success when subjected to prolonged drought stress. Despite the above findings on increased water holding capacity of the substrate using superabsorbents and their assistance to drought stress tolerance of plants, Savi et al. (2014) observed a decrease in water holding capacity of substrate amended with hydrogel after only fine months from the start of the experiment. Further improvement of physical and chemical characteristics of both hydrogels and substrate were suggested. Another study by Panayiotis et al. (2011) stated that perlite, even though prone to disintegration over time, can be suitable for green roof systems constructed in semi-arid climates due to increasing water holding capacity of the substrate. Therefore, other media components which improve water retention such as natural and synthesized superabsorbents as described by Olszewski et al. (2010) and Savi et al. (2014) should be investigated as a potential component for growing media in dry regions. 11

Many reviewed studies have measured the influence of only a few growing medium components or properties and have not pursued this further in terms of different experimental setups with varied substrate formulations and under different environmental scenarios. This should be considered for future research design and implementation.

3.3.2 Growing media depth Several studies reviewed in tables 1 and 2 concluded that overall performance and survival of plants is promoted by increased growing medium depth (e.g. Boivin et al., 2001; Durhman et al, 2007; Dunnett et al., 2008; Thuring et al., 2010; Morris et al. 2011; Panayiotis et al. 2011; Papafotiou et al. 2013; Molineux et al., 2015; Ondoño et al., 2016, Morris et al. 2011). In most studies, substrate depth was even more influential factor on growth and survival of plants compared to growing media types (Molineux et al., 2015, Papafotiou et al., 2013, Panayiotis et al., 2013 see Table 1) or combination of media types and irrigation treatments (e.g. see Panayiotis et al., 2011 in Table 1). However, in some cases (e.g., Papafotiou et al., 2013 in temperate climate) shallower substrate (75 mm) with addition of components such as compost and sporadic irrigation provided similar results as deeper (150 mm) substrate or other factors such as season of planting may affect more than substrate depth in plant success (Getter and Rowe, 2007). Research in dry semiarid climates (listed in table 1) showed substrates deeper than 100 mm can guaranty survival and growth of plants in normal conditions (Ondono et al., 2016, Razzaghmanesh et al., 2014 a,b). In drought conditions, addition of water retention additives along with deeper substrate could provide better plant growth outcomes (Ntoulas et al., 2013). Findings of studies conducted in Mediterranean climates (Panayiotis, et al., 2003; Benvenuti and Bacci, 2010; Panayiotis et al., 2011; Papafotiou et al., 2013) in hot, dry summers and mild, wet winters also confirmed that a deeper substrate is a critical factor for promoting establishment, growth, and survival of vegetation. Conversely, across cool continental climates, sedums and plants from Crassulaceae family generally performed well in shallow substrates (about 75-100 mm) with or without additional irrigation (Durhman et al., 2007; Rowe et al., 2012; Bovin et. al., 2001). This is most likely because sedums are more adaptable to shallow substrates and would outcompete other species. Further, extreme temperatures and lesser rainfall events generally hinder the success of plants in dry and hot climates. Therefore, these factors need to be adjusted either by increasing the depth of the growing media or by utilizing supplementary irrigation. Studies have found that when an adequate watering regime is combined with shallower substrates, the success of plant species is typically better (VanWoert et al., 2005a; Panayiotis et al., 2011; Papafotiou et al., 2013; Kanechi et al., 2014). Moreover, others have observed that plant survival, especially during the establishment phase, can be achieved by careful timing of irrigation (Thuring et al., 2010). 12

When designing green roofs, it is important to be mindful of different variables. For example, a deeper substrate can increase the chance of plant survival by adding more space for roots to develop, store more water, therefore, reduces the chance of plant to experience drought stress (Thuring et al., 2010), have better insulation properties and create habitat for more diverse flora and fauna. However, increasing the depth of growing media also imposes a greater demand on the structural load-bearing capacity of the roof (Dunnett and Kingsbury, 2010) or may increase weed invasion (Nagas et al., 2013), may not always improve water availability for plants especially in substrates with very small size particles (Fassman et al., 2013). Therefore, alternatives might be to modify physical attributes of the substrates through changing or adding certain components. For example, adding organic materials (Nagase and Dunnett, 2011), controlling particle size of the substrate (Graceson et al., 2013) or adding water retention additives (Farrell et al., 2013, Olszewski et al., 2010, Savi et al., 2014) can increase water holding capacity of the substrates.

3.3.3. Thermal characters of growing media As discussed earlier, physiological performances of plants are highly dependent on temperature and extreme air and substrate temperatures and are limiting factors of plant growth and survival. Root system in most vascular plants has much a narrower range of temperature fluctuations compared to their aboveground section (Simmons, 2015). Generally, a 4-30̊C is an optimum range for the roots of common plants in green roofs. Above the upper temperature, certain root processes such as transpiration and synthesis of secondary metabolites will be reduced and eventually on 48̊C root death occurs (Xu and Huang, 2000, Sutton et al., 2012). This threshold is same for CAM plants. In many hot climates, root surface temperatures can easily exceed this critical temperature. In Texas, root temperature even in early spring at 56̊C (Simmon et al., 2008), mid 50̊C in Florida (Sonne, 2006), 90̊C in Melbourne (Williams et al., 2010) were recorded. This suggests that in hot climate root system, especially on top of the substrate, is challenged by high temperature. Selecting plants with high tolerance to substrate temperature fluctuations (Savi et al., 2016, table 1), defining growing medium components and depth with the aim to reduce thermal conductivity and heat capacity in green roofs in extreme environments seems necessary. Research evidence has observed that deeper substrate better reduces temperature fluctuations and results in better plant coverage (see Getter et al., 2009 in Table 1). Major mineral components suggested by F.L.L. (2008) as components for green roof systems are processed (expanded clay or shale), recycled (brick, tile) or naturally occurring materials such as scoria and pumice. However, these usually have high thermal conductance increasing the risk for root to heat injury. One way to reduce thermal conductivity of these components would be to combine with organic material or other lightweight materials such as perlite or vermiculite, which have less thermal conductivity, while still enhancing water retention of the substrate in hot climates (Simmons, 2015). 13

Climates with low-temperature regimes may also provide challenges for green roof plants and media. Chilling injury to the root system or the whole plant can also be seen in temperate climates with cold winters. To overcome this challenge, using tolerant plant and growing media components resistant to frost, therefore, stable against breakdown and damage to the longevity of the substrate have been recommended by guidelines (ASTM international, 2014). Frost resistance should be tested according to the testing standards prior to substrate selection (ASTM international, 2014).

