The more gardens that crop up on roofs and along walls, the more benefits people find. The first adopters — in urban areas facing air pollution, stormwater management and heat island issues — turned to green roofs for environmental reasons. Striving to maximize every inch, creative gardeners started experimenting beyond sedums, giving edible plants new homes on green roofs and living walls and expanding the possibilities of hyper-local food production.
“It provides you with all the benefits you’d get from a green roof, plus the added value of sourcing produce,” says Jessie Benhazl, founder and CEO of Green City Growers, the organization that maintains a half-acre rooftop farm above Whole Foods in Lynnfield, Mass. “This year, we produced more than three tons of produce off of a 17,000-square-foot rooftop farm, so you can produce a lot.”
What grows on the roof?
Determining what plants grow best on rooftops is an ongoing experiment, and vegetables add a new challenge.
“What you can grow or not grow on a roof is mostly dependent on your soil depth,” Benhazl says. “The big difference between green roofs and food roofs, growing sedums and growing vegetables, is the soil depth. We typically suggest 12 inches, but if the roof can only structurally hold 8 inches, that’s going to completely amend what you grow.”
Extensive roofs, which have less than 6 inches of growing medium, work for leafy greens and herbs, but intensive roofs with more than 6 inches of growing medium hold more possibilities for vegetables. With enough soil, rooftops can grow root vegetables – especially miniature varieties of carrots, cucumbers, beets and eggplants. First, ensure the building has the structural integrity to support any garden, which can add 100 pounds per square foot.
The historic cottage at the Civic Garden Center of Greater Cincinnati can only support three inches of soil on its sloped roof – enough to grow sedums with minimal maintenance. But growing vegetables in a foot of soil on the flat roof of its Green Learning Station has been more difficult.
“We’ve learned that the soil they were offering for vegetable gardening on the roof wasn’t rich enough,” says Bennett Dowling, a horticulturist at the Civic Garden Center, explaining that the porous mix of shale and compost, which was intended to drain easily without blowing away, left vegetables dry. “Kale and cabbage seemed to do fine, but things that like richer soil, like tomatoes and peppers, had trouble up there.”
Instead, he grew herbs – a safe choice for most green roofs and living walls because they thrive in shallow media and warm, “Mediterranean” environments.
Rooftop growing conditions
Because roofs and walls both expose plants to sunnier, windier and warmer environments than ground level, mitigating the conditions is key to successful crops. Irrigation is a prime concern in both settings, says Leslie Herndon, who heads living wall projects at Greenscape, Inc. as the landscaping firm’s senior client relations manager and floriculturist.
“Green walls and green roofs help mitigate (urban heat islands and stormwater runoff),” Herndon says. “However, in the process, those plants need more water than they would have needed on the ground. They’re lifted up. They’re dealing with reflective heat and sunlight. It’s a hot, hot environment.”
That heat can be both good and bad. Windy City Harvest, the urban agriculture arm of the Chicago Botanic Garden, maintains the Midwest’s largest farm-to-fork rooftop garden above the McCormick Place Convention Center. In fact, part of the garden is directly above the kitchen of the food service company, SAVOR…Chicago, which causes hot spots.
“The growing season is longer because you’re getting heat from the building, giving you a really unique microclimate on the roof,” says Angela Mason, director of Windy City Harvest. “That presents opportunities and challenges. Some crops grow much quicker, but we really have to watch the vegetables because the temperature swings create problems.”
By lengthening the growing season, rooftop gardens enable food production where traditional gardens can’t exist, and also when they can’t. Windy City Harvest was still harvesting in mid-December, expecting to reap about 4,500 pounds of produce from the 20,000-square foot plot.
Living on the roof
Isolated rooftop environments remove most garden pests, but some are crucial to the ecosystem. That’s prompting green roofs to welcome pollinators by adding beehives. Windy City Harvest maintains three beehives at McCormick Place, housing 20,000 honeybees that produce 50 pounds of honey a year.
“Bees are so important to the ecosystem and food production in particular that we want to make sure we’re educating people about that,” says Mason, who provides transitional job training programs at the farm.
But bees aren’t the only residents on the roof of McCormick Place. The garden is also home to 2,000 worms that create 200 pounds of vermicompost annually.
“We have two vermicompost systems on site to compost landscape waste from the production farm and food scraps from the kitchen,” Mason says. “We use the vermicompost to build up the fertility level, because there’s not high levels of organic matter in traditional rooftop growing media.”
Opportunity from above
Though restaurants and food service providers were the first to take food to new heights, hospitals, schools, housing complexes and community centers are also incorporating edibles in green roofs and living walls. As the popularity spreads, garden centers can position themselves as experts, providing education as well as plants and supplies.
Bachman’s, Inc., which operates six retail floral, gift and garden centers in the Twin Cities, as well as a landscaping division, nursery and greenhouses, has partnered with LiveRoof and LiveWall to grow more than 120 green roofs in the last 10 years – including two on its flagship store in Minneapolis.
But in many markets, homeowners struggle to find their own materials and advice for green roofs.
“There still aren’t any garden centers in this market that have taken advantage of that – that are selling a residential version of a living wall – and that just blows my mind,” Herndon says. “If you don’t stay on top of some of the trends that are happening in landscaping, then you lose an opportunity.”
While garden centers already have many of the required components, “There’s so much that goes into a green roof that’s outside of traditional gardening,” Dowling says. “It’s more than just putting in a garden. It’s a matter of garden centers trying to just be more educated about the limitations of green roofs, but also the opportunities they provide, and being able to walk customers through that.”
Brooke is a freelance writer and frequent contributor to Garden Center magazine based in Cleveland, Ohio.