Introduction
In recent years, the importance of green space in urban areas has been changing. A new vision of urban centers incorporates more green space, such as parksReference Nowak1, and a more human-friendly environment mixing traditional urban centers of industry, commerce and residence with food productionReference Nugent2, Reference de Zeeuw, Guendel and Waibel3. Implementation of this vision has been facilitated by the introduction of more environmentally friendly technologies and the introduction of policies and programs which promote their useReference Cheney, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey4–Reference Liptan, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey8. One such technology being incorporated into development is green roofing.
Definition of green roofs
Green roofing is a technology enabling growth of vegetation on rooftops, effectively replacing green space lost during building construction. Conventional green roofs generally consist of a number of layers including a root barrier to prevent damage to the underlying structure; a drainage layer to facilitate the removal of excess water; a filter fabric to prevent the drainage layer from clogging with media; growing media; and vegetationReference Getter and Rowe9. Others may be composed of growing modules or vegetated mats. Green roofs vary in depth of growing media and vegetation, but are generally divided into two categories: extensive (those with <15 cm of media) and intensive (those with >15 cm of media)Reference Kula7. Extensive roofs are usually planted with ground cover or succulent species that require little maintenance after establishment, while intensive roofs can support herbaceous perennials, shrubs and even treesReference Getter and Rowe9, Reference Dvorak and Volder10 but typically require continued inputs. Design and plant selection depend on the purpose of the project and the environmental benefits to be achieved. Uses of green roofs range from functional storm water and energy management roofs to park-like amenities open to the public, to food production.
Factors determining plant survival include media composition and depthReference Getter and Rowe11, Reference Rowe, Monterusso and Rugh12, incoming solar radiationReference Getter, Rowe and Cregg13, climate and most importantly soil moistureReference Durhman, Rowe and Rugh14–Reference Monterusso, Rowe and Rugh16. Insufficient moisture can be remedied by altering media composition and depth or with irrigation if available. Growing media used on commercial green roofs are often engineered and comprise mostly lightweight materials such as heat-expanded slate or shaleReference Getter and Rowe9. Media composition depends on manufacturer and available materials, but it is designed to be lightweight while maintaining the ability to support plant life. Organic matter content may vary and is beneficial for plant growth, but when it decomposes it may leach nutrients, resulting in runoff water-quality issues, and is inconvenient to replace. Economics dictates that substrate composition will depend on materials that are locally available and can be formulated for the intended plant selection, climatic zone and anticipated level of maintenance.
Benefits of green roofs
Use of green roofs in urban development has been shown to provide a number of benefits including reduced air and noise pollution, carbon sequestration, increased habitat and biodiversity, increased roof lifespan, storm water retention, energy savings and mitigation of the urban heat islandReference Getter and Rowe9, Reference Getter, Rowe and Cregg13, Reference Alexandri and Jones17–Reference Wong, Tay, Wong, Ong and Sai26. Computer modeling predicts that an increase in green roof area, as could take place with the technology's incorporation into urban agriculture, will amplify these benefitsReference Bass, Krayenhoff, Martilli, Stull and Auld27. Moreover, the introduction of green roofs to city development plans could promote infill development (the redevelopment of vacant areas within urban centers) and reduce spending on the development of new infrastructure such as roads and sewer linesReference Loder and Peck22. Although infill development largely benefits municipalities and companies, expansion of the green roof industry would increase employment and economic growth in urban centers, benefiting populations that typically turn to urban agriculture.