3.3.4. Water retention characters of growing media The ability of green roofs to retain water is varied amongst green roof types, but drainage and retention layers (Monterusson and Rowe, 2005), growing media depth and composition as described earlier, also plants physiology and forms (Dunnett and Kingsbury, 2004, Dvorak and Volder, 2010) have effects on this attribute. The recommended range varies from 20% volume or more in extensive single layer green roofs to more than 45% volume in intensive green roofs. To avoid water logging in should not exceed 65% volume in any type of green roof system (F.L.L., 2008, page 62, also see ASTM international, 2014). Climate also plays an important role in water retention of green roofs and this needs to be addressed in future green roof guidelines. Current green roof standards such as German F.L.L. (2008) are based on years of extensive research in a relatively small area in Germany with a temperate climate and, therefore, might not be applicable to different climate zones. FLL recommendations should be taken with care and only as guidelines for any other region or climate. Based on F.L.L. (2008) a large range of porous mineral aggregates such as expanded clay, expanded shale, scoria and pumice, recycled brick and tile have been suggested as main components of green roofs and have worked well in green roofs of temperate zones (e.g. Panayiotis et al., 2011; Bates et al., 2015b; Graceson et al., 2014; Rayner et al., 2016) or some cool continental climates (e.g. Thuring et al., 2010; Eksi et al., 2015; Rowe et al., 2012; Olszewski, 2011). However, these may not provide appropriate water retention for the green roofs in other regions such as dry climates; hence, may not work for plants in their green roofs. Because of relatively shallow depths, usually free drainage nature of the substrate and harsh climatic conditions on the roofs compared to the ground levels, water stress is an important limiting factor in plant growth and establishment in green roofs of dry regions. Therefore, increasing substrate waterholding capacity in different ways has been suggested to reduce drought stress and mortality in plants (Olszewski, 2011). The inclusion of small size particles has been suggested to increase substrate water-holding capacity (Young et al., 2014) but it is still unclear how this physical property may affect tolerance of plants to drought (Nagas and Dunnett, 2011). It is proven that a a lot of small particles can increase the saturated weight of the substrate, reduce substrate permeability, air-filled porosity and increase water 14

logging, which may negatively affect plant health and survival (FLL. 2008). Further, gaining high water-holding capacity through smallest size particles does not necessarily guaranty high amount of available plant water as a large amount of such water will be strongly connected to the substrate particles; hence, not available for the plants (Rabbani KheirKhah and Kazemi, 2015). Therefore, existing guidelines and standards may need revision and consideration on how best to optimize particle size distribution for various substrate components or combinations of them to provide optimum available plant water in green roofs. F.L.L. (2008) has defined granulometric distribution range for vegetation substrates used in single and multiple layer intensive or extensive green roof systems (pages 59 and 60). Research is in support of increased organic materials (usually more than suggested ranges by F.L.L. (2008) in green roofs as a way to increase the water holding capacity of the substrate and provide nutrients for the plants, hence, enhance plant establishment (Molineux et al., 2009). However, care should be taken in creating organic-rich growing medium as it may biodegrade, provide bad smell or be sensitive to fire in some climate conditions (ASTM international, 2014). In dry climates biodegradation may be slow but in temperate or wet climates or in green roofs with complementary irrigation, organic material are more subject to faster biodegradation and reduction of longevity of the substrate, reduction in saturated hydraulic permeability and porosity and even transfer of fine particles to geotextiles and clogging of the drainage (ASTM international, 2014). Decomposition and biodegradation of growing media can result in reducing effective root volume (Simmons, 2015). To avoid putrefaction of organic materials especially in intensive green roofs ( ≥ 35 cm depth) a distinction of upper and lower substrate (multiple layer construction) might be necessary with a lower substrate consisting of less organic materials with less water holding capacity (F.L.L., 2008). Using water retention additives has been suggested as another strategy for increasing water-holding capacity of the green roof substrates (Olszewski et al. 2010, Young et al., 2015) and in some cases in drought conditions in semi-arid climate proved more effective than organic matter in plant success (see Ntoulas et al. 2013, table 1). Young et al. (2015) in their study observed that using polyacrylamide gels could increase drought resistance of plants in green roofs through increasing available plant water during dry spells without increasing excessive shoot growth. It appears these components can cause a small continuation of growth during early stages of drought. This is explained by the fact that they are able to provide more substrate available water for plants during early drought stages. It should, however, be noted that some types of water retention additives may retain water in the soil but such water may not be available to plants depending on the type of the water retention additive, plant species type or components of the substrate (Farrell et al., 2013). Further, while water retention additives compared to organic materials may look more stable components, they still should be considered with care as permanent components in green roof systems. Young et al. (2015) confirmed that the effect of polyacrylamide gels might decrease over 15

time and over multiple periods of dry and wet cycles. Savi et al. (2014) also observed that irrespective of substrate depth and its composition, hydrogels lose their water-holding capacity over time compromising the performance of plants and the green roofs, especially in dry conditions. As a more cost effective and long term solution for increased drought tolerance of plants in green roofs, it is recommended to use substrates that encourage slower but more sustainable growth of plants (Young et al., 2015). Understanding relationship between coarser particle sizes and smaller particle sizes in well-defined ratios may promise such growth and increased resistance to drought in green roof systems.

