Water in green roof substrates – linking physical measurements to plant performance Tobias Emilsson, ZinCo GmbH, (tobias.emilsson@zinco‐greenroof.com), Germany Christian Berretta, University of Sheffield, (c.berretta@sheffield.ac.uk), United Kingdom Ralf Walker, ZinCo GmbH, (tobias.emilsson@zinco‐greenroof.com), Germany Virginia Stovin, University of Sheffield, (v.stovin@sheffield.ac.uk), United Kingdom Nigel Dunnett, University of Sheffield, (n.dunnett@sheffield.ac.uk), United Kingdom
Abstract The University of Sheffield and ZinCo GmbH are currently undertaking the largest international research project on green roofs to date. The project “Green Roof Systems” is revisiting the fundamental basis of green roof system design and is funded under the EU FP7 Marie Curie (IAPP). The substrate is a key component in a green roof in relation to plant performance and stormwater management. Green roof substrates are required to fulfil some specified performance criteria, such as those defined by e.g. the FLL. The thresholds have generally been developed through evidence‐based practice and discussion between independent researchers and green roof companies during more than 30 years. Green roof substrates are primarily based on inorganic material amended with an organic material. Our aim is to investigate the possibility of using novel substrate formulations that fulfil technical requirements for density and permeability, but at the same time supply water to plants during extended drought periods and have a capacity to retain and detain stormwater during heavy storms. We have been developing a framework for testing the impact of amendments on substrate performance. The substrates have been investigated using the FLL methodology which has been compared to measurements on soil moisture World Green Roof Congress, 19-20 September 2012, Copenhagen Page 1
retention curves. We are also developing a bioindicator test‐protocol for substrate quality. This paper will explain our approach to testing substrate amendments, and methods for investigating substrates. We are presenting some preliminary results on relationships between substrate amendments, water holding capacity and plant survival. Methodological considerations in relation to FLL test, pressure‐ plate extractions and bioindicator methods are discussed.
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Background Industrial Context The major development of green roof system technology has taken place at applied research centres, companies and universities in Germany during the last 20‐30 years. Almost every aspect of green roof systems has been investigated, from substrate performance to plant composition and stormwater runoff. The research has resulted in guidelines and recommendations for installation of functioning green roof systems. The last 10 years have shown a dramatic increase in green roof research at an international level. Our current project is the largest international green roof project to date, involving more than 15 people from an academic institution, University of Sheffield and a commercial partner, ZinCo Gmbh. The project has a long time span, running over 4 years, and focuses on knowledge transfer and the development of research and technology. The project is funded under the European Union People program as a Marie Curie Industry Academia Partnerships and Pathways (IAPP) programme. Our approach has been to put together a thorough intensive project revisiting the most important aspects of green roof technology with a primary focus on water. The joint approach with an academic and commercial partner means that we are able to investigate problems and questions of scientific, technological and commercial interest. The aim of the project is to redevelop some of the fundamental scientific bases for green roof design and functioning and to open some of the black boxes of green roof functioning. We have identified water as the most important parameter for green roof performance and green roof design. Water is the most important variable for plant survival but also a key aspect when looking on proposed impacts of green roofs in an urban context, i.e. storm water retention and detention. We are investigating the water aspects of green roofs in relation to three work packages (WP). WP 1 is focused on broadening the available plant palette that can be used in green roofs of different substrate thicknesses. In this work package we are investigating the water variable directly through an irrigation treatment but also indirectly through varying substrate depths. In WP 2, which is the primary focus of this presentation, we are looking at water storage of conventional and novel substrate formulations as well as water storage capacity in different green roof system components. The last workpackage, WP 3, is aimed at developing a synthesis of water in green roofs, by looking on the combined effect of different green roof components as well as developing generic stormwater models and analysis of environmental performance. The water storage capacity of substrates has a clear industrial interest as it will influence how easily a roof can be established, i.e. the survivability and development of the green roof vegetation. It is also interesting from a commercial standpoint as substrate water storage capacity will influence urban runoff which in turn is a strong argument for installing green roofs. Water storage is also particularly important during the establishment phase of a green roof where finding a good balance between growth and survival is of great importance.