Energy savingsReference Fang28–Reference Wong, Tay and Chen31 and mitigation of the urban heat islandReference Alexandri and Jones17, Reference Bass, Krayenhoff, Martilli, Stull and Auld27, Reference Wong, Tay and Chen31–Reference Memon, Leung and Chunho34 in particular have great practical benefit to poorer communities who are most likely to benefit from urban agriculture. Green roofs can reduce energy consumption of a building by 2 to 39%Reference Sailor29, Reference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias30 and by between 12 and 87% for the top floorReference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias30 during the summer. These reductions are primarily due to reducing the amount of direct solar radiation that reaches the roof and the amount of heat transferred into the buildingReference Fang28, Reference Wong, Tay and Chen31. The extent of the reduction is dependent on the extent of vegetative cover on the roofReference Wong, Tay, Wong, Ong and Sai26, Reference Fang28, Reference Sailor29, the existence of other insulationReference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias30, the thickness of the green roof media, irrigation of the roof and climateReference Sailor29. Energy savings during the winter are negligible in warm climates, but can be seen in cooler climatesReference Sailor29, Reference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias30. Energy savings in very humid climates are also expected to be lower due to reduced evapotranspiration and the associated cooling effectsReference Alexandri and Jones17.
Green roofs have been shown to reduce ambient temperaturesReference Wong, Tay and Chen31, an effect that is projected to increase with increasing roof surfaces and the inclusion of green walls (plants grown on walls using a variety of training and planting systems) in urban developmentReference Alexandri and Jones17, Reference Bass, Krayenhoff, Martilli, Stull and Auld27. Modeling predicts a corresponding reduction in urban temperatures and increase in thermal comfortReference Alexandri and Jones17, Reference Bass, Krayenhoff, Martilli, Stull and Auld27. The end result of reduced temperatures and therefore energy use is a reduction in the use of conventional air-conditioningReference Alexandri and Jones17, Reference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias30. This would benefit low-income neighborhoods for two reasons. Urban neighborhoods with higher summer temperatures than surrounding urban areas are correlated with lower incomes, higher poverty ratesReference Harlan, Brazel, Prashad, Stefanov and Larsen32 and impoverished individuals aged 65 and olderReference Johnson and Wilson33 in cities such as Phoenix, AZ and Philadelphia, PA. These individuals are more at risk to environmental hazards, such as extreme heat events, due to a lack of resources enabling them to cope with the hazards, such as air-conditioningReference Harlan, Brazel, Prashad, Stefanov and Larsen32. Implementation of green roofs would not only reduce energy bills in such neighborhoods but also reduce the occurrence of extreme heat and the need to air-condition in the summer time, freeing up limited funds for other uses and improving the quality of life of individuals in these at-risk areas and populations.
Food production in an urban setting
Food production can be added to the benefits provided by green roofsReference Loder and Peck22, Reference Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard, Hickey, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey35 (Fig. 1) and expanded through the incorpoation of this technology into urban agriculture. Urban agriculture is defined as horticultural, agricultural or farming activities carried out on small plots of land in and around urban centersReference Enete and Achike36–Reference Vagneron38. Individuals in urban centers around the world participate in urban agriculture for reasons, such as poverty, unemployment, food insecurityReference Nugent2, Reference van Averbeke39, high prices of market food, income or asset diversification and supplementary employmentReference Nugent2. These motivational factorsReference Nugent2, Reference de Zeeuw, Guendel and Waibel3, Reference Enete and Achike36–Reference Peters, Bills, Lembo, Wilkins and Fick41; limitations such as land availability, land use and ownership rights, physical and economic access to inputs and potential food contaminationReference Enete and Achike36–Reference van Averbeke39,Reference Agbenin, Danko and Welp42, Reference Thornton43; and geographic and climatic factors unique to each urban center shape urban agricultureReference Nugent2, Reference de Zeeuw, Guendel and Waibel3, Reference Enete and Achike36, Reference Vagneron38. It is likely that a number of these limitations could be alleviated through the use of green roofs, while maintaining the benefits expected by urban farmers. Currently, policy is under reform in a number of cities worldwide resulting in accommodation for urban agricultureReference de Zeeuw, Guendel and Waibel3. These policy changes present an opportunity and could be guided to include farming on green roofs and expedite the inclusion of green roofs and other alternative technologies in urban farming with great benefit to urban populations.
Vegetable production on a green roof in East Lansing, MI.
