Urban agriculture is viewed by some people as a viable alternative to industrial food production, but does it really offer more advantages than disadvantages?

Urban agriculture advocates charge that industrial agriculture depends on unsustainable practices such as profligate use of fresh water, harmful pesticides and fertilizers, and long supply chains, for example.

Urban agriculture, they say, provides fresher and healthier food that supports local economies, and does not depend on petroleum-based pesticides and fertilizers. The RUAF Foundation, for example, is a globally focused non-profit organization that advocates for urban agriculture.

According to RUAF, by 2020, “85% of the poor in Latin America, and about 40-45% of the poor in Africa and Asia will be concentrated in towns and cities.”

Population growth and increasing urbanisation cause many people who live in cities in developing nations to struggle with livable housing, sufficient employment and food security. Urban agriculture enables people to grow their own food despite lack of income or purchasing power. Some can grow extra food to sell or barter.

Urban agriculture also creates community bonds and social interaction, and is valuable as a relaxing physical activity in more developed areas. Ecological factors can also benefit from urban agriculture. Developing cities often struggle with disposal of urban waste, and urban agriculture can turn some forms of waste into productive resources.

In the US, a non-profit organization called Growing Power has been doing urban agriculture since 1993. Founded in Milwaukee, Wisconsin by former professional basketball player Will Allen, Growing Power uses intensive methods to produce over 450,000 kilograms annually of broccoli, lettuce, eggplant, kale, chard, carrots, tomatoes, and other vegetables. In addition, the organisation raises goats and uses aquaponic systems to produce tilapia and perch.

The organization provides training for would-be urban farmers, as well as demonstrations and technical assistance with the development of community food systems. The group also has growing facilities in Madison, Wisconsin and Chicago, Illinois.

Milwaukee, a small city of about 600,000 people, has seen flat population growth for the past two decades. Is the Growing Power model, apparently successful in that city, adaptable to dense and fast-growing urban areas across the globe? The United Nations estimates that by 2050, two thirds of the world’s nine billion people will live in cities. Can urban agriculture provide enough food for densely populated cities?

Blogger and urban planner Chris Kok does not believe so. In a blog post, Kok slammed urban agriculture (not urban gardens or community gardens) for using valuable urban land in a non-intensive manner.

“My problem is with industrial scale agriculture in urban areas. With the exception of a handful of cities, such as Detroit, there are better uses for land than agriculture,” he wrote.

One part of the problem he labelled the “festishization of the local foods movement” that values food grown close by.

“How close is close enough? Ultimately there are going to be decreasing returns on localism as there are with anything else,” Kok wrote. “It is much more important to focus on obtaining food from the local region than it is to be obtaining food from the neighborhood.”

In addition, urban agriculture displaces dense urban housing, which might contain 30 units or more per acre, Kok said. Housing that gets pushed to the outer fringes (suburban or exurban) of a city by urban agriculture will be built less densely, so where 30 units of housing may occupy an acre of urban land, only one to four units of housing will occupy an exurban acre. For that reason, he said, “A loss of 1 acre of development in an urban area could result in 30 acres or more of new greenfield development.”

Another blogger, Zane Selvans, calls urban agriculture an “awful idea” and offers an hypothetical example of his small city attempting to grow its own food locally. Using the benchmark of one-half hectare per person needed to grow food, Selvans determined that all arable land in the county would be needed to produce food for the city’s residents. Even though his city, Boulder, Colorado, is relatively small and not dense, with only 100,000 residents, urban agriculture “can never scale up to feed even a small fraction of the citizenry.”

Fast-growing cities in the developing world face an even tougher challenge.

“For those cities, no plausible increase in farming intensity can hope to feed the population, even if every square meter of the city were dedicated to food production,” Selvans noted.

In dense urban areas, urban agriculture may be just one piece of the puzzle, along with larger regional farms. Another option may offer more promise. Rooftop agriculture makes use of previously unused space, produces food and oxygen, consumes carbon dioxide, decreases the urban heat island effect, decreases runoff, and also can decrease buildings’ energy use.

One objective analysis of an urban green roof concluded that due to the project:

• The green roof intercepted about 78 per cent of all rainfall during a 10-month monitoring period, or more than 104,000 litres.
• Runoff from the green roof contains much less nitrogen and other pollutants than typical urban runoff.
• Most rainfalls of under 25 millimetres result in no runoff at all.
• The building’s energy use decreased by 10 per cent in the winter months.
• On the hottest summer days, the green roof’s temperature can be up to 59 degrees Fahrenheit cooler than neighboring conventional roofs.

Green roofs combined with urban agriculture are not new, but Dr Sara Wilkinson, associate professor at the School of the Built Environment at the University of Technology, Sydney (UTS), has undertaken a study funded by the city to ascertain best practices. Her study built three different types of gardens on three campus rooftops. Herbs and vegetables were grown in raised beds, vertical gardens, and wicking beds.

Wicking beds grow plants in a container, with geotextile fabric separating the plants and soil from the water in the reservoir below. In this case, the wicking bed was the clear winner, with the lowest cost, the least need for water and the most abundant harvest.

With Australia’s highly urbanised cities, rooftop agriculture may offer a promising alternative to conventional agriculture, which will face increasing challenges from climate change. The Intergovernmental Panel on Climate Change has predicted higher temperatures, more extreme droughts and flooding, and more variable rainfall in the coming decades.

Southern Australia and the Murray-Darling Basin will see less rainfall, for example, while areas in the north-east and north-west will see more intense rainfall. Rooftop farms, with their soils absorbing water, and their masses of edible plants cooling and cleaning the air, could provide an antidote to the perils of both climate change and industrial agriculture.