Green roofs have been touted as a means of saving energy, improving water management and expanding natural space within urban environments. Can they also help save native plants? National Geographic grantee Clark DeLong is working to answer that question, particularly with regard to species in the U.S. mid-Atlantic region. The botanist, who is based at the University of Maryland’s Plant Science and Landscape Architecture department, answered some questions about his work via email.
What inspired you to look at using green roofs as conservation space?
I spent two years working in the horticulture field before entering University of Maryland as an undergrad. During this time, I began to realize that the vast majority of the species that we use in our landscapes and urban developments do not originate from North America. These species are termed “exotic,” or “non-native,” due to the fact they did not exist in the North America prior to introduction by humans. Examples include everything from turfgrass to Forsythia. The majority of plants grown for ornamental reasons are not a part of the natural ecology. I began to conduct my own research outside of work hours and quickly began to realize the ecological impact this has on ecosystems.
Native plant species form complex relationships with native insects that evolve over long periods of time to use them as their food source. If you replace those native plants with exotics, you sever the link and the insects dependent on those native plants no longer can provide their young with a place to feed and grow. In turn, when you drive away the insects, you do the same to the larger animals (such as birds) that feed on the insects.
We have impoverished vast tracts of North America with our choices of ornamental vegetation around homes and places of work. My overarching goal is to help conserve native plant species by transitioning the horticulture field towards their use instead of the exotics we use today. Green roofs are a relatively new industry in the United States, but the use of exotics is already well entrenched.
There are a number of habitat types in the Mid-Atlantic that resemble green roofs in their environmental conditions. Many of these habitat types are geographically limited and particularly vulnerable to human disturbance. Planting the species native to these ecosystems on green roofs will provide additional habitat space for them and protect them should any damage occur to wild populations.
What types of plants are you trying to save, and why?
This project primarily focuses on species that grow in geographically limited habitat types such as shale barrens and limestone fields. These habitats typically feature dry terrain that can be rocky and/or subject to extreme heat with few surrounding trees. Many of the plants in these habitats have adapted to tolerate drought well. I think my best species I am working with right now is Packera antennariifolia (formerly Senecio antennariifolius), also known as shale barren ragwort: it’s highly ornamental, with dusky grey foliage that reaches only three or four inches off the ground, and starry yellow flowers that appear in spring. It is geographically limited to shale barren habitats in the mid-Appalachians, from West Virginia to southern Pennsylvania. Because shale barrens generally cover small areas of up to a dozen acres and can be separated from each other by miles of forest, their plant denizens are particularly vulnerable to extinction. Shale barren ragwort is one of those plants, and it’s considered endangered in Pennsylvania.
What are the benefits of green roofs in an urban environment?
Green roofs provide a diverse selection of benefits in urban environments. There primary purpose from a regulatory perspective is to reduce storm water runoff from the roofs on which they are situated. This is of particular importance due to large amount of impermeable surface in urban areas which cause enormous damage through runoff, siltation and pollution into urbanized watersheds. Green roofs retain a large portion of the water that falls on them during storms and return it to the atmosphere through evaporation and transportation by plants growing on them. They passively cool buildings and reduce the urban heat island effect, thus saving energy used to heat and cool buildings. They make urban roofs more aesthetically appealing and can raise housing values when they are overlooked by office or apartment space. With the addition of native species conservation and habitat space may be able to be added to the list of benefits that green roofs provide to urban areas.
What type of results do you hope to see as a result of your work, and can you say when?
I hope to see the plants that I am evaluating introduced to the horticulture trade for use on green roofs in the mid-Atlantic. Widespread planting of these species in the region they are native to will help conserve the species and in essence expand their potential habitat to include urban areas. The plants will be introduced to the trade as my project wraps up. In addition, I am working to quantify the effect that different plant species have on the performance characteristics of a green roof. I have sensors placed under representatives of five species that I am working with to determine if species contribute differently to the water-holding capacity and insulative properties of green roofs. This data should be available toward the end of fall of 2013.