The Role of Extensive Green Roofs in Sustainable Development

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1 JOBNAME: horts 41#5 2006 PAGE: 1 OUTPUT: July 12 18:12:24 2006 tsp/horts/118440/01418 HORTSCIENCE 41(5):12761285. 2006. the ecosystem, but it can affect human health as well. For example, untreated urban The Role of Extensive Green Roofs in runoff onto public beaches has caused surf- ers twice as many health problems than Sustainable Development beaches not exposed to urban runoff (Dwight et al., 2004). Kristin L. Getter1 and D. Bradley Rowe2,3 Michigan State University, Department of Horticulture, A212 Plant & Soil Sciences Bldg., East Lansing, MI 48824 Energy conservation and the urban heat island Additional index words. vegetated roof, ecoroof, stormwater management, energy conserva- Because water is not retained in the soil tion, plant evaluations as a result of runoff from these surfaces, the quantity of water available for evapotrans- Abstract. As forests, agricultural fields, and suburban and urban lands are replaced with piration is reduced. Therefore, a great deal impervious surfaces resulting from development, the necessity to recover green space is of incoming solar energy that would have becoming increasingly critical to maintain environmental quality. Vegetated or green been used to evaporate water is instead roofs are one potential remedy for this problem. Establishing plant material on rooftops transformed into sensible heat (Barnes et provides numerous ecological and economic benefits, including stormwater manage- al., 2001). In addition, many impervious ment, energy conservation, mitigation of the urban heat island effect, and increased surfaces tend to be heat-absorbing struc- longevity of roofing membranes, as well as providing a more aesthetically pleasing tures. The albedo of a surface is a measure environment in which to work and live. Furthermore, the construction and maintenance of the incoming solar radiation that is of green roofs provide business opportunities for nurseries, landscape contractors, reflected off the surface and thus is not irrigation specialists, and other green industry members while addressing the issues of absorbed and transformed into heat energy. environmental stewardship. This paper is a review of current knowledge regarding the The albedo of urban surfaces is generally benefits of green roofs, plant selection and culture, and barriers to their acceptance in the 10% lower than the albedo of rural surfaces United States. Because of building weight restrictions and costs, shallow-substrate (Oliver, 1973). extensive roofs are much more common than deeper intensive roofs. Therefore, the Loss of vegetation and water resulting focus of this review is primarily on extensive green roofs. from the creation of impervious surfaces combined with the heat-absorbing properties of such structures results in higher internal Before human development began dis- sorbed, whereas only about 25% is absorbed building temperatures and ambient air tem- turbing natural habitats, soil and vegetation in cities (ScholzBarth, 2001). Excessive peratures outside the building compared constituted part of a balanced ecosystem that runoff increases the chances for flooding with the surrounding suburbs or country- managed precipitation and solar energy ef- downstream as stormwater exceeds channel side. According to the USEPA (2003), urban fectively. In natural areas, much of the rain- capacities, resulting in the probability of air temperatures can be up to 5.6 C warmer water infiltrates into the ground or is returned property damage and human harm. A high than the surrounding countryside. In an to the atmosphere via evapotranspiration, volume of stormwater runoff can also over- urban heat island effect situation, even night thus absorbing rainwater and performing whelm municipal sewer systems. Combined air temperatures are warmer because built a cooling function for excess solar loads. As sewer systems consist of a single pipe that surfaces absorb heat and radiate it back the human population began expanding, takes wastewater and stormwater to treat- during the evening hours. In Berlin, temper- more construction ensued, which disturbed ment plants. When stormwater exceeds ca- atures on a clear windless night were 9 C these natural habitats. Cities, towns, and pacity, the combined sewage can overflow higher than in the countryside (Von Stulpnagel suburbs all add more impervious surfaces as into relief points, resulting into raw waste et al., 1990). we construct buildings, roads, and parking being dumped into our rivers, an event called Because of the health effects associated lots. In the United States, it is estimated that a combined sewage overflow (CSO). In New with excess heat, the Environmental Pro- 10% of residential developments and 71% to York City, about half of all rainfall events tection Agency (EPA) has developed a Heat 95% of industrial areas and shopping centers result in a CSO event. These CSO events Island Reduction Initiative (HIRI). Through are covered with impervious surfaces (Fer- dump 40 billion gallons