Tuesday, March 4, 2014
Native Plant Myth 2 Native Plants are Not as Tough as Exotic Plants
Few issues in the gardening world generate as much heat as the debate about native plants. As a result, native plants have developed their own dogma. It’s that dogma that I want to set straight. So I’m here to bust some of the top myths about native plants. Let the smackdown continue:
Myth 2: Native plants are not as tough as exotic plants.
This is one I hear all the time among landscape architects. “This site is too brutal for natives,” a colleague said recently. He was referring to an urban parking lot that would not be irrigated. Implicit in the assumption is the belief that natives are somehow weaker and more delicate than exotics plants.
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| Wild some natives like trillium may not be tough enough for urban areas, others are. |
It’s easy to understand where this mythology comes from. A forest of mostly native species gets razed for an office park. The client expresses a desire to use mostly natives on the new site, perhaps as a way to mitigate the fact that an energy-sucking office park just ate a forest. But the conditions have changed now. The precious native ephemerals such as tiarellas, trilliums, and geraniums that thrived under the cool woodland canopy will no longer survive on the edge of a sunny parking lot, especially once the maintenance crew salts it in winter. So the designer reverts to a “tried and true” palette of juniper, berberis, and euonymous to green the parking islands.
This line of thinking is not limited to designers. Senior research scientist of the Arnold Arboretum, Peter Del Tredici wrote, “My advice is simple: don’t limit your planting designs to a palette of native species that might once have grown on the site. Imposing such a limitation on diversity not only reduces the aesthetic possibilities for the landscape, but also its overall adaptability.”
The assumption made in both cases is that the only native plants appropriate for the site are those that used to be there hundreds of years ago. If you leave anywhere east of the Mississippi, that probably means some kind of woodland. Of course, many of these plants would be poorly suited to harsh urbanized conditions. But what about native plants adapted to harsh conditions?
I remember hiking through some of Georgia’s granite rock outcrops and marveling at the ability of native plants to live in utterly desolate conditions. Beautiful patterns of mosses, grasses, and cedars grew in the slimmest pockets of anaerobic soil. These plants withstood blazing heat, drought, periodic inundations, and infertile soil, and deep frosts— conditions remarkably similar to urban environments. And it’s not just granite outcrops. All over the country, native plants thrive in horrifically inhospitable environments. When we limit our understanding of native plants to a few precious woodland floor plants, we lose sight of their potential in human disturbed landscapes.
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| Native vegetation on harsh urban-like conditions of granite outcrops in Heggies Rock in Georgia. Painting by Philip Juras. |
I remember hiking through some of Georgia’s granite rock outcrops and marveling at the ability of native plants to live in utterly desolate conditions. Beautiful patterns of mosses, grasses, and cedars grew in the slimmest pockets of anaerobic soil. These plants withstood blazing heat, drought, periodic inundations, and infertile soil, and deep frosts— conditions remarkably similar to urban environments. And it’s not just granite outcrops. All over the country, native plants thrive in horrifically inhospitable environments. When we limit our understanding of native plants to a few precious woodland floor plants, we lose sight of their potential in human disturbed landscapes.
This summer I interviewed Mark Simmons, a research ecologist with the Lady Bird Johnson Wildflower Center. Mark and his colleagues have done a slate of new research that challenges the assumption that native plants are somehow weaker than exotics. Simmons has a revolutionary vision of what urban spaces can be: a place where native plants and ecosystems provide ecological services for humans such as stormwater management, pollution filtration, and habitat creation. “Native plants, as we’re finding, are a hugely untapped resource,” says Simmons.
This vision is backed with cutting-edge research. In 2008 Simmons completed a study that compared the performance of different types of green roofs. Simmons compared 24 different experimental rooftops. The study showed that green roofs with native plants outperformed green roofs with mostly sedums. The native plants captured stormwater and cooled the surfaces better than exotic sedums.
A second study compared the performance of exotic versus native turfgrasses. Early research demonstrates that the lawns composed of a mix of native grasses outperformed the non-native lawn. The native lawn better conserves water, resists disease, and handles foot traffic than the non-native lawn. In addition, the native lawns were indistinguishable in appearance from the non-native lawn.
Simmons’ research shatters stereotypes of natives as weak, underperforming plants. For example, the Texas Department of Transportation was initially hesitant to substitute native grass seeding for the more tried and true exotic Bermuda grass. Simmons tested a way to artificially increase the density of native wildflowers. Not only did the native grasses and wildflowers grow better, but they also reduced populations of the invasive bastard cabbage which grew alongside the Bermuda grass.
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| Test plots of native turfgrass outperform exotic species. |
Simmons’ research shatters stereotypes of natives as weak, underperforming plants. For example, the Texas Department of Transportation was initially hesitant to substitute native grass seeding for the more tried and true exotic Bermuda grass. Simmons tested a way to artificially increase the density of native wildflowers. Not only did the native grasses and wildflowers grow better, but they also reduced populations of the invasive bastard cabbage which grew alongside the Bermuda grass.
Simmons is optimisitc about future applicaitons of native ecosystems in urban environments: "Native ecosystems have the potential to improve almost any urban environmental problem."
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