By 2050, the world’s population is predicted to reach 9.8 billion and food production through conventional agriculture has been straining the global environment. Moreover, urbanization has continued to reduce farmland capacity and farm labor. The rise in urbanization also means that more food needs to be transported into inner cities, and the increasing food miles demand greater efficiency in food preservation. Various calls have been made to consider farming within urban settings to reduce food miles and serve as an important supplement to rural agriculture. One attractive scheme is rooftop farming. Particularly, by 2040, China is estimated to have around two million hectares of rooftop area, providing an opportunity to utilize at least some percentage of this urban space for food production.
From 2012, the group led by Dr. OW, David W. started a research direction on urban rooftop farming to solve the problems of food safety and security, which are raised by the expansion of population and urbanization. In 2016, they published a paper entitled “Rooftop production of leafy vegetables can be profitable and less contaminated than farm-grown vegetables” (Liu et al., 2016 Agronomy for Sustainable Development 36: 41) to demonstrate the feasibility and the advantages of rooftop farming in Guangzhou city. However, even though their prior analysis showed that it could be profitable to produce safe and nutritious leafy vegetables using a single layer hydroponics set-up, the profit margin was not particularly attractive. Multilayer production might be a way to increase yield and earnings, and from 2015, Dr. OW, David W., Dr. SU, Yun-Lin and their colleague tested the production of leafy vegetables using a two-layer hydroponic system. Since light quality in the bottom tank was less optimal, the addition of a bottom tank to the hydroponic setup doubled the growing space but actually did not correspondingly double vegetable yield. Supplemental electrical lighting could be used for the bottom-layer production, but sunlight is free and in most developing countries, electricity is limiting. Thus, to increase the necessary lighting, they opted for a cost effective route of using reflectors to direct ambient light to the bottom tank (Figure 1). With the use of the reflector-assisted double-layer hydroponic systems, a special planting scheme that takes into consideration of consumer preference was deduced to be able to produce 6310 kg vegetables from the 150 m2 rooftop screen house, and depending on the cost of rent, this would translate to 84–162% of the 2018 Guangzhou average income. Moreover, if ～86 % of Guangzhou’s rooftop space is used, this planting scheme has the capacity to provide 0.3 kg of vegetables per day to satisfy the vegetable needs of the city’s 14 million inhabitants. These findings may help incentivize investments in rooftop farming. Hopefully, this study as well as other studies could help further the development of this type of urban farming and could solve the problems caused by the expansion of population and urbanization.
The related study was published in Urban Forestry & Urban Greening ((2020): 54: 126766; impact factor: 4.021) as a research article entitled “Increasing effectiveness of urban rooftop farming through reflector-assisted double-layer hydroponic production”. The funding was provided by the Ministry of Science and Technology of China (2016YFD0101904) and the Chinese Academy of Sciences (KSCX2-EW-J-28; QYZDY-SSW-SMC010). For further reading, please refer to https://doi.org/10.1016/j.ufug.2020.126766.
Figure 1. Roof screen house and reflector-assisted two-layer hydroponic system.