Despite the possible benefits from incorporating green roof technology into urban agriculture, there are a number of potential issues that must be addressed. These include installation and maintenance costs, weight limitations, media composition and depth, cultural practices, potential water-quality issues of effluent and how food production would influence the other known benefits attributed to green roofs. These are not only factors that may limit the use of green roofs, but would also limit their viability for widespread use in urban agriculture. Further research and innovation may present solutions to these problems. The goal of this review is to examine some of the possible benefits and barriers to incorporating green roof technology into urban agriculture. In doing so, we shall first examine the current state of urban agriculture and then introduce potential benefits or limitations of using green roof technology. Due to the lack of published studies on the subject, we will also discuss future research needs. We have therefore divided the review into sections containing common themes; economic benefits and food security, economic barriers, access to resources and policy, human health and environmental health concerns.
Economic Improvement and Food Security
Economic development brought about by participation in urban agriculture comes in a variety of forms, including the supplementation of family income, job creation and freeing up funds previously used to purchase food. Crop choice and scale affect the extent to which urban agriculture contributes to the income of a householdReference Nugent2, Reference Graefe, Schlecht and Buerkert37–Reference van Averbeke39. Rice, for example, is a staple in many parts of the world and can provide income security for an urban farmer's householdReference Vagneron38, but production of vegetables may yield higher market pricesReference Graefe, Schlecht and Buerkert37, Reference Vagneron38. Animal husbandry, another form of urban agriculture, can provide high profitsReference Nugent2, Reference Graefe, Schlecht and Buerkert37, Reference Vagneron38, but may require much higher investmentsReference Vagneron38. In some cases, social capital can be generated by a household, by giving away food that could not be soldReference Nugent2. The impact of urban agriculture on employment is highly variable, depending on the economic status of urban farmersReference Nugent2, Reference Graefe, Schlecht and Buerkert37. For households in both developing and developed countries that do not produce for sale, or sell only their excess produce, urban agriculture frees up funds for other usesReference Nugent2, Reference Enete and Achike36, Reference Vagneron38, Reference van Averbeke39. The prevalence of urban agriculture increases when poverty increases and when costs of purchasing food surpass that of growing itReference Nugent2. This can be an important measure in stretching household budgets, allowing for the purchase of other itemsReference Nugent2, Reference van Averbeke39 or some economic freedom for women where household budgets are male-controlled, as was found in Pretoria, South AfricaReference van Averbeke39. In addition, economic concerns are an incentive for consumers who assume that purchasing local produce increases economic returns to local farmers through shortened supply chains and better market accessibilityReference Hu and Ding40, Reference Peters, Bills, Lembo, Wilkins and Fick41.
Food security, the second major driver of urban agriculture, is affected by both quantity and quality of food available to a household. Even in locations where urban agriculture does not contribute significantly to employment, food security is of major concern to urban farmersReference Nugent2. Currently, food in the US must be shipped an average of 74 km (49 miles) to consumers in urban centersReference Peters, Bills, Lembo, Wilkins and Fick41. It has been estimated that under ideal conditions the agricultural products of the entire state of New York could not supply the agricultural needs of more than 55% of New York CityReference Peters, Bills, Lembo, Wilkins and Fick41. This suggests that providing adequate food supplies for urban centers with growing populations may not even be possible on a regional scale, and costs associated with shipping may affect the food security of the urban poor. Producing agricultural goods within urban centers is one method of reducing the ecological footprint of urban centersReference de Zeeuw, Guendel and Waibel3, Reference Peters, Bills, Lembo, Wilkins and Fick41 and ensuring urban dwellers, access to food. Food insecurity, or the lack of access to adequate food for an active and healthy lifeReference Alexandri and Jones17, is not just a problem in the developing world, but in the US as wellReference Nugent2, Reference Enete and Achike36, Reference Widome, Neumark-Sztainer, Hannah, Haines and Story44 (Fig. 2). Food insecurity can be temporary or chronicReference de Zeeuw, Guendel and Waibel3 and is associated with a variety of problems in adolescents, who are at higher risk than young childrenReference Widome, Neumark-Sztainer, Hannah, Haines and Story44. A perceived or actual need to improve food security and a lack of ability to rely on food from rural areas can result in the use of urban agricultureReference de Zeeuw, Guendel and Waibel3, Reference Graefe, Schlecht and Buerkert37, which has been shown to improve the quantity and quality of food available to low-income urban households under a variety of conditionsReference Nugent2, Reference de Zeeuw, Guendel and Waibel3, Reference Enete and Achike36, Reference Graefe, Schlecht and Buerkert37, Reference Widome, Neumark-Sztainer, Hannah, Haines and Story44.