3.3.5.Guidelines and standards Most of the laboratory and greenhouse studies listed in Table 2 were conducted according to at least one of the established and recognized testing procedures for green roofs, e.g. FLL guidelines or ASTM. However, this is not the case for all field investigations listed in Table 1. Therefore, it is advisable that future field research considers applying their methods based on the standard green roof guidelines. This approach would allow more meaningful comparison of experimental findings and could, therefore, be used as a guide for future studies, which incorporate different climates. A lack of information which allows comparison of results using existing guidelines, hinders the development of a progressive and authoritative body of knowledge (Dvorak and Volder, 2010), particularly in the context of growing media design across different climates. The most comprehensive of all readily available guidelines and standards (F.L.L., 2008) has influenced green roof industry throughout Europe and the UK. These guidelines recommend ranges for various physical and chemical properties of growing media and describe precise testing procedures for the optimal performance of green roof substrate. FLL guidelines are a product of years of research and practical experiences derived from a relatively small area in Germany and are the most comprehensive source in the field. However, not all recommendations will be applicable to other climate regions but they are a useful starting point. In 2011, the GRO Green Roof Code (GRO Technical Advisory Group, 2011) was developed, based on the FLL Guidelines, as a UK-specific code of best practice to accommodate UK climate conditions. Its purpose is to guide green roof design, specification, installation, and maintenance. For example, GRO recommends 80 mm as a minimum substrate depth for extensive green roofs (GRO Technical Advisory Group 2011, p. 14), instead of the minimum 60 mm recommended by FLL guidelines (F.L.L. 2008, p. 43). From tables 1 and 2, it is evident that USA and Canada are also experiencing substantial growth in the green roof market. However, our study identified only a few papers (Carson et al., 2012) referring to the American Society for the Testing of Materials (ASTM) standards for green roof growing media. 16

These standards focus on: test methods for saturated water permeability of granular drainage media (ASTM International 2011a); determination of dead and live loads associated with green roof systems (ASTM International, 2011b); test methods for maximum media density and associated moisture content when the system is drained (ASTM International, 2011c); guidelines for selection, installation, and maintenance of plants for green roof systems as well as growing media composition (ASTM International, 2006); and the use of expanded shale, clay and slate in the growing media and drainage layer (ASTM International, 2012). Another guideline is ASTM international (2014) which provide knowledge on green roof plants. This guide is relatively comprehensive in the area and has some cross climate comparisons compared to F.L.L. (2008) but it is criticised for providing rigorous testing procedures and apparatus for determination of numerous characteristics of growing media while not providing recommendations on ranges for these characteristics, indicating that further improvements to these standards are necessary. In Australia, there is only one standard AS 4419-2003 (Standards Australia, 2003b), which sets performance criteria for soils and inorganic/organic matter blends for use in landscapes including rooftops. Additionally, a recent book by Leake et al. (2014) recommends the use of both AS 44192003 and methodology of AS 3734-2003 (Standards Australia, 2003a) to meet the requirement for application on green roofs. However, because of their broad coverage, these standards assess growing media high in organic matter and, therefore, often not suitable for rooftop usage. Furthermore, growing medium for green roofs are high in mineral components, thus its coarseness can impose some problems when using standard test procedures for natural soils. Often the equipment and instrumentation used for natural soil analysis are not designed for testing coarse material, therefore, the testing methodology and equipment for green roof growing media may need adaptation.

4. Conclusion and recommendations This study demonstrated that strategic selection of components and growing media depths are crucial for plant success in green roof systems in any climate region of the world. While it is obvious that growing media components, depth, and attributes affect plant performances in green roofs, the extent to which these components and their ratios affect vegetation in different climates is still unknown. Without such knowledge, it is a challenge to design substrates for green roofs in different climates. To obtain good understanding, growing medium should be designed to provide good water-holding capacity, thermal performance and other attributes that meet climatic and design requirements of the specific region. Based on this review greater body of work on plant performance in different growing media in every world climate with greater interest in more challenging yet less investigated dry climate, is required. Compared to other climate regions dry climate has low precipitations, hightemperature fluctuations and is, therefore, more prone to high evapotranspiration rates and greater plant stresses. These studies should be conducted in accordance with widely accepted guidelines and 17

standards to allow cross-comparison across world’s regions. Further, in warmer and dryer climates green roof design guidelines and standards need to be developed. To reduce drought stress on green roof plants water-holding capacity of the substrate should be defined with care. Based on F.L.L. (2008), water holding capacity is the most important reference value related to plants, which directly affects the amount of water available to plants. The percentage of pores and inclusion of water absorbents should be carefully investigated for each climate region. The increased water-holding capacity of the substrate along with increased depth and organic matter can enhance plant growth, survival, and establishment. However, the threshold for any of these factors might depend on the roof design attributes, aims, and climatic conditions of the specific region. For example, if the roof system does not allow heavy substrate loads then increased organic matter and water retention additives should be considered as priorities, especially in arid and semi-arid climates. However, if a heavy load of the substrate is not a matter of concern, to achieve appropriate water holding capacity, increased substrate depth and porosity might be a more sustainable option than adding water retention additives or increasing organic content, especially in wet climate regions. To reduce thermal stresses to plants and their root system in extreme climate conditions (very cold or very hot temperature), it is highly recommended that future investigations for thermal characteristics of growing media look into specific traits of each climate region. Based on the review, some previous studies have been undertaken either within highly controlled laboratory or greenhouse environments or in the field as prototype-scale systems. While investigations in controlled environments are valuable in the research stage to provide systematic knowledge on the effect of individual factors on system performance, they may have limitations in determining actual field outcomes. Therefore, it is recommended using greenhouse studies as preliminary or specific tests and to couple them with field research. Also, most research has been conducted within limited time frames highlighting the need for long-term monitoring studies to inform the changes in such a dynamic system over time and particularly over growing seasons. Use of alternative substrate materials, the ratio of organic to mineral components, the relationship between particle size distribution, available water, and survival and stability of different vegetation forms and types in different climates especially in hot and dry climates are just some examples of further required research. In a world where natural resources are being rapidly depleted, designing climate-adaptive green roof systems can integrate architecture and nature to create sustainable building solutions as a part of future urban forms.