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Problem Green roof substrates are developed and marketed on a very competitive market. The bulk components in green roof substrates are generally porous mineral materials that are mixed with varying amounts of organic materials. There has been extensive research on various inorganic materials that could be suitable for green roofs, including volcanic material, recycled materials and engineered materials (Roth‐Kleyer 2001). There are numerous combinations that could be suitable for use on green roofs. The porous materials are normally amended with some kind of organic material such as peat, compost or other types of green waste, in order to increase water holding capacity, aeration and nutrient exchange. The final characters of the green roof substrate are related to cost of substrate and to the widely‐adopted FLL guidelines (FLL 2002). The FLL guideline is the main document and specification of green roof system performance. The guidelines have been developed by the independent organisation Forschungsgesellschaft Landschaftsentwicklung und Landschaftsbau since its first edition in 1984 (FLL 2002). The FLL guidelines are aimed at maintaining a minimum quality for new green roof systems that are being installed in Germany. If the threshold values that are specified in the guidelines are followed, one can be confident that the individual components will not fail. The guidelines also ensure that the purchaser of a green roof has a reduced risk and that possible conflicts regarding system quality can be handled. The FLL guidelines specify certain substrate characteristics that have to be met to prevent failure of the roof vegetation and prevent harm to the actual building and sealing membrane. The substrate has to be designed to maintain positive plant growth without posing a risk to the building envelope. Achieving sufficient water holding capacity without increasing costs is the starting point for most substrate formulations. Additional considerations such as limited nutrient runoff and structural stability in respect to frost and organic turnover also needs to be taken into account. The origin of organic components in green roof and other horticultural substrates have been debated. The main criticism has been related to the inclusion of peat in substrates as this is seen as a non‐renewable resource that cannot be harvested without permanent damage to wetland ecosystems. There are also regulations in place that restricts the use of recycled organic materials such as compost and sludge. Thus, organic components are of vital importance for green roof substrates as they increase water holding capacity and increase the amount of nutrient exchange site but harvest and/or processing of peat can be problematic (Alexander and others 2008; DEFRA 2012). In our investigation we are trying to investigate 1) the impact of different organic materials and amendments on water holding capacity and permeability of green roof substrates and 2) more precise methods to characterise water storage in substrates. Learning Objectives:
Increased understanding of the effect of some substrate amendments on water holding capacity and permeability
Outlooks on alternative measurements on substrate water content
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Approach We have been investigating the impact of 8 synthetic or inorganic amendments, and 6 organic amendments in a total of 63 different combinations and application rates. This paper presents a subset of substrates focusing on organic amendments. Our investigated substrates were mixed using a concrete mixer. The substrates were mixed in relation to volume based recipes but the actual mixes were created based on weight which was recalculated to volume using the density of the materials. Standardised densities of the materials were determined by sieving the material through a 20mm grid down into a 20 litre container where it was allowed to settle under gravity (Deutsches Institut für Normung 2000). All investigations were done on substrates based on crushed brick as the main inorganic component. The crushed brick mix was amended with a small amount of pumice to increase the water holding capacity of the mix to around 30% (v/v). This mix of crushed brick and pumice (RT+) was used as a reference for further mixing. We also used a standardised green roof substrate including organic material as a reference mix (RM) within the project. This reference mix contained 85% RT+ and 15% organic material. The organic material in this RM was 2/3 coco coir and 1/3 composted bark. The main analysis of substrates was performed using the methods as described in FLL (2002). Additional measurements of water holding capacity of substrates was investigated with pressure plate extraction as described by Werdin (2011). Amended substrates were investigated using ‐3, ‐5, ‐10 and ‐15 bar pressure. RM was investigated at ‐0.35, ‐1, ‐2, ‐3, ‐4, ‐5, ‐10 and ‐15 bar Pressure. Plant growth was investigated in a controlled climate experiment, a phytometer experiment. The setup included 3 different plant species (Sedum sexangulare, Bromus erectus and Leucanthemum vulgare), in RM substrate and 10 other amended crushed brick based substrates. In this paper, we will show some preliminary results from 3 interesting mixes. The main idea behind the phytometer experiment is related to plant survival and development during establishment. It is generally known that the first year and especially the first few weeks after plant establishment are critical in a green roof installation. It would be ideal if the installer would not have to undertake additional watering after plant establishment. Thus, plants should not be stimulated to excessive growth by a substrate with a very high WHCmax as this growth can become difficult to sustain in draught periods. The substrate should on the same time be able to supply the plant with water for an extended period to support plant survival during extended periods without precipitation. This could be achieved by reducing evaporative losses from the substrate or through amending the substrate with material that have novel ways of storing water and thereby keeping evaporative losses down at a minimum. This approach allows us to identify how much of an amendment that is required in order to achieve the same WHCmax and it is also a good basis to compare the pF‐curves and plant establishment in the amended substrates. All substrates that was developed to be used in the phytometer had the same WHCmax as determined with FLL methodology. In our case the target value was set to 35% V/V as this is the minimum required amount according to FLL for an Extensive Green Roof. Thus, differences in survival would be dependent on how tightly water was bound to the substrate World Green Roof Congress, 19-20 September 2012, Copenhagen Page 5
and not to total amounts as measured with the FLL WHCmax. Plants from B. erectus and L. vulgare were produced from seeds, grown in sand and transplanted to the substrate under investigation at the start of the experiment. S. sexangulare were established from cuttings direct in the test substrate. The substrates were tested in 4 replicates per substrate and species combination in small containers, 26 by 16 cm, installed 5 cm thick substrate. Every container was planted with 12‐14 plants depending on species. The containers were soaked at time of establishment and irrigated regularly for two weeks after which they were left to dry for 35 days. Plant aesthetics and container weight was recorded on a weekly basis. Biomass was determined at the end of the experiment. Green house climate was set according to the OECD manual.to 22°C ± 10°C and relative humidity 70 % ± 25 % (OECD 2006).