A community garden in Detroit, MI.
Green roofs are already utilized to improve the economic circumstances and food security of urban farmers (Fig. 3). EcoHouse, in St. Petersburg, Russia is an example of a rooftop garden project which provides jobs to, and increases cash flow among, individuals living within the apartment complexReference Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard, Hickey, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey35. The project also provides those residents with a reliable source of vegetablesReference Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard, Hickey, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey35. Another example is the green roof of the Fairmount Hotel in Vancouver, a portion of which is devoted to a kitchen garden, saving the hotel approximately $30,000 a yearReference Loder and Peck22. The rooftop garden on Earth Pledge's New York office is used not only as a source of food but also as a promotional tool for the group's organic local produce campaignsReference Cheney, Arpels, Chrisman, Sommerfield, Towers, Berkowitz, Brainard and Hickey4, Reference Loder and Peck22. Similar community garden projects have been developed in other cities, such as the Multnomah County Green Roof Project in Portland, ORReference King45 and several community-scale gardens in Chicago, ILReference Coffman and Martin46. A green roof at Trent University in Peterborough, Ontario, is producing vegetables for a local restaurant, The Seasoned Spoon Café, which was started as a healthy fast-food alternativeReference Blyth and Menagh47.
A herb garden on a green roof in Grand Rapids, MI.
Economic Barriers
Economic barriers to urban agriculture include inadequate access to or knowledge about marketsReference Nugent2 and insufficient laborReference Nugent2, Reference Bass, Krayenhoff, Martilli, Stull and Auld27, inputs such as fertilizersReference Vagneron38, quality seedsReference Graefe, Schlecht and Buerkert37 and credit or subsidy for startup costs or inputsReference Nugent2, Reference Enete and Achike36–Reference Vagneron38. These barriers are due to limited resources of urban farmers and will not be affected by the introduction of green roof technology to urban agriculture. An additional barrier introduced by the use of green roof technology is the cost of green roof installation and maintenance.
Installation of a green roof can be $32 m−2 more expensive than a conventional roof for roof structure aloneReference Wong, Tay, Wong, Ong and Sai26. Installation of green roof systems can vary from two to six times more expensive than conventional roof systems, depending on the design of the roof systemReference Wong, Tay, Wong, Ong and Sai26. Factors that impact the cost of a green roof include ease of access for installation, structural integrity of the building, type of drainage system, depth and composition of media, inclusion of an irrigation system and the use of a modular, mat or conventional built-up continuous roof system (C. L. Rugh, personal communication, February 5, 2010). Maintenance costs of a green roof also depend on roof design, as intensive roofs tend to require more care than extensive roofs. Maintenance of the roofing layers themselves is comparable to that of a conventional roof due to the longer lifespan of green roofsReference Wong, Tay, Wong, Ong and Sai26.
Tapping into incentive programmes, such as those used in Portland, OR6 and Chicago, IL5 which provide reductions in storm water removal fees and grants to help subsidize installation, may help. Other programmes, such as those geared toward improving the availability of fresh fruits and vegetables in urban centers and improving healthy eating habits in urban youth may also be sources of funding. It is also possible that locally made materials could be used to construct green roofs, reducing the cost of installation, but generation of policy promoting the use of green roofs and subsidizing their installation will be of greater importance in low-income areas than in high-income areas. Evaluation of locally available materials would also be necessary to determine both their suitability and their impact on green roof installation costs.