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5. Acknowledgement The authors would like to acknowledge editorial comments by Maxine Godley. The comments of the editors and the anonymous reviewers of the journal are much appreciated.

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Rowe, DB and Monterusso, MA, Rugh, CL 2006, 'Assessment of heat-expanded slate and fertility requirements in green roof substrates', HortTechnology, vol. 16, no. September, pp. 471-477. Sailor, DJ and Hagos, M 2011, 'An updated and expanded set of thermal property data for green roof growing media', Energy and Buildings, vol. 43, no. 9, pp. 2298-2303. Savi, T., Borgo, A.D., Love, V.L., Andri, S., Tretiach, M., Nardini, A., 2016, Drought versus heat: What's the major constraint on Mediterranean green roof plants? , Science of the Total Environment, vol. 566-567, pp. 753-760. Savi, T, Marin, M, Boldrin, D, Incerti, G, Andri, S and Nardini, A 2014, 'Green roofs for a drier world: Effects of hydrogel amendment on substrate and plant water status', Science of The Total Environment, vol. 490, pp. 467-476. Schneider, A., 2014, Fusco, M., Bousselot, J., 2014, Observations on the Survival of 112 Plant Taxa on a Green Roof in a Semi-Arid Climate, Journal of Living Architecture, vol. 1, no. 5, pp. 10-30. Schroll, E, Lambrinos, J, Righetti, T and Sandrock, D 2010, 'The Role of Vegetation In Regulating Stormwater Runoff From Green Roofs In A Winter Rainfall Climate', Ecological Engineering, vol. Article in, no. 4, pp. 595-600. Schwarz, T. 2005, 'Worauf es bei der Pflanzenauswahl ankommt - Pflanzen für extensive Dachbegrünungen', 2005. Simmons, M.T., Gardiner, B., Windhager, Tinsley, J. (2008) Green roofs are not created equal: the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate, Urban Ecosystems, vol. 11, pp. 339-348. Simmons, MT., Windhager, S and Tinsley, J 2008, 'Green Roofs Are Not Created Equal: The Hydrologic and Thermal Performance of Six Different Extensive Green Roofs and Reflective and Non-Reflective Roofs in A Sub-Tropical Climate', Urban Ecosystems, vol. 11, pp. 339-348. Simmons, Mark. T., 2015, Chapter 3: climate and microclimates: challenges for extensive green roof design in hot climates, pages: 63-80, In: Sutton, R.K. (editor) (2015) Green roof ecosystems, Springer. Sivakumar, MVK, Das, HP and Brunini, O 2005, 'Impacts of Present and Future Climate Variability and Change on Agriculture and Forestry in the Arid and Semi-Arid Tropics', Increasing Climate Variability and Change, vol. 70, no. 1-2, pp. 31-72. Solano, L, Ristvey, AG, Lea-Cox, JD and Cohan, SM 2012, 'Sequestering zinc from recycled crumb rubber in extensive green roof media', Ecological Engineering, vol. 47, pp. 284-290. Sonne, J. (2006) Evaluating performance aspects of a Florida green roof, In: 15th annual symposium on improving building energy systems efficiency in hot and humid climates, Orlando. Standards Australia 2003a, AS 3743 - Potting mixes, Standards Australia, Sydney, NSW.

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Standards Australia 2003b, Australian Standard ™ Soils for landscaping and garden use, Standards Australia International Ltd, Sydney. Susca, T., Gaffin, S.R. and Dell’Osso, G.R. (2011) Positive effects of vegetation: urban heat island and green roofs, Environmental Pollution, vol. 159, no. 8-9, pp. 2119-2126. Sutton, R. K., 2008, Media modifications for native plant assemblages on green roofs. In: paper presented at the seventh annual greening rooftops for sustainable communities conference, Awards and trade show. Sutton, R., Harrington, J.A., Skabelund, Lee, MacDonagh, Peter, Coffman, Reid R. and Koch, Gord, 2012, Prairie-based green roofs: literature, templates and analogues, Journal of Green Buildings, vol. 7: 143-172. Thuring, C and Dunnett, N 2014, 'Vegetation composition of old extensive green roofs (from 1980s Germany)', Ecological Processes, vol. 3, no. 1, 2014/01/28, pp. 1-11. Thuring, CE, Berghage, RD and Beattie, DJ 2010, 'Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions', HortTechnology, vol. 20, no. 2, pp. 395-401. Trewartha, G 1967, An introduction to weather and climate. Ed. 2. 545 p, New York: McGraw-Hill., AH Robinson, and EH Hammond. VanWoert, ND, Rowe, DB, Andresen, JA, Rugh, CL, Fernandez, RT and Xiao, L 2005a, 'Green Roof Stormwater Retention', Journal of Environment Quality, vol. 34, no. 3, p. 1036. VanWoert, ND, Rowe, DB, Andresen, JA, Rugh, CL, Xiao, L. 2005b, 'Watering regime and green roof substrate design affect Sedum plant growth', HortScience, vol. 40, no. 3, pp. 659-664. Vijayaraghavan, K and Joshi, UM 2015, 'Application of seaweed as substrate additive in green roofs: Enhancement of water retention and sorption capacity', Landscape and Urban Planning, vol. 143, pp. 25-32. Voyde, E, Fassman, E and Simcock, R 2010, 'Hydrology of an extensive living roof under sub-tropical climate conditions in Auckland, New Zealand', Journal of Hydrology, vol. 394, no. 3-4, pp. 384-395. Voyde, E, Fassman, E, Simcock, R and Wells, J 2010, 'Quantifying evapotranspiration rates for New Zealand green roofs', journal of Hydrologic Engineering, vol. 16, no. 6, June 2010, pp. 395-403. Werthmann, C 2008, 'Water and Green Roofs in Dry Climates - A Speculation', Applied Sciences, pp. 1-9. Williams, NSG, Hughes, RE, Jones, NM, Bradbury, DA and Rayner, JP 2010a, 'The Performance of Native and Exotic Species for Extensive Green Roofs in Melbourne, Australia', paper presented at the II International Conference on Landscape and Urban Horticulture 881, 2010. Williams, NSG, Rayner, JP and Raynor, KJ 2010b, 'Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia', Urban Forestry & Urban Greening, vol. 9, no. 3, pp. 245-251.

