As urban populations grow, urban agriculture is increasingly viewed as a local source of food and a way to help combat inequitable access to food. But little is known about how productive urban agriculture is compared to traditional, rural farming. A new study has found, the pursuit of urban gardeners and hydroponics can meet and sometimes exceed the yields of rural farms.
“Despite its growing popularity, there’s still a lot we don’t know about urban agriculture, such as whether yields are comparable to conventional agriculture, or even which 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 Future Earth.
The new study compiled urban agriculture studies from 53 countries to find out which plants grow best in cities, which growing methods are most effective, and which space can be used for growth. Researchers have found that urban yields for some crops, such as cucumbers, tubers and lettuce, are two to four times higher than conventional farming. Many other urban crops studied are produced at the same or higher rates than in rural settings. Cost efficiency remains an open but important question.
Most studies of urban agriculture focus on green spaces, such as private and community gardens, parks and farm growing operations. Payen’s work includes “gray” spaces — areas of cities that are already built but could be used for growth, such as rooftops and building facades. In the green and gray spaces, the study examined a set of plants grown in soils versus hydroponics, horizontal versus vertical farming, and natural versus controlled conditions.
“Surprisingly, there are some differences between the total yield of indoor and outdoor green spaces, but there are clear differences in the suitability of crop types in different gray spaces,” said Payen. Some plants like lettuces, kale and broccoli are more naturally suited to growing vertically indoors than others. “You can never put apple trees in a five- or ten-layer high growth chamber,” he said, “although we found one study that was able to grow wheat in layers like that.”
Some crops, such as water vegetables (for example, tomatoes) and leafy greens, do well in a hydroponic environment. And plants grown in fully controlled environments can grow throughout the year, allowing harvests to occur more times per year than in open environments, leading to higher annual yields. But scientists need to continue studying these systems to devise effective agricultural solutions.
The finding that urban agriculture can have the same or greater yields than conventional agriculture “is exactly what we in the urban agriculture research community expected,” said Erica Dorr, an environmental scientist at AgroParisTech who is not involved in study.
Growing data
Current 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 getting a handle on how much food a city can produce for itself is difficult without access to information like the data presented in Payen’s study. Payen and his colleagues are part of an interdisciplinary team building evidence and understanding of the value of urban growth for food security, health and the environment.
“This is the first step,” Payen said. Once scientists have accurate estimates for urban yields, they can map a city’s potential growing areas and calculate how much food can be produced there. “That’s the strength of this dataset, for planners and policymakers to see if it’s worth investing in roof gardens or greenhouses, for example, or if hydroponic systems are better.” Future studies can also use the data to estimate the potential of cities to meet future food demand and the likelihood that cities will become self-sufficient in terms of food products.
Lack of data sometimes hinders progress in implementing effective, practical urban agriculture.
“As we engage and talk to various stakeholders, such as government agencies and local councils, we realize that the absence of solid, comprehensive data on urban agricultural yields prevents them from sustaining and supporting development and implementation of urban food growth,” said Payen. “We need to understand realistically how much this form of growth can contribute to food security to help make the business case for it.”
Scaling is also a key point for Dorr. “What percentage of a city’s vegetable and fruit consumption can be provided by urban agriculture? Studies have estimated this, but with lower quality yield data” than this new study provides. But, “we need to consider not only how much food can be produced, but what the cost to the environment is.”
It remains to be seen whether growing food in cities has a smaller or larger overall carbon footprint than conventional agriculture; the answer probably varies. Researchers are also studying how food grown in cities can be affected by pollution. And some crops included in the literature review study lacked numbers to be included in the statistical analysis, pointing to a need for more research on urban crops such as fruits and cereals. .