Access to Resources and Policy
Availability of land, especially land of adequate quality, is the main obstacle affecting urban agricultureReference Nugent2, Reference de Zeeuw, Guendel and Waibel3. Land scarcity and uncertainty in maintaining access to available land are due to competition with other development uses, primarily building constructionReference Nugent2, Reference Graefe, Schlecht and Buerkert37, Reference Vagneron38. These other uses are often more economically profitable and are therefore preferred by land ownersReference de Zeeuw, Guendel and Waibel3, Reference van Averbeke39, Reference Thornton43. Land use and investments in urban agriculture by urban farmers are impacted by resource use rights. Under current systems there are often no formal leasing agreements between land owners and urban farmers cultivating vacant lotsReference Nugent2. Rights of urban farmers are often minimalReference de Zeeuw, Guendel and Waibel3, uncertainReference Thornton43, and frequently transient due to changing land uses and termination of informal use agreementsReference van Averbeke39, Reference Thornton43. Lack of formal agreements over water use rights has led to conflict between municipalities and urban farmersReference van Averbeke39.
Incorporation of green roofs into new development would increase the potential agricultural area and remove competition with urban development that reduces the willingness of urban farmers to invest in urban agricultureReference Nugent2, Reference Vagneron38. Currently, flat rooftops comprise as much as 85% of the roof area in downtown and commercial areasReference Carter and Jackson20. In larger urban areas, this could add up to a great deal of space and potential for productive use if these existing roofs were retrofitted into green roofs. Buildings must, however, have the structural integrity to support the added weight in a worst-case scenario, regardless of whether roofs are designed new or retrofittedReference Rowe, Getter, Aitkenhead-Peterson and Volder48. Some existing roofs may not be suitable for retrofitting without incurring considerable costs in structural support. Many flat roofs have load capacities of only 146 kg m−2 (30 ft−2), which could be exceeded by as little as 7.6 cm (3 inches) of growing mediaReference Kortright49. This means that the flat roof area of existing buildings, which could be used for urban agriculture, is not accurately represented by the flat roof area of a city. More information on which roofs can support the additional weight of a green roof and the minimum depth of media necessary for agricultural production will enable more accurate estimates of how much roof area could be used. Despite these limitations, land owners could take advantage of this potential, enabling them to utilize more profitable development and then generate secondary profits through rental agreements with urban farmers.
Development of flat roof space into agriculturally productive areas could facilitate formalization and standardization of rental, leasing or use agreements between land owners and urban farmers. Access to roof space is limited and would require urban farmers to negotiate with building owners to gain access to the green roof space. This would be a reversal of the current use of vacant lots for urban agriculture, whose absent owners are unaware of agricultural activities or unwilling to take measures to keep urban farmers off the landReference Nugent2, Reference de Zeeuw, Guendel and Waibel3. Although this could create problems for urban farmers if green roof owners are unwilling to rent the space due to zoning or building code issues that might ariseReference Dillion50, Reference Sutton51, it could also empower urban farmers. Formal, legally binding use arrangements would grant urban farmers recourse should the green roof owner break the agreement. Such formal and empowering leasing agreements could also encourage farmers to increase investments in urban agriculture, increasing productivity and food security. Urban farmers with informal arrangements do not currently have this level of power and securityReference de Zeeuw, Guendel and Waibel3, Reference van Averbeke39, Reference Thornton43. Formalized rental and leasing agreements could easily be extended to include access to the buildings, water supply. This would be greatly beneficial to those farmers who have expressed willingness to pay for clean water where no clean water source currently existsReference Graefe, Schlecht and Buerkert37, but will increase the costs of farming for most urban farmers. Rainwater collected from an unused portion of the roof may also provide an added source of clean water for irrigation.