27

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28

Table 1.Summary of green roof research across climate zones by location, depth and composition of growing media Climate zone

Location of research site

Depth(s) of growing media

Number of investigated growing media blends and composition

Major findings

Reference

Dry (Arid and Semiarid) Zone

Bsh

Hot Semiarid

Bsk

Dry Semiarid

Murcia, Spain

50 mm

1)compost:soil:brick=1:1:3

No irrigation resulted in a failure of the plants. Substrate depth

100 mm

2) compost:brick=1:4

was more relevant then substrate type for plant growth.

1) proprietary substrate with various percentages of

No findings were correlated to substrate composition and/or depth.

heat expanded clay, peat, perlite and vermiculite

Most plants survived in the defined substrate.

Fort Collins, Colorado, USA

Adelaide, Bsk

Dry Semiarid

Australia

100 mm

1) Type I: crushed brick, scoria, coir fibre and 100 mm

composted organics

Medium Type I performed better than Medium Type II in both

300 mm

2) Type II: scoria, composted pine bark and hydro-

growing media depths.

(Ondoño et al., 2016)

(Bousselot, 2010)

(Razzaghmanesh et al., 2014a,b)

cell flakes Greater survival of succulents was observed over growing seasons.

Denver, Bsk

Dry Semiarid

Colorado, USA

1) expanded shale: compost and composted bark = 80

Deeper substrates supported a variety of tested species including

: 20 (% by Vol.)

shrubs.

(Schneider, et al., 2014)

Four substrate types: 1)S15: Pum60:P20:Z5, 2)S15: Pum60:C20:Z5, 3)S30: Pum40:P20:Z10, Bsk

Dry Semiarid

Athene, Greece

75 mm, 150

4) S30: Pum40:C20:Z10, where S, sandy loam soil;

Perlite amended deep substrate resulted in the least drought stress

mm

Pum, pumice; P, peat; C, compost; and Z, zeolite in

and highest cover rate on Zoysia matrella.

(Ntoulas et al., 2013)

volumetric proportions as indicated by their subscripts. Temperate Zone

Cfa

Humid

Sydney,

Subtropical

Australia

100 mm

1) 100 % mineral (scoria, Bayswater sand, coarse

200 mm

sand)

300 mm

2) 80% organic (coir, composted pine bark) : 20% mineral (clinker ash)

29

Plant loss was observed in shallower substrate regardless of substrate type and fertilizing treatment. 100% mineral mix was the worst performer.

(Morris et al., 2011)

3) 50% organic (coir, composted pine bark) : 50% mineral (scoria, clinker ash)

1) substrate (lapillus, pomix, zeolite, 2.9% organic matter) : 0,3% hydrogel* 2) substrate (lapillus, pomix, zeolite, 2.9% organic Cfa

Humid Subtropical

Trieste, Italy

80 mm

matter) : 0,6% hydrogel*

Plants performed better in substrates with added hydrogels even in

120 mm

3) substrate only

shallower substrate depth.

(Savi et al. 2014)

* cross-linked polyacrylic acid-potassium salt, STOCKSORB 660 medium, Evonik Industries

Cfa

Humid Subtropical

Trieste, Italy

100 mm 130 mm

The depth of the substrate has great influence on substrate N/A

(Savi et al. 2016)

temperature, therefore determines the resistance of root system to sustain heat stress.

1) amended soil (red gravel : vermiculite : bark Cfa

Humid Subtropical

Kobe, Japan

100 mm

compost = 3:4:3)- AS

Theaddition of OM showed to be the main driver for overall plant

2) furnace bottom ash-FBA

performance in shallow systems.

(Kanechi, et al. 2014)

3) turf mat (dried block of sod)- TM Cfa

Cfb

Humid

College Station,

Subtropical

Texas, USA

Marine-Mild Winter

Sheffield, UK

Commercial substrate (Rooflite© drain) in modular

shallow substrate, even when not irrigated, can still support a

system

growth of well-chosen and tested plants.

100 mm

1) LECA (light expanded clay aggregate) : green

Visual, structural and ecological plant traits were better in a low-

200mm

waste compost : medium loam = 50:35:15

input 200mm deep growing medium.

114 mm

(Dvorak and Vordel, 2012) (Dunnett et al., 2008)

1) commercial substrate (crushed brick with less than 4% organic matter) Cfb

Marine-Mild Winter

Sheffield, UK

80 mm

2) commercial substrate (crushed brick with less than

Higher organic content in dry conditions did not show greater

(Nagase and Dunnett,

4% organic matter) + 10% organic matter

plant growth.

2011)

3) commercial substrate (crushed brick with less than 4% organic matter) + 25% organic matter

30

4) commercial substrate (crushed brick with less than 4% organic matter) + 50% organic matter Cfb

Marine-Mild

Stuttgart,

Winter

Germany

Substrates in older extensive green roofs tends to acidify, various

Various on aged roofs

gradually building up the more organic matter and are shallower in

(Thuring and Dunnett,

depth than younger green roofs.

2014)

1) natural soil, lava, organic material (Vegtec Cfb

Marine-Mild Winter

proprietary substrate) Malmo, Sweden

40 mm

Organic material used (peat) rapidly decomposed, therefore not

2) crushed roof tiles and low organic content

suited for use on green roofs. The more stable organic material

3) crushed roof tiles and high organic content (no

should be used.

