During the partial federal shutdown in December 2018 and January 2019, news reports showed furloughed government workers standing in line for donated meals. These images were reminders that for an estimated one out of eight Americans, food insecurity is a near-term risk.

The conversation.com reported that in California, where I teach, 80 per cent of the population lives in cities. Feeding the cities of the nine-county San Francisco Bay Area, with a total population of some 7 million involves importing 2.5 to three  million tonnes of food per day over an average distance of 500 to 1,000 miles.

This system requires enormous amounts of energy and generates significant greenhouse gas emissions. It also is extremely vulnerable to large-scale disruptions, such as major earthquakes.

And the food it delivers fails to reach one  of every eight  people in the region who live under the poverty line – mostly senior citizens, children and minorities. Access to quality food is limited both by poverty and the fact that on average, California’s low-income communities have 32.7 per cent fewer supermarkets than high-income areas within the same cities.

Many organizations see urban agriculture as a way to enhance food security. It also offers environmental, health and social benefits. Although the full potential of urban agriculture is still to be determined, based on my own research I believe that raising fresh fruits, vegetables and some animal products near consumers in urban areas can improve local food security and nutrition, especially for underserved communities.

Urban farming has grown by more than 30 per cent in the United States in the past 30 years. Although it has been estimated that urban agriculture can meet 15 to 20 per cent of global food demand, it remains to be seen what level of food self-sufficiency it can realistically ensure for cities.

One recent survey found that 51 countries do not have enough urban area to meet a recommended nutritional target of 300 grams per person per day of fresh vegetables. Moreover, it estimated, urban agriculture would require 30 percent of the total urban area of those countries to meet global demand for vegetables. Land tenure issues and urban sprawl could make it hard to free up this much land for food production.

Other studies suggest that urban agriculture could help cities achieve self-sufficiency. For example, researchers have calculated that Cleveland, with a population of 400,000, has the potential to meet 100 percent of its urban dwellers’ fresh vegetable needs, 50 percent of their poultry and egg requirements and 100 percent of their demand for honey.

Although urban agriculture has promise, a small proportion of the food produced in cities is consumed by food-insecure, low-income communities. Many of the most vulnerable people have little access to land and lack the skills needed to design and tend productive gardens.

Cities such as Oakland, with neighborhoods that have been identified as “food deserts,” can lie within a half-hour drive of vast stretches of productive agricultural land. But very little of the twenty million tons of food produced annually within 100 miles of Oakland reaches poor people.

Paradoxically, Oakland has 1,200 acres of undeveloped open space – mostly public parcels of arable land – which, if used for urban agriculture, could produce 5 to 10 percent of the city’s vegetable needs. This potential yield could be dramatically enhanced if, for example, local urban farmers were trained to use well-tested agroecological methods that are widely applied in Cuba to cultivate diverse vegetables, roots, tubers and herbs in relatively small spaces.

In Cuba, over 300,000 urban farms and gardens produce about 50 percent of the island’s fresh produce supply, along with 39,000 tons of meat and 216 million eggs. Most Cuban urban farmers reach yields of 44 pounds (20 kilograms) per square meter per year.  An organic farm in Havana, Cuba, that produces outputs averaging 20 kilograms (44 pounds) per square meter per year without agrochemical inputs. Miguel Altieri, CC BY-ND If trained Oakland farmers could achieve just half of Cuban yields, 1,200 acres of land would produce 40 million kilograms of vegetables – enough to provide 100 kilograms per year per person to more than 90 percent of Oakland residents.

To see whether this was possible, my research team at the University of California at Berkeley established a diversified garden slightly larger than 1,000 square feet. It contained a total of 492 plants belonging to 10 crop species, grown in a mixed polycultural design.

In a three-month period, we were able to produce yields that were close to our desired annual level by using practices that improved soil health and biological pest control. They included rotations with green manures that are plowed under to benefit the soil; heavy applications of compost; and synergistic combinations of crop plants in various intercropping arrangements known to reduce insect pests.

Research plots in Berkeley, Calif., testing agroecological management practices such as intercropping, mulching and green composting. Miguel Altieri, CC BY-ND

Achieving such yields in a test garden does not mean they are feasible for urban farmers in the Bay Area. Most urban farmers in California lack ecological horticultural skills. They do not always optimize crop density or diversity, and the University of California’s extension program lacks the capacity to provide agroecological training.

The biggest challenge is access to land. University of California researchers estimate that over 79 percent of the state’s urban farmers do not own the property that they farm. Another issue is that water is frequently unaffordable. Cities could address this by providing water at discount rates for urban farmers, with a requirement that they use efficient irrigation practices.

In the Bay Area and elsewhere, most obstacles to scaling up urban agriculture are political, not technical. In 2014 California enacted AB511, which set out mechanisms for cities to establish urban agriculture incentive zones, but did not address land access.