There are, however, two additional potential outcomes of rental agreements for the use of green roof space. First is the exclusion from farming and water resources of resource-poor urban farmers unable to pay rent for green roof space. Second is the reallocation of ground-level space and water sources currently used by farmers able to pay for rental agreements. The former could result in greater problems associated with poverty and food security in urban areas, but the latter could grant a larger number of urban dwellers access to land and water and therefore the economic opportunities and additional food security provided by urban agriculture. If the latter is the outcome, it would mean a better quality of life for a greater number of urban dwellers. This will be of particular importance as populations become increasingly urban, both worldwide and in the USAReference van Averbeke39, 52, 53, and doubts about the ability of rural areas to agriculturally support these growing urban populations also increaseReference van Averbeke39, Reference Peters, Bills, Lembo, Wilkins and Fick41, Reference Widome, Neumark-Sztainer, Hannah, Haines and Story44.
Human Health Concerns
Access to fertilizers is especially important as space limitations in urban agriculture require more intensive farming and greater fertilizer use per area than rural areasReference Enete and Achike36. Often resource-poor urban farmers will use inexpensive and easily accessible fertilizers, such as manures or municipal wastes, which can lead to an increase in soil heavy metal and pathogen concentrationsReference Enete and Achike36, Reference Graefe, Schlecht and Buerkert37, Reference Agbenin, Danko and Welp42, Reference Zhuang, McBride, Xia, Li and Li54, Reference Srinivas, Rao and Kumar55. Heavy metal and pathogen contamination of food are the primary human health concerns associated with urban agriculture. Sources of contamination include soils in which crops are grown, water used for irrigation and air pollutants. In many cases, the land most readily available to urban farmers is contaminated with heavy metals from a variety of industrial and mining sources, which can lead to contamination of the agricultural productsReference Hu and Ding40, Reference Agbenin, Danko and Welp42, Reference Zhuang, McBride, Xia, Li and Li54. In addition, resource-poor urban farmers often cannot afford to pay for clean irrigation water even if a source existsReference Nugent2, Reference Graefe, Schlecht and Buerkert37, Reference Vagneron38. Atmospheric deposition of contaminants during production, transportation and marketing of produce also leads to elevated levels of heavy metalsReference Hu and Ding40, Reference Srinivas, Rao and Kumar55–Reference Voutsa, Grimanis and Samara57.
The extent to which heavy metal contamination affects the safety of vegetables depends on several different factors, including the vegetable species, the part of the plant that is eaten and the type of heavy metalReference Zhuang, McBride, Xia, Li and Li54, Reference Srinivas, Rao and Kumar55, Reference Voutsa, Grimanis and Samara57, Reference Yang, Zhang, Li and Jiang58. On average, fruit vegetables accumulate lower quantities of heavy metals than leafy or root vegetablesReference Zhuang, McBride, Xia, Li and Li54, Reference Voutsa, Grimanis and Samara57, Reference Yang, Zhang, Li and Jiang58. Leafy vegetables are a major source of heavy metal dietary intakeReference Srinivas, Rao and Kumar55, due to high rates of translocation, transpiration and growth, as well as high surface area in close proximity to contaminated soil and irrigation splashReference Zhuang, McBride, Xia, Li and Li54. High-surface-area vegetables, such as cauliflowerReference Sharma, Agrawal and Marshall56, and those that spend more time in the fieldReference Voutsa, Grimanis and Samara57, are also known to accumulate higher concentrations of heavy metals through atmospheric deposition. Dietary intake of heavy metals can lead to accumulation in the human body because they are non-biodegradableReference Arora, Kiran, Rani, Rani, Kaur and Mittal59, causing their negative effects to become apparent only after years of exposureReference Srinivas, Rao and Kumar55, Reference Arora, Kiran, Rani, Rani, Kaur and Mittal59. Among resulting health problems are a variety of cognitive disruptionsReference Arora, Kiran, Rani, Rani, Kaur and Mittal59, Reference Chimbira and Moyo60, nervous, cardiovascularReference Zhuang, McBride, Xia, Li and Li54, Reference Sharma, Agrawal and Marshall56, Reference Chimbira and Moyo60, kidney, bone and liverReference Zhuang, McBride, Xia, Li and Li54, Reference Sharma, Agrawal and Marshall56 diseases, as well as cancerReference Arora, Kiran, Rani, Rani, Kaur and Mittal59.