(Emilsson, 2008)

data on percentage was presented.) Cfb

Cfb

Marine-Mild

Melbourne,

Winter

Australia

Marine-Mild

Melbourne,

Winter

Australia

150 mm

125 mm

1)10mm scoria : 7mm scoria : coarse sand : coir peat

Majority of tested species showed not to be suitable for extended

= 50% : 20% : 20% : 10%

dry periods in tested substrate type and depth.

1)scoria (10 mm minus) : scoria (7 mm aggregate) :

To maximize survival of plants in hot and arid climates

coarse washed sand (1–2 mm) : horticultural grade

supplementary irrigation and selection of plants with increased

coir = 50 : 20 : 20 : 10

succulence is recommended.

(Williams et al., 2010a)

(Rayner et al., 2016)

Test I: 55 Cfb

Marine-Mild Winter

mm Surrey, UK

Test II:

The most successful growing medium contained clay pellets and

Tested 16 different aggregates.

crushed brick perhaps because of their good water holding

55 mm

capacity and stable pH for survival of wildflowers.

80mm

Cfb

Marine-Mild Winter

Newport, UK

150 mm

(Molineux et al., 2015)

1)Nine crushed brick and crushed tile inorganic

Volume to a mass ratio between organic and inorganic components

substrates

and particle size distribution of the growing medium were

2) Composted green waste (fine(0 – 10mm) and

important factors for increasing growth and survival of plants.

(Graceson et al., 2014)

coarse(10-25mm) grades 1) crushed brick, 2) crushed demolition aggregate, Cfb

Marine-Mild

Birmingham,

Winter

UK

110mm

3) solid municipal waste incinerator bottom ash aggregate,

Pure crushed brick was a material that supported the most diverse plant assemblage. In addition, water availability in a substrate was likely to be limiting factor for plant growth in short-term studies.

4) 1:1 mix of 1 and 2 5) 1:1 mix of 3 and 2

31

(Bates et al., 2015b)

Cfb

Csa

Marine-Mild

Birmingham,

Winter

UK

Interior Mediterranean

Pisa, Italy

110mm

Mulch treatments:

Higher OM content was not the best treatment over a longer

1) sandy loam(with approx.3% OM)

period, as tested plants showed to be less resilient to drought. The

2) mature compost(sandy loam with 6% OM)

substrate with low OM content supported more diverse and stable

3) no mulch control

brownfield vegetation.

Commercial substrate (AgriTERRAM©):

Almost 100% survival rate was achieved in 200mm with

150 mm

lapil : pumice : zeolite : peat = 35% : 35% : 5% :

hydroperlite, as a drainage layer, highlighting the significance of

(Benvenuti and Bacci,

200 mm

15%with 80mm hydroperlite (IGROPERLITE©) as

its increased water holding capacity and prolonged water

2010)

drainage layer

availability to plants when drought stressed.

1)100% sandy loam soil Csa

Interior Mediterranean

2)sandy loam soil : urea-formaldehyde resin foam = Athens, Greece

(Bates et al., 2015a)

280 mm

60%:40% 3)sandy loam soil : peat : perlite = 50%:30%:20% 4)peat : urea-formaldehyde resin foam = 60%:40%

Growing medium type 1 and 2 had a similar good growth rate of tested species. The addition of perlite to sandy loam improved growth and flowering and decreased substrate weight load at field

(Panayiotis et al., 2003)

capacity. Plant growth wasn’t influenced by reduction of substrate depth,

Csa

Interior Mediterranean

Athens, Greece

75 mm

1) grape marc

and water use efficiency was improved with the addition of grape

150 mm

compost:soil:perlite (2:3:5, v/v)

marc to the substrate. To improve plant growth in semi-arid

2) peat:soil:perlite(2:3:5, v/v)

conditions soil was added to increase water-holding capacity at

(Papafotiou et al., 2013)

low tensions. 1)pumice : perlite : compost : clinoptinolite zeolite = Csa

Interior Mediterranean

Athens, Greece

75 mm

50:20:20:10

150 mm

2)sandy loam : pumice : perlite : compost :

Growing medium containing sandy loam had higher moisture content available to plants during dry period implying satisfactory

(Panayiotis et al., 2011)

water holding capacity at low tensions.

clinoptinolite zeolite = 15:40:20:20:5 Cool Continental Zone Fine grade heat-expanded clay : medium grade heat expanded clay : coarse-grade heat-expanded clay : Humid Dfa

Continental Hot Summer, Wet All Year

arboretum and greenhouse waste compost in ratios

Ambler, Pennsylvania, USA

64 mm

(% by volume):

Shallow substrates containing heat-expanded clay tends to dry fast, even after 1 day without irrigation. Water holding capacity of

1) 10:60:10:20

components was the most relevant trait for plant survival.

2) 20:50:10:20 3) 30:40:10:20 4) 40:30:10:20

32

(Olszewski, 2011)

5) 50:20:10:20 6) 60:10:10:20 Heat-expanded slate : grade sand : Michigan peat : aged compost (aged poultry manure and composted yard waste) mixed in

Humid Dfa

Continental Hot Summer, Wet All

(% by volume): Michigan, USA

100 mm

of heat-expanded slate (80%) and a small amount of fertilizer. For

1)60:25:10:5

native perennials and grasses, the deeper substrate with added OM

2)70: 18.75:7.5:3.75

Year

Succulents could sustain in substrates with relatively high content

(Rowe and Monterusso, 2006)

and additional irrigation was necessary.

3)80: 12.5:5:2.5 4)90: 6.25:2.5:1.25 5)100:0:0:0 Humid Dfa

Continental Hot Summer, Wet All

Michigan, USA

Year

Dfa

Summer, Wet All

Heat-expanded slate : grade sand : Michigan peat :

50 mm

dolomite : composted yard waste : composted turkey

The deeper substrate and careful plant selection are important for

75 mm

litter = 40:40:10:5:3.33:1.67

successful development of Crassulacean species.