Analysis The initial FLL tests showed that amending the RT+ substrate with 10 to 15% organic material in the form of peat, composted bark or biochar increased water holding capacity of the substrate as compared to the unamended substrate (Figure 1). There was no statistical difference in WHCmax between the three amendment rates of composted bark, biochar or peat. The different materials behaved significantly differently (ANOVA, F 2,21 = 27.145 p<0.001) with biochar having the least effect on the water holding capacity. The permeability of the substrates increased with biochar addition but decreased when amending the RT+ substrates with peat or composted bark (Figure 2). The permeability was significantly effected by amendment type (ANOVA, F 8,21 = 36.23 p<0.001) and all substrate mixes with biochar had significantly higher permeability than all the other substrates (Tukey test < 0.05). The variability in the data was large and no significant relationships could be established between level of organic amendment and permeability. However, WHCmax and mod. Kf show a strong negative correlation as permeability decreased with increasing water holding capacity (Spearman’s rho, N 30, correlation coefficient ‐0.855, p <0.01) (Figure 3).
Figure 1: Maximum water holding capacity (WHCmax % V/V) for biochar, composted bark and peat amended to a standardised crushed brick material (RT+) in 10, 12.5 and 15% of total World Green Roof Congress, 19-20 September 2012, Copenhagen Page 6
volume. Circles represent mean values and error bars +/‐ 2 SE. The unamended reference mix had a WHCmax of 30.26% V/V which is represented by the grey line.
Figure 2: Water permeability (Mod Kf mm/min) for biochar, composted bark and peat amended to a standardised crushed brick material (RT+) in 10, 12.5 and 15% of total volume. Circles represent mean values and error bars +/‐ 2 SE. The unamended reference mix had a Mod Kf of 203.9 mm/min which is represented by the grey line.
Figure 3: Waterholding capacity (WHCmax % V/V) was found to be negatively correlated to water permability (Mod Kf mm/min) for biochar ( ), composted bark ( ) and peat ( ). The pressure plate analysis revealed that there is a difference between substrates amended with different types of organic material, in relation to remaining water content when the substrate is extracted to very high negative pressures such as ‐15 bar, i.e. what is commonly World Green Roof Congress, 19-20 September 2012, Copenhagen Page 7
known as the permanent wilting point. Peat amended substrates and the RM substrate show a larger remaining water reservoir at ‐15 bar extraction pressure as compared to completely unamended substrates (Table 1). Comparison between the FLL WHCmax and the pressure plate extractions also shows that there is on clear relationship between the tightly bound water in the substrate and the measurement of water at field capacity which is investigated in the FLL methodology. It might be that the substrates that we are investigating have very similar characteristics for the tightly bound water and that the amendments have their largest impact on the readily available water. Additional measurements on the substrates that were used in the phytometer experiment, thus substrates that have been developed to have similar FLL WHCmax values, are underway. These measurements will also include measurements on lower pressures that would more closely match the values for WHCmax from the FLL methodology. Table 1: Available water in substrate RM, substrate amended with 15% peat, and RT+ after a) Fll WHCmax measurements b) pressure plate extraction at ‐3, ‐5, ‐10 and ‐15 bar, and c) available water between ‐3 to ‐5bar, ‐5 to ‐10 bar and between ‐10 to ‐15. Table show mean values +/‐ 2 SE WHCmax (%V/V) FLL
RM 15% peat RT+
3.00
Mean 40.77 37.32
SE 0.35 0.69
Mean SE 14.79 0.34 18.07 0.98
30.26
0.22
14.10 0.47
Pressure (bar) 5.00 10.00
15.00
Mean SE Mean SE Mean SE 11.02 0.32 7.67 0.33 7.13 0.15 11.03 0.17 8.30 0.06 7.30 0.17 8.43 0.07
9.05 0.25
5.85 0.15
Available water between pressures (bar) 3-5 5-10 10-15 Mean 3.77 7.04
Mean 3.35 2.73
Mean 0.53 1.00
5.68
-0.63
3.20
