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Considering global trends such as climate change and resource scarcity, a major challenge of future cities will be to reduce urban footprints. Moreover, cities have to become or remain livable for their inhabitants and offer social and economic opportunities. Thus, reconnecting food production and cities offers promising potential. The diffusion of urban farming reflects a rising awareness of how food and farming can shape our cities. A growing number of urban farming projects exist in and on urban buildings, including open rooftop farms, rooftop greenhouses and indoor farming. These projects are characterized by the non-use of land or acreage for farming activities. We use the term 'Zero-Acreage Farming' (ZFarming) to represent these farms. The objective of this paper is to: (1) illustrate and systemize present practices of ZFarming and (2) discuss specific novelties of ZFarming in the wider context of urban agriculture. We analyzed 73 ZFarms in cities of North America, Asia, Australia and Europe using a set of criteria, and developed a typology of ZFarming, complemented by in-depth interviews with pioneers in rooftop farming in New York. The results illustrate that ZFarming generates innovative practices that may contribute to a sustainable urban agriculture. Besides growing food, it produces a range of non-food and non-market goods. It involves new opportunities for resource efficiency, new farming technologies, specific implementation processes and networks, new patterns of food supply and new urban spaces.

How can buildings be combined with agricultural production and what are the major potential benefits and challenges for the introduction of zero-acreage farming (ZFarming) in Berlin from the relevant stakeholders' perspectives? These questions were explored through a series of interviews and stakeholder workshops held between 2011 and 2013. The aim was to identify the most suitable building-integrated farming model for the Berlin metropolitan area and to develop guidelines for the model's successful and sustainable implementation through a stakeholder-driven approach. This paper provides an aggregated synthesis of the outcomes derived from the qualitative interviews and stakeholder workshops. As the results reveal, the stakeholders perceive potential benefits and challenges related to the issue of ZFarming in all dimensions (economic, social, environmental and political). OPEN ACCESSSustainability 2015, 7 4512They largely agreed on the importance of focusing on local resources, using energy-efficient production-including social and educational aspects-and developing new market structures when introducing ZFarming to the city of Berlin. The stakeholders identified urban rooftop greenhouses (RTG) as the most promising farming model for Berlin. In a joint collaboration of all stakeholders, a manual for RTG was developed within the participatory innovation process that addresses the identified problems and challenges associated with future implementation and governance of RTG in Berlin and beyond.

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