40 mm Michigan, USA

70 mm 100 mm

Year

Continental Hot Summer, Wet All

Deeper substrates with 70mm and 100mm had better moisture Sand : silt : clay =

retention capacity and daily temperature fluctuations were not as

86: 10: 4 (% by vol.)

severe as in 40mm, which resulted in better average coverage of

Michigan, USA

Year

25 mm

Heat-expanded slate : grade sand : Michigan peat :

50 mm

dolomite : composted yard waste : composted turkey

75 mm

litter = 40:40:10:5:3.33:1.67

Michigan, USA

Year

Dfb

the other hand, plants which survived in shallow depths after

(Rowe et al., 2012)

Recommended range of compost for high yield performance (80%

Continental Hot Summer, Wet All

Deeper substrates support better overall performance of plants. On seven years formed stable communities.

(% by volume)

Humid Dfa

(Getter et al, 2009)

12 Sedum species.

Humid Dfa

(Durhman et al, 2007)

(% by volume)

Humid Continental Hot

25 mm

Humid

Central

30 mm

Continental Mild

Pennsylvania,

60 mm

6 substrate components including sand and heat

> OM > 60%) of two vegetable plants (cucumber and pepper) was

expanded shale and increasing percentages of

much higher than the recommended range by F.L.L. guideline

commercial compost (0, 20, 40, 60, 80, and 100%)

(<20 %).

2 commercial substrate types:

None of the herbaceous plants survived in shallowest substrates in

33

(Eksi et al., 2015)

(Thuring et al., 2010)

Summer, Wet All

USA

120 mm

1) expanded shale, 2)expanded clay

drought stress conditions. Expanded clay showed better

Year

characteristics than expanded shale, especially for Sedums.

Humid Dfb

Continental Mild Summer, Wet All

Quebec City, Quebec, Canada

Year Humid Dfb

Continental Mild Summer, Wet All Year

50 mm

Commercial growing medium (Sopraflor):

Herbaceous plants grown in deeper substrates performed better

100 mm

mineral aggregate : organic matter = 60 : 40 (% by

during winter months. Min. 100 mm deep substrate was

150 mm

volume)

recommended. Tested species used water rapidly through evapotranspiration

Central Pennsylvania,

(Boivin et al., 2001)

89 mm

Commercial growing medium(Gerick Corp., Ohio)

when water was available. However, after a couple of days without water, evapotranspiration dropped and was similar to

USA

evaporation from the non-planted test plots.

34

(Berghage et al., 2007)

Table 2. Summary of studies investigating green roof media composition in controlled environments Location of research site

Growing media components

Study environment

1) heat-expanded shale 50% Northern

2) sphagnum peat moss 20%

Colorado, USA

3) perlite 20%

Major findings

Standard method used

Reference

in the test

Succulent species retained more soil moisture over a Greenhouse

longer period compared to herbaceous plants. Also, the

N/A

(Bousselot et al., 2011)

revival of succulents was greater than that of herbaceous

4) vermiculite 10%

plants.

1)crushed porcelain : municipal compost = 80 : 20 (% by vol) 2)foamed glass : municipal compost = 80 : 20 East Lansing, Michigan, USA

3) heat-expanded shale substrate (24%

Plant grown in heat-expanded shale showed best results in Greenhouse

hayditeA (0.07–2.38 mm) and 24% haydite B

terms of plant growth, biomass accumulation, and plant

FLL recommendations

(Eksi et al. 2016)

stress for Sedum album and Ocinum × citriodolum.

(2.38–9.51 mm)) : 2NS sand : municipal compost = 48 : 32 : 20 (% by vol) Three substrates: 1)Scoria mix:

Increased survival of five succulent species was related to

scoria ≤8 mm: scoria 7mm aggregate : coir = Melbourne, Australia

60 : 20 : 20 (% by vol)

Greenhouse

reduced biomass under drought. The recommendation

2)Crushed roof tile mix:

made to use plants with low water use and high leaf

≤7 mm crushed roof tile :coir = 80 : 20

succulence in substrates with high WHC.

FLL recommendations

(Farrell et al., 2012)

3)Bottom ash mix: bays water sand : filter coir = 60 : 20 : 20 1) Scoria based mix: Melbourne,

scoria ≤8 mm: scoria 7mm aggregate : coir =

Australia

60 : 20 : 20 (% by vol)2) crushed terracotta

Overall, silicates showed better results in terms of Greenhouse

roof tiles:

1) aerolite black scoria 7 mm minus block mix

Australia

: 7 mm red scoria aggregate : coir = 60 : 20 :

FLL recommendations

(Farrell et al., 2013a)

aestivum and Lupinus albus, especially in scoria. On the other hand, hydrogels improved WHC for scoria only.

≤7 mm crushed roof tile :coir = 80 : 20 Melbourne,

increased time until wilting and root biomass for Triticum

Greenhouse

Granite outcrops showed greater flexibility in water usage, then succulents. Careful plant selection with

35

(Farrell et al., 2013b)

20

favourable plant physiological characteristics in mind is strongly recommended.

12 growing media blends containing following components: Ambler,

1) heat expanded coarse slate

Pennsylvania,

2) heat expanded fine slate

USA

3) waste compost

Laboratory and

The addition of OM and hydrogel was recommended for

greenhouse

shallow green roof systems and areas prone to wind

FLL recommendations

(Olszewski et al., 2010)

erosion and high evapotranspiration.

4) hydrogel (polyacrylamide-based)

Heat-expanded slate : grade sand : Michigan Michigan, USA

peat : dolomite : composted yard waste :

Deeper substrates showed best results for moisture Greenhouse

content and growth of Sedum spp.. Shallow substrates

composted turkey litter = 40:40:10:5:3.33:1.67

with water retaining fabric did support plant growth better

(% by volume)

then shallow substrates on his own.

