But getting a handle on just how much food a city could produce for itself is difficult without yield information like the data presented in Payen's study. The finding that urban agriculture can have similar or greater yields to conventional agriculture 'is exactly what we have been waiting for in the urban agriculture research community,' says Erica Dorr, an environmental scientist at AgroParisTech who was not involved in the study.Ĭurrent estimates suggest that between 5% to 10% of legumes, vegetables and tubers are grown in urban settings, and between 15% to 20% of global food is produced in cities. But scientists will need to keep studying these systems to plan cost-effective agriculture solutions. And crops grown in fully controlled environments can be grown throughout the year, allowing harvests to happen more times per year than in open-air environments, which leads to higher annual yields. Other crops, like watery vegetables (e.g., tomatoes) and leafy greens, performed well in hydroponic environments.

'You can't exactly stack up apple trees in a five- or ten-layer high growth chamber,' he says, 'though we did find one study that managed to grow wheat stacked up like that.' Certain crops like lettuces, kale and broccoli are more naturally suited to be grown vertically in indoor spaces than others. 'Surprisingly, there were few differences between overall yields in indoor spaces and outdoor green spaces, but there were clear differences in the suitability of crop types to different gray spaces,' Payen says. In both green and gray spaces, the study examines a suite of crops grown in soils versus hydroponics, horizontal versus vertical farming, and natural versus controlled conditions. Payen's work includes 'gray' spaces - places in cities that are already built but could be used for growing, such as rooftops and building facades. Most studies on urban agriculture have focused on green spaces, such as private and community gardens, parks and field growing operations. Cost efficiency remains an open but important question. Many other urban crops studied are produced at similar or higher rates than in rural settings.

The researchers find that urban yields for some crops, like cucumbers, tubers and lettuces, are two to four times higher than conventional farming. The new study compiles studies on urban agriculture from 53 countries to find out which crops grow well in cities, what growing methods are most effective, and what spaces can be utilized for growing. 'Despite its growing popularity, there's still quite a lot we don't know about urban agriculture, like whether the yields are similar to conventional agriculture, or even what crops are commonly grown,' says Florian Payen, an environmental scientist at Lancaster University and lead author of the study, published today in AGU's journal Earth's Future.

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