The interesting part of substrate design is whether this measured difference in water storage can be related to the plant experienced water environment and if the small differences in moisture content can be translated into performance variables. The phytometer experiment showed dramatic loss of water from the substrates during the first week. Plant aesthetics decreased in a similar fashion and all non‐succulent plants were dead after 35 days following onset of the drought treatment. The results indicate that high FLL water holding capacity does not directly result in prolonged survival or increased plant aesthetics. The RM substrate in the phytometer experiment had 5% higher water holding capacity as compared to the other investigated substrates, but this did not translate into increased aesthetics or survival (Fig 4). As mentioned before, the substrates that were used in the phytometer experiment are currently being investigated in pressure plate experiments to determine the effect of tightly bound water on plant performance and survival.
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Figure 4: Mean plant aesthetics of Bromus erectus, Leucanthemum vulgare and Sedum sexangulare during a 5 week drying treatment in controlled climate. Points represent mean values and error bars +/‐ 2 SE. RM ( ), Peat amended substrate ( ), Composted bark amended substrates ( )
Results and Business Impacts Key Findings There are numerous ways to achieve a good green roof substrate. We used the same type of crushed brick and pumice as the basis for our mixes but there is substantial research on characters of different inorganic materials (Roth‐Kleyer 2001). These can be combined with a range of virgin or recycled organic materials. The increasing concerns regarding use of peat in green roof substrates can be alleviated by using other organic materials. Our initial research show that composted bark behaves similarly to peat when looking on water storage capacity. Additional work is needed to determine its role in nutrient exchange and ability to sustain plant growth (Benito and others 2005). We have been able to determine a strong negative relationship between water holding capacity and water permeability. This relationship is rather trivial but we have also been able to establish that the strength of negative relationship differ between different substrate amendments. Thus, a wise combination of different organic material can increase water holding capacity of the substrates without severely reducing permeability. In any case, all the substrates presented in this study have high permeability rates, well above FLL target values. Biocoal or biochar has received a lot of attention in the last years for its positive effects on e.g. aeration, water holding capacity, water permeability and nutrient exchange (Sohi and others 2010). We did not see a dramatic improvement of water holding capacity when amending our crushed brick based substrates with biochar. The biochar material that we used as amendment had a similar grain size distribution as the crushed brick. The small effect of biochar in our green roof substrate could be due to the fact that our crushed brick matrix might have had a higher affinity for water than the actual amendment. In a normal World Green Roof Congress, 19-20 September 2012, Copenhagen Page 9
soil this would have been reversed with biochar being the component with high affinity for water. We still need to investigate how the water is held within a biochar substrate in pressure plate extractions and growth tests to increase our understanding on how the biochar might influence plant growth. We have also shown that there is a real need to develop more detailed ways of measuring water storage in green roof substrates. There has been a growing interest in using alternatives methods during the last years and this is something that we can confirm in our study (Fassman and Simcock 2011). We did not find a clear relationship between WHCmax from FLL methods and pressure plants extractions or between WHCmax and performance in growth test in our phytometer. We are currently developing these measurements for substrates that were designed to have very similar WHCmax as described in the FLL methodology. A key point would be how these differences relates to aesthetics in different plant species and how reliable values or useful the measurements from the FLL methodology are for substrate optimisation for increased plant performance.
Business Impacts Substrate design is a key aspect of the green roof market. There are viable alternatives to peat. Composted bark might one of many viable options. Biochar is probably not a good alternative on its own, as the sole organic amendment in a mix. It could play a role as a minor or secondary organic component that maintains permeability and aeration in dense mixes. The results from our phytometer experiment will also be used to link measurements for evapotranspiration from a range of different substrates to water retention curves, and plant survival. This information will be vital in developing substrates with improved establishment success with reduced need for supplemental irrigation.