1)50% pumice (4–10 mm), 2)30% zeolite(1–8 mm)

N/A

Plants decreased their transpiration rate and improved Greenhouse

3)20% composted softwood bark fines

water retention, when stressed, improving water retention

FLL recommendations

(Vanwoert et al., 2005b)

(Voyde et al. 2010)

of substrate providing better stormwater storage recovery. Large brick substrates had lower WHC than the small

Sheffield, UK

1) small brick at 2–5mm particle dia

brick substrates. Green waste compost increased shoot

2) large brick of 4–15mm dia

and root growth, shoot nitrogen concentration and

3) conifer bark compost

Greenhouse

root:

shoot

ratio

compared

to

bark.

4) green waste compost

Polyacrylamide gel increased water holding capacity and

5) Polyacrylamide gel “SwellGelTM”

increased shoot growth slowly.

v/v) 2)green waste compost (20%v/v) 3)presence or absence of polyacrylamide gel

N/A

(Young et al., 2014)

Substrates with coarser particle sizes encouraged slower

1)brick particle sizes 2-5 mm or 4-15 mm (80% Sheffield, UK

decreased

but more sustainable growth and more drought tolerance Greenhouse

of plants. Water retention additives enhanced drought tolerance of plants without excessive plant growth.

36

FLL recommendations

(Young et al., 2015)fd./

Appendix A - Glossary of Terms Activated carbon Arkalyte Autoclaved aerated concrete

A form of processed carbon riddled with small, low-volume pores that increase the surface area available for adsorption or reactions. Usually derived from charcoal. Clay heated to 1000° C (see also Expanded clay) A lightweight, precast, foam concrete building material which simultaneously provides structure, insulation, and fire- and mold-resistance. Commercially available calcined diatomaceous earth containing <1% crystalline silica

Axis

(cristobalite) and <1% crystalline silica (quartz), prepared by kiln-firing at ~1,800°C, resulting in a porosity of 82%, pore sizes ranging from 0.1 to 1.0 μm and a pH of 7.

Biochar Bottom ash

Clinker Coir Diatomaceous earth Dolomite

Charcoal produced by heating organic material at a high temperature in limited oxygen. Stable product, very rich in carbon and is used to lock carbon into the soil. Part of non-combustible residue (fly ash and clinker) of combustion in a furnace or incinerator. Non-combustible fragment that can be found in ash residue after burning heating fuels such as coal or wood. A natural fibre extracted from coconut husk. Soft, crumbly, porous sedimentary deposit formed from the fossil remains of diatoms, a type of hard-shelled algae. Very light due to high porosity. Translucent mineral consisting of a carbonate of calcium and magnesium. Lightweight ceramic shell with honeycomb core produced by firing natural clay to temperatures of

Expanded clay aggregate

1100 - 1200 °C in a rotating kiln. Rounded pellets are characterized by its light weight, high permeability, high durability and excellent sound and thermal insulating properties.

Expanded shale

Expanded slate

Material manufactured by heating shale which dramatically expands its volume. Suitable for use as lightweight aggregate. Slate whose subjection to exfoliation results in a porous material suitable for use as a lightweight aggregate.

Fibre Life compost

Commercially available compost. By-product of an anaerobic digestive process

Friedlaender Ton (FT)

Bentonite-like clay mineral, named after German town, Friedland

Grodan hydroponic media

A form of silica that is melted and spun into fibres producing light, water absorbing material.

Haydite

Shale heated to 1000° C (see also Expanded shale)

Hydrocell flakes

Hydrogel Hydroretentor Lapilli (singular: Lapillus)

A commercially available horticultural foam flake, very light-weight with excellent water holding ability. Hydrocell is biodegradable and used as a soil enhancer and aerator. A network of natural or synthetic polymer chains that is hydrophilic. Hydrogels are highly absorbent polymers that can contain over 99.9% water. Soil amendment with good water holding capacity Size classification term for fragmental material that falls during a volcanic eruption. By definition lapilli range from 2 mm to 64 mm in diameter. Commercially available mineral called pozzolan, a crystalline, porous aluminosilicate. Its

Lassenite

composition includes aluminium, silicon, and oxygen. It has remediative possibilities and capacity as soil amendment.

Lava

Mycorrhizae

Peat

Refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. A symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a vascular plant. An important component of soil life and soil chemistry. An accumulation of partially decayed vegetation. Peat forms in wetland conditions, where flooding obstructs flows of oxygen from the atmosphere, slowing rates of decomposition.

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Perlite

Pomix

Pumice

Roofsoil (Vegtech)

An amorphous naturally occurring volcanic glass with relatively high water content. It has unusual property of greatly expanding when heated sufficiently. Commercially available growing media consisting of white peat, limestone, pumice, lapilli and added nutrients. A volcanic rock, typically light-coloured that consists of vesicular rough textured volcanic glass, which may or may not contain crystals. Commercially available growing media based on natural soil, lava, organic material and other components, but the exact composition is proprietary. Product of Vegtech.

Roof shingles

A roof covering made of wood, slate, ceramic, etc.

Scoria

Highly vesicular, typically dark coloured volcanic rock that may or may not contain crystals.

Silica

A hard, unreactive, colourless compound, SiO2, occurs as mineral quartz or sand.

Urea-formaldehyde resin

A non-transparent thermosetting resin, light but very stable. Used for improving physical

foam

properties, such as water retention capacity and aeration of horticultural substrates.

Vermiculite

A hydrous, silicate mineral that expands greatly when heated.

WHC

Water holding capacity, an attribute of soil or any substrate which show the total amount of water a soil or substrate can hold at field capacity. A microporous, aluminosilicate minerals commonly used as commercial absorbents. Zeolite can

Zeolite

absorb up to 55% of its weight in water and slowly release it for plant uptake. Also, provides source of slowly released potassium.

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