Rethinking rooftops

Rooftop farming is intensive agriculture using engineered soil and irrigation on building roofs. Commercial rooftop farming is an emerging practice at the intersection of agriculture and urban planning, where economic and environmental returns are equally important. Rooftop farms can take many forms, including gardens, high tunnels or climate-controlled greenhouses. Rooftop farming enables production of hyper-local food and its associated social and educational benefits in urban areas where land is unavailable or prohibitively expensive for farming.

The basics of rooftop farms

Municipal zoning laws and building height restrictions are barriers to rooftop farms, but some cities are beginning to revise their codes to accommodate urban agriculture and rooftop farms. In New York City, for example, it became easier for property owners to obtain the city’s approvals for constructing rooftop farms in 2012 when a citywide planning initiative known as “Zone Green” removed the zoning impediments for incorporating ornamental and agricultural uses on rooftops. In this new context, the Brooklyn Grange, a 1.5-acre commercial rooftop farm, was constructed atop an 11-story building of the former Brooklyn Navy Yard. For this construction, $592,730 was funded by the Community-Based Green Infrastructure Program of the NYC Department of Environmental Protection based on the expectation that the farm could reduce drainage volume and nutrient load to the East River while producing fresh vegetables for local consumption. While commercial rooftop farming is still in its early stages, New York City alone has 0.9 billion square feet (about 21,000 acres) of flat roof surface, 14 percent of which is considered suitable for large-scale (>10,000 ft2) commercial farms (Ackerman et al. 2013; Acks 2006).

Rooftop farms can increase revenue through diverse social and cultural programs, not just vegetable production. For example, the Brooklyn Grange became a popular destination for environmental and agricultural tourism, and is used for exercise classes, weddings, photo shoots, music events and organic food tasting. Also, the Grange offers farming internships as well as environmental education programs that have engaged approximately 40,000 K-1 students. These programs emphasize participation of immigrants, refugees and other under-represented groups, which are subsidized by the municipal programs for green-job training and diversity.

Growing media in rooftop farms

Load-bearing capacity of the building is among the constraints specific to the design of rooftop farms. In comparison to using field soil, it is relatively easy to specify engineered soil within the weight limitation. These engineered soils can be categorized into the following two types: expanded shale, clay and slate (ESCS); or potting soil (PS). ESCS are common base materials for green roof soil products, which meets the industrial standards for rooftop landscape construction, including ASTM (American Society for Testing and Materials) standards (Ampim et al. 2010). ESCS soils can be useful for rooftop farming if they have sufficient water-holding capacity and nutrients (Whittinghill et al. 2013).

The second type is potting soil, i.e. container media. If peat, coconut coir and other organic materials are used as base material, potting soils can be lighter and have greater water holding capacity than ESCS soils (Harada et al. 2017). Whether ESCS or potting soils are used, the precise management of organic amendments becomes challenging because organic matter lost to decomposition must be replenished. Also, lightweight materials must be self-knitting (ex. peat, coconut coir) because lightweight granules (ex. vermiculate, perlite) can easily be lost by wind erosion.

Production in rooftop farms

Precise management of irrigation and fertilizer inputs can reduce the drainage loss of water and nutrients, which enhances both environmental and economic returns of rooftop farming. Unlike in-ground agriculture, for example, rooftops do not have immediate access to water in the ground level. Thus far, irrigation has used municipal water, which is both expensive and competes directly with human consumption.

The economics of rooftop farms

The economic viability of urban agriculture relies on the sales of quick-turn/high-yielding leafy greens and fruit-bearing vegetables with high market value. These crops also have high demands of water and nutrients, which can make rooftop farming prone to drainage loss of water and nutrients. While the yield of rooftop farms can exceed in-ground agriculture, efficiency of water and nutrients use can be lower. Optimizing these inputs is an exciting area for research at the junction of science and practice (Harada et al. 2017; Sanyé-Mengual et al. 2015). The development of soil mixes with enhanced water-holding capacity combined with drainage recycling systems are central to this effort.

Rooftop greenhouse considerations

The use of climate-controlled greenhouses allows for more intensive year-round vegetable production on rooftops. Design of such facilities involves a complex process of navigating zoning laws, load-bearing capacity of buildings and rooftop access. Rooftop greenhouse design also takes into account the potential for beneficial heat from the underlying building as well as greater wind speed than similarly sited ground-based greenhouses. Because of the challenges in site selection and design, some rooftop greenhouses have found it easier to incorporate rooftop farms into new construction projects rather than retrofit existing buildings. Once constructed, operating a rooftop greenhouse is remarkably similar to traditional greenhouses other than getting loads up and down through the building.

Because greenhouses allow for control of temperature, light and relative humidity, rooftop greenhouse operations usually focus on producing one particular class of vegetables to optimize production efficiency and output. Two examples of rooftop greenhouses include Sky Vegetables based in the Bronx and Gotham Greens with four locations including: Queens, two in Brooklyn, and Chicago. Both Sky Vegetables and Gotham Greens focus on leafy greens and herbs using NFT (nutrient film technique). In a rooftop setting NFT is favored over deep water culture (DWC, raft/pond hydroponics) because DWC uses a large volume/weight of water. Both operations sell to local restaurants and supermarkets.

While rooftop agriculture has its challenges, we may be seeing a lot more of it as the human population becomes increasingly urban. By 2050, 70 percent of the global population is projected to live in urban areas.

Acknowledgements Financial support was received from the USDA National Institute of Food and Agriculture, Multistate Research Project NE-1335: Resource Management in Commercial Greenhouse Production. Yoshiki Harada (yh535@cornell.edu) is a recent PhD graduate, Tom Whitlow (thw2@cornell.edu) is an associate professor, and Neil Mattson (nsm47@cornell.edu) is an associate professor and extension specialist within the School of Integrative Plant Science at Cornell University.

References Ackerman K, Dahlgren E, Xu X (2013) Sustainable Urban Agriculture: Confirming Viable Scenarios for Production, Final Report No.13-07 vol NYSERDA No. 13-07 [Online] (Accessed on April 01, 2016).

Acks K (2006) A framework for cost-benefit analysis of green roofs: initial estimates Green Roofs in the Metropolitan Region: Research Report Columbia University Center for Climate Systems Research

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