Urban agriculture has crawled up walls and fire escapes onto rooftops across North America. As we cultivate our rapidly greening skyline, we’re hungry to learn the potential of the blossoming Rooftop Agriculture movement. On March 5th I had the privilege of speaking at the Philadelphia Flower Show – the world’s largest and longest-running flower show – about this very topic. Here’s an excerpt from the talk (not to be reproduced without permission):
We’ve all seen veggies growing in community gardens. Some of us may have seen edibles peaking out of window boxes, growing in the public-right of way (that strip between the sidewalk and the street), maybe in vacant lots, or even on rooftops. Rooftop agriculture has tremendous potential to feed our cities and our soul.
Right here in Philly, there are over 16,000 acres of rooftop . If 0.5% of these roofs were converted to farms and veggie gardens, it would exceed the amount of area Philly currently has under production with all the urban farms and community gardens combined . In 2008, Philly’s community gardens alone produced over 2 million lbs of summer vegetables . Just think how much food we could grow right here across our skyline.
Ok, so I’m kind of nerdy, and I actually did the calculation. If 6.5% of Philly’s total roof area was converted to hydroponics (around 1,050 acres), we’d grow enough food to feed our 1.5 million residents. I’ll say that again. If 6.5% of our rooftops grew food hydroponically, we could provide every Philadelphian with fresh produce. Holy moly.
Now, I’m a green roof designer. Part of my job is taking idealistic scenarios and slapping on some pragmatism. The first thing to consider when talking about green-washing a city’s skyline is weight. Not every roof is strong enough to support a farm or garden. Each roof, in fact, is designed to support a specific amount of weight, or load… Most buildings are designed to meet the load requirements set forth by the local Building Code. Some buildings can hold more weight than expected because they were designed to support higher loads or they were built back when the requirements were more strict. Other buildings can support less weight than expected because they’ve become weaker over time. The bottom line is that each building can support a specific load. If you’re going to grow food up there, you need to know what that load is, and only a structural engineer can tell you.
The second pragmatic consideration is how, exactly, are all these people with strong roofs going to grow food up there? I mentioned hydroponics earlier but that’s just one way to skin the cat. Containers and raised beds are used most often on top of homes. They’re also used on top of apartment buildings, office buildings, churches, schools, community centers; anywhere where home or community gardening takes place. Farming (which I define as growing food for sale or direct use in the commercial building below like restaurant or hotel) often utilizes a different approach. Row farming and hydroponics are the most common rooftop farming strategies.
Each of these methods (container gardening, raised bed production, row farming, and hydroponics) has different costs, yields, and longevity. In general, container gardening is the least expensive and provides the lowest yields. Hydroponics requires the largest initial investment but provides the highest yields, and often highest profit margin. Each production method caters to a different user group and produces unique benefits.
… Communities benefit from rooftop agriculture as they do with all other types of urban agriculture. Fresh food access is the first benefit that comes to mind… When food is grown locally, in an urban or peri-urban farm, fewer dollars are spent on transportation and fossil fuels. There’s less air pollution from this decreased transportation, and often times less packaging and food waste. Urban agriculture also fosters community building. Gardening with neighbors and friends in a community garden or neighboring properties is an incredible way to make friends, exchange knowledge, and share food. Urban agriculture also allows us to teach kids how to garden and appreciate healthy food choices. Sometimes they’re the ones teaching us. More formal educational opportunities are possible when classrooms are taken outside to learn in the garden.
The main community benefit that distinguishes rooftop agriculture from ground-level urban agriculture is that rooftop agriculture takes place in a safe, secure, convenient location. To give an example, Chicago’s South Side is a dangerous place. Urban gardens weren’t successful because kids didn’t feel safe outdoors. In 2006 the Gary Comer Youth Center opened its doors, with an 8,000 sf educational rooftop farm built for neighborhood youth to learn about gardening and healthy eating. For the first time these kids could play and learn outside in a safe, secure, convenient environment.
… The last thing I want to touch on is the urban food system. I mentioned feeding our city with hydroponics at the beginning of the talk. That’s because hydroponics offers the highest yields of any rooftop growing method. Rooftop farming companies like Gotham Greens, in Brooklyn and Queens, are run by savvy CEOs, with large staffs and multiple locations. Production relies upon high tech equipment that’s highly calibrated, with the ideal growing conditions provided for each type of crop. And boy do these hydroponic greenhouses grow a lot of food! In fact, Gotham Green’s rooftop greenhouses grow 20-30 times more food than ground-level farming, while using 20 times less water.
… Hydroponics is an incredible form of agriculture, but by now you should understand the diverse value of all the different types of production. All types of rooftop agriculture are important. Collectively they address community and business needs, while keeping food miles to a minimum. While rooftop agriculture will continue to expand as people like you and me recognize its value, we must remember that rooftop agriculture works in concert with other types of urban agriculture to create the urban food system. The system is like a web.
Diversity in a food system equals resilience. So as our urban food system continues to diversify, it will get stronger and stronger. The backbone of this system, however, is rural agriculture. You didn’t think I’d say that, did you?! Well it’s true. Rural farms play a critical role in the national food system, so we must always acknowledge that urban agriculture will not replace rural agriculture. We’re simply diversifying the system.
So while you’re gardening your rooftop and indulging in its delicious rewards, remember that you’re garden is part of something much, much bigger.
 Forsyth, Phil. “Up On the Roof: Expanding Urban Food Growing.” Permaculture Activist, no. 79 (2011).
 Kremer, P., DeLiberty, T. “Local Food Practices and Growing Potential: Mapping the Case of Philadelphia.” Journal of Applied Geography, 31:4 (2011): 1252-1261.
 Popp, Trey. “The Park of a Thousand Pieces.” The Pennsylvania Gazette, July/August (2011).