This is why cities can’t grow all their own food

If every homeowner in Seattle ripped up their lawn and replaced it with edible plants, the resulting crop production would be enough to feed just one percent of the city’s residents, according to a new study by researchers at the University of Washington.

Previous studies in various cities have surveyed land currently in use for urban agriculture, identified vacant parcels that could grow food, and estimated the percentage of specific categories of food such as eggs or vegetables that might be produced within city limits.

The new study, published in the journal Urban Forestry & Urban Greening, takes a more comprehensive approach. “This is the first systemized way of looking at all the different crops a city could grow, as well as looking at the nutrition and actual amount of food people need to survive,” says senior author L. Monika Moskal, associate professor of environmental and forest sciences.

The researchers mapped the distribution of buildings, impervious surfaces like parking lots, trees, grass, and water throughout Seattle, and calculated the amount of solar radiation received by each location. Then they used national nutritional guidelines and existing data on crop yields to determine the city’s capacity to fill its residents’ bellies.

They chose nine crops that are well suited to Seattle’s climate – beets, squash, potatoes, carrots, dry beans, barley, kale, hazelnuts, and apples – and calculated the amount of each that would supply the proper amount of calories, protein, carbohydrates, fat, and micronutrients. So, for example, 40 percent of the city’s growing space would be devoted to hazelnut orchards, 14 percent to fields of barley, and three percent to carrot patches.

Of course, these aren’t the only edibles that grow well in Seattle: prior to European settlement, the region’s Native American tribes made good use of abundant wild food sources. “I didn’t consider any traditional or wild plant foods, based on the scarcity of data regarding the amount of yield one could get,” says lead author Jeff Richardson, a postdoctoral researcher in the School of Environmental and Forest Sciences.

“We choose not to look at animal protein, but it could certainly be argued that including salmon and other fish caught in Puget Sound, Lake Union, and Lake Washington could substantially increase the food available,” he adds.

The researchers zeroed in on the city’s 175,032 single-family homes because most of these parcels likely have yards that could be converted to food production relatively easily. If all the grass in these residential zones were replaced with food crops, Seattle could grow enough to feed about 6,000 people, or roughly one percent of its population, they found.

If all of Seattle’s land in full sun were planted out with crops, it would produce just over 21 percent of the food necessary. But this would require installing gardens on every rooftop, as well as ripping up streets and other impervious surfaces to plant vegetables. And at a certain point, a city without a functioning street grid isn’t really a city anymore.

If grassy areas throughout Seattle (not just in residential zones) were converted to agriculture, this would yield four percent of the city’s food needs. The tradeoffs here aren’t trivial – where would the kids play soccer? – but the authors say that this number represents a reasonable estimate of Seattle’s maximum food crop production capacity (MFCPC).

The MFCPC is different for each city, and depends not just on growing space but also light, nutrients, water, temperature, and the climate suitability of specific crops. “The higher the population density of the city, the lower the MFCPC, so big sprawling cities will be at an advantage,” says Richardson, which highlights the conflict between the sustainability goals of local food and urban density.

Richardson also farms 10 acres in the Skagit Valley, about an hour north of Seattle, and though he doesn’t directly say so, his results do suggest that if city residents want local food, they should protect farmland nearby.

Amid growing interest in urban agriculture and concern about “food miles” traveled from farm to plate, the study is, at first glance, sobering. One to four percent – that’s it? But even if city-grown food can’t supply all nutritional needs, vegetables like kale, spinach, chard, and lettuce have lots of nutrients, can be grown in small spaces, tolerate partial shade, and could increase access to fresh produce in “food deserts” where its availability is currently limited. So maybe cities can’t do it all – but don’t count them out entirely. – Sarah DeWeerdt | January 26, 2016

Source: Richardson J.J. and L.M. Moskal.“Urban food crop production capacity and competition with the urban forest.” Urban Forestry & Urban Greening DOI: 10.1016/j.ufug.2015.10.006

Header image:International District Community Gardens, Seattle, WA. Credit: Danny Woo/Wikimedia Commons