Increasing effectiveness of urban rooftop farming through reflector-assisted double-layer hydroponic production

While the expansion of population and urbanization has been placing additional demands on food production, it has also been reducing the amount of arable land and increasing the costs of food transport. Urban farming using space that does not compete for other urban uses has been proposed as a possible solution to alleviate this problem. In many cities, rooftops can be suitable for farming, especially for leafy vegetables that are not as easily transported from distant locations. Our previous study using a single layer hydroponics set-up within a 150 m2 rooftop screen house in Guangzhou, China, showed that safe and nutritious leafy vegetables can be profitably grown. However, the profit margin could only reach up to 53 % of the average 2015 Guangzhou income and recovery of set-up cost would take 2.5–3.5 years. To increase profit margin, this study tested using a low-cost reflector-assisted two-layer hydroponic system. Based on the production tests of 10 leafy vegetables, and depending on the type of vegetables, light reflectors were found to increase yield of the sun-shaded bottom tank to 31–76 % of the level achieved by the top tank. 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 84–62% of the 2018 Guangzhou average income, with recovery of set up costs reduced to 0.9–1.7 years. 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 urban rooftop farming.