The Role of Extensive Green Roofs in Sustainable Development

As forests, agricultural fields, and suburban and urban lands are replaced with impervious surfaces resulting from development, the necessity to recover green space is becoming increasingly critical to maintain environmental quality. Vegetated or green roofs are one potential remedy for this problem. Establishing plant material on rooftops provides numerous ecological and economic benefits, including stormwater management, energy conservation, mitigation of the urban heat island effect, and increased longevity of roofing membranes, as well as providing a more aesthetically pleasing environment in which to work and live. Furthermore, the construction and maintenance of green roofs provide business opportunities for nurseries, landscape contractors, irrigation specialists, and other green industry members while addressing the issues of environmental stewardship. This paper is a review of current knowledge regarding the benefits of green roofs, plant selection and culture, and barriers to their acceptance in the United States. Because of building weight restrictions and costs, shallow-substrate extensive roofs are much more common than deeper intensive roofs. Therefore, the focus of this review is primarily on extensive green roofs. Before human development began disturbing natural habitats, soil and vegetation constituted part of a balanced ecosystem that managed precipitation and solar energy effectively. In natural areas, much of the rainwater infiltrates into the ground or is returned to the atmosphere via evapotranspiration, thus absorbing rainwater and performing a cooling function for excess solar loads. As the human population began expanding, more construction ensued, which disturbed these natural habitats. Cities, towns, and suburbs all add more impervious surfaces as we construct buildings, roads, and parking lots. In the United States, it is estimated that 10% of residential developments and 71% to 95% of industrial areas and shopping centers are covered with impervious surfaces (Ferguson, 1998). Today, two-thirds of all impervious area is in the form of parking lots, driveways, roads, and highways (Water Resources Group, 1998), and this loss of natural areas causes many problems. Volume of stormwater runoff Because an impervious surface cannot absorb precipitation, this water flows off surfaces and reduces infiltration into groundwater. In forests, ~95% of rainfall is absorbed, whereas only about 25% is absorbed in cities (Scholz–Barth, 2001). Excessive runoff increases the chances for flooding downstream as stormwater exceeds channel capacities, resulting in the probability of property damage and human harm. A high volume of stormwater runoff can also overwhelm municipal sewer systems. Combined sewer systems consist of a single pipe that takes wastewater and stormwater to treatment plants. When stormwater exceeds capacity, the combined sewage can overflow into relief points, resulting into raw waste being dumped into our rivers, an event called a combined sewage overflow (CSO). In New York City, about half of all rainfall events result in a CSO event. These CSO events dump 40 billion gallons of untreated wastewater into New York’s surface waters annually (Cheney, 2005). Quality of stormwater runoff Impervious surfaces collect pollutants such as oil, heavy metals, salts, pesticides, and animal wastes. During runoff events, these contaminants may wash into waterways. Novotny and Chesters (1981) described the quality of urban runoff water as approaching that of treated sewage or even worse. Research supports the link between runoff from impervious surfaces and the reduction of water quality in streams. Even 10% of a land area covered with impervious surfaces can have an effect on stream quality (Ferguson, 1998). Runoff that contains a large amount of organic matter can also cause eutrophication of the surface waters, reducing oxygen availability and resulting in loss of aquatic species (Barnes et al., 2001). Not only does polluted runoff impact the ecosystem, but it can affect human health as well. For example, untreated urban runoff onto public beaches has caused surfers twice as many health problems than beaches not exposed to urban runoff (Dwight et al., 2004). Energy conservation and the urban heat island Because water is not retained in the soil as a result of runoff from these surfaces, the quantity of water available for evapotranspiration is reduced. Therefore, a great deal of incoming solar energy that would have been used to evaporate water is instead transformed into sensible heat (Barnes et al., 2001). In addition, many impervious surfaces tend to be heat-absorbing structures. The albedo of a surface is a measure of the incoming solar radiation that is reflected off the surface and thus is not absorbed and transformed into heat energy. The albedo of urban surfaces is generally 10% lower than the albedo of rural surfaces (Oliver, 1973). Loss of vegetation and water resulting from the creation of impervious surfaces combined with the heat-absorbing properties of such structures results in higher internal building temperatures and ambient air temperatures outside the building compared with the surrounding suburbs or countryside. According to the USEPA (2003), urban air temperatures can be up to 5.6 C warmer than the surrounding countryside. In an urban heat island effect situation, even night air temperatures are warmer because built surfaces absorb heat and radiate it back during the evening hours. In Berlin, temperatures on a clear windless night were 9 C higher than in the countryside (Von Stulpnagel et al., 1990). Because of the health effects associated with excess heat, the Environmental Protection Agency (EPA) has developed a Heat Island Reduction Initiative (HIRI). Through this program, the EPA works with researchers, community groups, and public officials to identify and implement methods that reduce heat islands. They cite that excessively hot air temperatures can result in physiological disruptions, organ damage, and death (USEPA, 2003). The Department of Health and Human Services Centers for Disease Control and Prevention (CDC) says that high-temperature events have caused more deaths in the United States each year than hurricanes, lightning, tornadoes, floods, and earthquakes combined. The CDC estimated that for the 23-y period between 1979 and 2002, 8966 deaths resulted from excessive heat exposure in the United States (Department of Health and Human Services, 2005). A potential solution: vegetated green roofs Green roof technology offers an alternative to spending millions of dollars to renovate Received for publication 23 Jan. 2006. Accepted for publication 21 Apr. 2006. Figure 3 drawn by Marlene Cameron. This paper is a portion of an MS thesis submitted by K.L. Getter. Graduate Research Assistant. Associate Professor. To whom correspondence should be addressed; email [email protected] 1276 HORTSCIENCE VOL. 41(5) AUGUST 2006 JOBNAME: horts 41#5 2006 PAGE: 1 OUTPUT: July 12 18:12:24 2006 tsp/horts/118440/01418