Conclusions Development of novel substrates for green roofs is an ongoing task. Substrates with increased hydrological performance are needed as well as substrates that have a low cost of installation. Development of alterative methods to FLL would be useful for increased understanding of water relations of green roof substrates even if the FLL guidelines have high practical relevance. Key Lessons Learned: There are several good alternatives to peat
Biochar does not increase water holding capacity in crushed brick substrates
Development of new methods is needed for greater understanding of water movement in green roof substrates.
New methodologies are needed to develop substrates that are optimised for establishment success.
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References Alexander, P. D., Bragg, N. C., Meade, R., Padelopoulos, G.,Watts, O. (2008) Peat in horticulture and conservation: the UK response to a changing world. Mires and Peat 3: 1‐10 Benito, M., Masaguer, A., De Antonio, R.,Moliner, A. (2005) Use of pruning waste compost as a component in soilless growing media. Bioresource Technology 96: 597‐603 DEFRA (2012). Consultation on reducing the horticultural use of peat in England (No. 101217). London: Department for Environment, Food and Rural Affairs. Deutsches Institut für Normung (2000). DIN EN 12580 Soil improvers and growing media ‐ Determination of a quantity. Berlin. Fassman, E. A.,Simcock, R. (2011) Moisture Measurements as Performance Criteria for Extensive Living Roof Substrates. Journal of Environmental Engineering: In press FLL (2002). Guideline for the planning, execution and upkeep of green‐roof site: Roof greening guidelines (4 ed.). Bonn: Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e. V. OECD (2006). Test No. 208: Terrestrial Plant Test: Seedling Emergence and Seedling Growth Test OECD. Roth‐Kleyer, S. (2001) Vegetationstechnische Eigenschaften mineralischer Substratkomponenten zur Herstellung von Vegetationstrag‐ und Dränschichten für bodenferne Begrünungen. Dach + Grün 10: 4‐11 Sohi, S. P., Krull, E., Lopez‐Capel, E., Bol, R.,Donald, L. S. (2010). Chapter 2 ‐ A Review of Biochar and Its Use and Function in Soil. In Advances in Agronomy (Vol. 105, pp. 47‐ 82): Academic Press. Werdin, J. (2011). Determination of moisture retention curves for a variety of green roof substrates by using the pressure plate method. Paper presented at the 1st national green roof student conference.
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Authors’ Biographies Dr Tobias Emilsson is working as Marie Curie senior research fellow at Zinco GmbH. His main work is currently focused on substrate design and water relations of extensive green roof substrates. Tobias Emilsson has a background in Plant ecology and a PhD in technology focused on Extensive green roofs. His previous work has involved vegetation development and nutrient runoff from extensive green roofs.
Dr Christian Berretta’s research focuses on hydrological and environmental processes monitoring and modelling in urban environment, Sustainable Drainage Systems (SuDS) to restore pre‐development hydrological condition while controlling targeted pollutants, and the assessment of source area runoff impact on aquatic ecosystems and human health. He developed his research activity at the University of Genoa, Italy (2001‐2007) and at the University of Florida, US (2007‐2011). He is currently working as Marie Curie senior research fellow at the University of Sheffield, UK.
Ralf Walker is the Head of R+D at ZinCo GmbH in Germany and has undertaken lots of developments in the field of Green Roofs. His background is in horticultural engineering, soil science and plant nutrition as well as in plants.
Dr Virginia Stovin is a Senior Lecturer in the Department of Civil and Structural Engineering at The University of Sheffield. Her work focuses on urban drainage structures and processes, most recently on the hydrological performance of SuDS. She is an enthusiastic proponent of SuDS retrofitting, and is a co‐author of the recently‐published CIRIA retrofitting guidance. As part of the Sheffield‐ based Green Roof Centre, she has constructed 11 green roof test beds. The long term records from these beds are being combined with laboratory studies to underpin the development of green roof hydrological performance modelling tools suitable for urban stormwater management planning.
Dr Nigel Dunnett is Director of The Green Roof Centre, Sheffield, UK, and Professor of Planting Design and Vegetation Technology in the Department of Landscape, University of Sheffield. He has a background in botany, horticulture and ecology. His work revolves around innovative approaches to planting design, and the integration of ecology and horticulture to achieve low‐input, dynamic, diverse, ecologically‐tuned designed landscapes, at small and large scale.
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