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Home > Solutions > Spring 2010 > Saving Some Green

Saving Some Green

Finding the right plants for Minnesota rooftops may help foster a greener skyline

By Sara Specht

Green roof

Minnesotans love their green spaces. They pride themselves on the state’s diverse array of life: the rich wetlands, lush forests and rolling prairies. But if you look from above its cities and towns, mostly what you’ll see is a barren wasteland of concrete, tar and lifeless stone. But in recent years some have begun to change that, transporting the displaced patches of green from the land to the rooftops above. Researchers from CFANS plan to show them how to make their roofs grow.

Living roofs aren’t a particularly new idea—sod roofs have been used in northern Europe for centuries. The modern concept of a green or eco-roof developed in Germany in the 1960s and gained popularity throughout Europe, moving into the U.S. in recent decades. Cities like Chicago, Seattle and New York have fitted landmark buildings with green roofs in their urban centers.

Minneapolis, too, has begun to build green in recent years. The Minneapolis Central Library and the Target Center Arena both feature high-profile living roofs, and the city is a leader at putting turf and greenery atop parking complexes. But in the rush to adapt to this earth-friendly trend, Minnesotans have run into a familiar dilemma: what about this weather?

Jonathon Hensley and John Erwin are testing Minnesota roof-worthiness of both native and exotic plants.“I was getting asked out to places like Lebanon Valley Nature Preserve, where they have a green roof on their recreation center, and almost the entire roof was dead,” says Department of Horticultural Science professor John Erwin. “They were using recommendations from Chicago and Germany. Of course the plant material wasn’t surviving here. It’s too cold here, and too dry.”

A majority of the construction and planting standards for modern green roofs come out of Chicago and Baltimore in the U.S., as well as from Germany and Toronto. Compared to Minnesota, those places have more consistent precipitation and less dramatic winter temperatures. As a result, the suggested plant material often requires significantly more time, resources and attention to sustain in this climate. The Minnesota Nursery and Landscape Association and a Joint Watershed Research Grant are sponsoring a project to create a guide for building a local green roof that is both attainable and sustainable.

Savings in the pocket

The biodiversity of the plants featured in the study serves a variety of purposes, from aesthetic value to air particulate filtration. In a patchwork garden atop Williamson Hall on the University of Minnesota’s east bank campus, dozens of plant varieties in individual trays grow or die under intense scrutiny. Digital loggers track details about not only survivability, but surface and root temperature and soil water retention, at 30-minute intervals around the clock, all year. A weather station nearby records daily weather information on the roof to paint a complete picture of how each plant survives and what its impact could be on a roof ecosystem. But why the sudden demand for roof-friendly plants?

“Most people don’t realize that one of the biggest impacts of a green roof in Minneapolis is that it cools the building in the hot summer,” says Jonathon Hensley, the horticultural science graduate student maintaining the rooftop facility for the research project. “It reflects light. But it also insulates the building. It saves on energy costs for both heating and cooling.”

In the winter months, layers of plant material and soil on a green roof add natural insulation that can provide a 26 percent reduction in a building’s heat loss. On a sunny, 80-degree-Fahrenheit day, the surface temperature can range from 120 degrees on a white roof to 180 degrees on a black tar roof; a roof with plant cover can remain about 85 degrees. Even with ample insulation on a roof to slow heating of the building’s interior, intake vents for air-conditioning units are often found on the roof. The colder incoming air from a green roof further lessens the cooling system’s burden.

Poppies from the Himalayan mountains have never been tested for green roof applications, but may be well suited to Minnesota's climate. That chilling effect can achieve a broader scope combating the summer’s urban heat island effect. Traditional building materials soak up and re-emit the sun’s heat, making cities typically 7 degrees F warmer than surrounding areas. Installing multiple green roofs in a concentrated area can actually cool the overall temperature in an urban area.

A green roof also can save money for both individuals and the city in handling stormwater runoff. Rain from heavy storms flows off standard roofs and into the sewer-stormwater systems, causing flooding and dumping sewage into nearby lakes and rivers. A green roof can absorb up to 75 percent of rainwater and slows the overall rate of runoff. Minneapolis is upgrading its water management infrastructure, using property taxes paid based on each roof’s surface area—taxes reduced or erased by the presence of a green roof.

“One great application for the data we’re gathering on these plants is to be able to measure the impact of each for planning a green roof,” Erwin says. “This plant with red leaves likely absorbs more light, while this one with silvery leaves reflects it. This one retains more water. They as rooftop plants would have different temperatures and serve different purposes.”

Preserving green spaces

Adding green roofs to a city’s skyline has benefits beyond the bottom line. Erwin and Hensley are testing an array of plant species, both native and exotic, to identify other values and uses within a sustainable roof garden, such as carbon sequestration, particulate capture, food cultivation and aesthetics.

In its first year, the East Bank roof garden hosted 68 plant varieties, from native prairie grasses and perennial plants to drought-tolerant succulents popular to green roofs. The plants were left to survive Minnesota’s climate for a year with little or no tending, and in the second year Hensley added 35 new species to the 30 that survived. Mediterranean culinary herbs, onions, mints and thymes joined tulips and bulb plants in the extended garden. The researchers also were testing some alpine plants specially suited to cold, dry mountaintop climates, like a poppy species from the Himalayas—plants that wouldn’t thrive in Chicago’s climate.

Sedum plant species spread low to the ground and retain water in their leaves, making them popular on green roofs.“All of these plants can have unique ecological effects if they survive,” Hensley says. “A great example would be in this context of a metro sprawl area with a lot of airborne emissions that become a problem when you limit the green spaces. We could build an eco-roof custom-tailored to filter some of the dense pollution in this area.”

Living roofs filter and retain pollutants and carbon dioxide out of the air and the captured rainwater runoff. At the same time, the restored green spaces host a return of wildlife and birds that have been displaced by construction. The researchers included native and flowering plants in the mix not only for aesthetic variety, but to invite honeybees and insect life into the city to promote biological diversity in urban centers around the region.

“People just love a green roof,” Hensley says. “It catches students’ eyes as they walk by and ask when we’ll finish planting the rest of the roofs. That is encouraging to me. Why should we just lose the green spaces when there’s an opportunity to put it on the roof instead of scrapping it like it’s worthless? Green roofs are going to be the way of the future.”

Building a Minnesota green roof

Once their research is complete, Erwin plans to have a list of species that will survive in Minnesota’s climate that nurseries and garden stores can use to produce a full commercial palette of green roof plants. He hopes Minneapolis will use the list to expand ground-based zoning codes that will help ensure that when someone moves their green space to the roof, it will live.

New buildings constructed with green roofs in mind often add a heavier weight capacity to the roof, along with an irrigation infrastructure. These can support a full garden and even trees, with a deep soil profile. Such roofs can cost 30-60 percent more to construct, but green roofs can double or even triple a roof’s lifespan. But in reality, Erwin says, all you need is a waterproof roof and trays with a few inches of soil and growing material.

“In the Twin Cities, it’s more likely that we will be dealing with an industrial building that will be retrofit to use a system that offers all the advantages with a smaller weight load,” he says. “We don’t want to pick plants that require a massive energy input. The plant materials have been defined by landscape architects, but horticulturalists have a better understanding of how to grow plants that survive.”


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