The use of green roofs in urban agriculture also has the potential to reduce health concerns. Green roofs have the potential to reduce use of contaminated land in urban agriculture due to the nature of their construction. In most cases, the media in which vegetables would be grown on green roofs are engineered instead of using local soils, and so initial contamination will be minimal. Green roof media are also less likely to accumulate heavy metals than ground soils due to their high permeability and low cation exchange capacity, which results in leaching of nutrientsReference Emilsson, Czerniel Berndtsson, Mattsson and Rolf61 and heavy metalsReference Berndtsson, Emilsson and Bengtsson62 into runoff water. This tendency for leaching would reduce the likelihood of vegetable contamination on green roofs if contaminated sources of water or fertilizers are used. Rental and water use agreements will also facilitate more widespread use of uncontaminated water sources for irrigation. Atmospheric deposition of contaminants may also be reduced during the production phase. It has been suggested that distance from the source of pollutants impacts the extent of heavy metal contamination due to atmospheric depositionReference Sharma, Agrawal and Marshall56. Most green roofs are several stories high, increasing the distance between crop production and such sources of pollution as major roadways and highways.
In addition to food contamination, concerns about urban agriculture include health problems due to improper handling of agrochemicals and urban waste, a potential increase in pests such as rodents and flies, which can contribute to the spread of diseases and the transmission of diseases from livestock to humans due to improper animal husbandry techniquesReference de Zeeuw, Guendel and Waibel3. Although these concerns may be avoidable through proper practices, they promote negative perceptions of urban agriculture. Formalized leasing agreements may provide greater oversight of agrochemicals and urban waste used in urban agriculture. Leasing agreements could include specifications for what, if any, agrochemicals or urban wastes can be used on the green roof, how they should be stored and used, and the consequences for the urban farmer if the specifications are not followed resulting in human injury or a health hazard. In some countries, organizations may already be in place which could monitor such agreements, such as the Occupational Safety and Health Administration (OSHA) in the USA. It is unlikely that the use of green roofs in urban agriculture will affect the keeping of large livestock, which would be impractical on rooftops.
Environmental Health Concerns
Despite the costs, urban farmers are currently producing vegetables on green roofs in natural soils and composts at a media depth of 30 cm or more. This practice potentially creates several problems, including the added weight to the roof, consistency of growing media, potential nutrient loads polluting effluent that discharges into our waterways from fertilizers and as compost decomposes, and the logistical practicality of adding compost every year on a roof several stories above the ground.
Water quality of runoff is another concern as nutrient leaching could cause problems downstream. Composition of the growing media is one aspect of this problem. Most commercial green roof media are formulated within the guidelines of the German FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) standards63. Years of experience have resulted in media that possess the chemical and physical properties to support plants, yet are lightweight, coarse enough to allow drainage at shallow depths, can be replicated and limit nutrient runoff when guidelines are followed. Natural soil is always variable and the addition of compost can lead to runoff quality issues. Even so, commercial green roof media generally contain about 15–20% organic matter which can result in nutrient loadingReference Berndtsson19, Reference Rowe23, Reference Hathaway, Hunt and Jennings64. Researchers in North CarolinaReference Hathaway, Hunt and Jennings64 found that concentrations of both N and P decreased with decreasing percentages of compost in the media. These results emphasize the point that growing media can have an immense effect on the quality of effluent. In addition, applications of fertilizers and pesticides to ensure plant growth can be very detrimental to water qualityReference Rowe23, Reference Emilsson, Czerniel Berndtsson, Mattsson and Rolf61. This is especially true for soluble fertilizers applied in liquid formReference Emilsson, Czerniel Berndtsson, Mattsson and Rolf61.
Studies performed on green roofs with conventional media have shown that nutrient leaching can initially be a problem, but overall, these media can have a positive effect on water qualityReference Rowe23, 65. The initial nutrient load likely is due to decomposition of organic matter that was incorporated into the original mix. Once established, and the organic matter reaches an equilibrium, vegetation and substrates can improve water quality of runoff by absorbing and filtering pollutantsReference Rowe23. The effect of plants and their root systems was evident when effluent from an unplanted green roof containing media was shown to have higher concentrations and totals of N and P than effluent from planted roofs65.
There is also the question of how much fertilizer is necessary to maintain agricultural productivity in green roof media. Experimentation has shown that non-succulents grown on green roofs require either additional organic matter in the media or fertilizationReference Rowe, Monterusso and Rugh12. The relatively high levels of fertilizer that may be necessary to produce vegetables could lead to high levels of nutrient leaching. This begs the question, are we trading the benefits of local food production for decreased water quality? If nutrient loading does turn out to be a problem, then green roofs could be coupled with other low-impact development practices such as rain gardens and bioswales (landscaping techniques designed to manage storm water), although these practices are not always possible in dense urban settings. This highlights the need to develop and use green roof growing media and cultural practices that minimize leaching of nutrients while still providing adequate physical and chemical properties for plant growth.
Conclusions
In addition to previously mentioned research needs, there are several areas where research on the use of green roof technology in urban agriculture is necessary before wide-scale use of the technology can be implemented. First, determination of what crops are suited to growth in green roof media will be necessary. Little is known about how growing vegetables on green roofs will impact the environmental benefits provided by green roofs. Many of the benefits are directly related to the amount of coverage achieved by the vegetation and the leaf area of the vegetationReference Heilman, McInnes, Gesh, Lanscano and Savage66. The coverage that would be achieved by vegetables will be very different than that of the ground covers and perennials traditionally used on green roofs because they are typically cultivated in rows. In addition, vegetable gardens would be replanted every year, whereas typical green roofs are populated with perennial species. Research on how this difference will impact energy savings and storm water retention, for example, will enable better assessment of this use of green roofs in areas where these benefits are of particular importance.
The effects of other environmental factors on crops, such as exposure to higher winds, should be determined for optimum crop selection. Although pollinators have been seen and kept on green roofs, an understanding of the efficiency and quality of pollination of vegetable plants on green roofs would enable better decision-making about which crops to grow and the necessity of bee keeping on vegetable-growing green roofs. Finally, economic evaluation of different crops may generate more information on how much economic impact this form of food production could have on both a small and large scale.
The incorporation of green roof technology into urban agriculture maintains the economic and food security benefits of urban agriculture, while eliminating some of the many difficulties faced by urban farmers around the world. The ideal case, where formalized use agreements with building owners and oversight by municipal authorities ensure greater space availability and healthier produce, is, however, only possible through the cooperation of all parties involved, something that may be difficult in areas where urban agriculture is viewed in a particularly negative light. The formalization of use rights required by use of green roof space by urban farmers represents an opportunity for farmers to achieve guaranteed access to quality land and irrigation water, providing security for their agricultural pursuits. For land and building owners, the formalization of use rights represents an opportunity to achieve greater economic success and some degree of oversight over the activities taking place. This combination of economic opportunity and oversight may have the added benefit of improving land and building owners’ attitudes toward urban agriculture, which could expedite policy reform. Municipal involvement will enable new insights into the benefits of urban agriculture and understanding the ways in which its negative impacts can be minimized.
The process, though difficult, could be made easier through the establishment of policy friendlier to urban agriculture, incentive and subsidy programmes for the installation of green roofs and research into reducing the initial cost of green roofs and minimizing the inputs necessary for productive agriculture on green roofs. The resolution of these issues will further enable a future in which urban areas are greener and healthier places to live. This future could utilize ideas about development that incorporate green space in the forms of green roofs, parks or agricultural plots, enabling a closer connection with nature and the production of food with the benefit of increased food security, especially for the urban poor.