of untreated waste- this program, the EPA works with research- guson, 1998). Today, two-thirds of all imper- water into New Yorks surface waters annu- ers, community groups, and public officials vious area is in the form of parking lots, ally (Cheney, 2005). to identify and implement methods that driveways, roads, and highways (Water Re- reduce heat islands. They cite that exces- sources Group, 1998), and this loss of natural sively hot air temperatures can result in areas causes many problems. Quality of stormwater runoff physiological disruptions, organ damage, Impervious surfaces collect pollutants and death (USEPA, 2003). The Department such as oil, heavy metals, salts, pesticides, of Health and Human Services Centers for Volume of stormwater runoff and animal wastes. During runoff events, Disease Control and Prevention (CDC) says Because an impervious surface cannot these contaminants may wash into water- that high-temperature events have caused absorb precipitation, this water flows off ways. Novotny and Chesters (1981) de- more deaths in the United States each year surfaces and reduces infiltration into ground- scribed the quality of urban runoff water as than hurricanes, lightning, tornadoes, water. In forests, ~95% of rainfall is ab- approaching that of treated sewage or even floods, and earthquakes combined. The worse. Research supports the link between CDC estimated that for the 23-y period runoff from impervious surfaces and the between 1979 and 2002, 8966 deaths re- reduction of water quality in streams. Even sulted from excessive heat exposure in the 10% of a land area covered with impervious United States (Department of Health and Received for publication 23 Jan. 2006. Accepted surfaces can have an effect on stream quality Human Services, 2005). for publication 21 Apr. 2006. Figure 3 drawn by (Ferguson, 1998). Runoff that contains Marlene Cameron. This paper is a portion of an MS thesis submitted by K.L. Getter. a large amount of organic matter can also 1 Graduate Research Assistant. cause eutrophication of the surface waters, A potential solution: vegetated 2 Associate Professor. reducing oxygen availability and resulting green roofs 3 To whom correspondence should be addressed; in loss of aquatic species (Barnes et al., Green roof technology offers an alternative email [email protected] 2001). Not only does polluted runoff impact to spending millions of dollars to renovate 1276 HORTSCIENCE VOL. 41(5) AUGUST 2006

2 JOBNAME: horts 41#5 2006 PAGE: 2 OUTPUT: July 12 18:12:24 2006 tsp/horts/118440/01418 outdated stormwater infrastructures and to power air conditioners. Green roofs involve growing plants on rooftops, thus replacing the vegetated footprint that was destroyed when the building was constructed. They are similar to other cool-roof technologies in that they have a high albedo (ranging, from 0.70.85), depending on water availability (Gaffin et al., 2005). Other cool-roof tech- nologies, such as white roofs, may start with an albedo of 0.8, but reflectivity can decline up to 11% from dust and debris accumula- tion. Conventional roof surfaces have much lower albedos, ranging from 0.05 to 0.25 (USEPA, 2005). Even though green roofs are not very common in the United States, they are not a new idea. One of the first above-ground gardens were the hanging gardens of Babylon built around 500 BC. During the Middle Ages and the Renaissance, roof gardens were mainly for the rich, although Benedictine monks were fond of them as well. In the 1600s to 1800s, Norwegians covered roofs with soil for insulation and then planted grasses and other species for stability. Early American settlers of the Great Plains also Fig. 1. An intensive green roof on the Coast Plaza Hotel in Vancouver, British Columbia. used this technique in the late 1800s because of a lack of timber (Osmundson, 1999). Germany is recognized as the place of origin for modern-day green roofs. In the 1880s, Germany experienced rapid industri- alization and urbanization. Inexpensive hous- ing was often built with highly flammable tar as the roofing material. A roofer named H. Koch developed a method to reduce the fire hazard by covering the tar with sand and then gravel. Seeds naturally colonized these roofs eventually to form meadows. As of 1980, 50 of these roofs were still intact and still completely waterproof (Kohler and Keeley, 2005). The Great Depression and World War II led to a general lull in roof greening. How- ever, Britain benefited from the camouflaging capabilities of green roofs by using them to cover military airfield hangars in the form of turf during the 1930s (Frith and Gedge, 2005). Despite the failing economy during the Depression, the first prominent US mod- ern green roof was built at the Rockefeller Center in New York City during that time (Osmundson, 1999). Today in the United States, green roofs are becoming less of a novelty, although other countries are far more advanced in the adoption of this tech- Fig. 2. Portion of a 10.4-acre extensive green roof on an assembly plant at Ford Motor Company in nology. In Germany, it is estimated that 14% Dearborn, Mich. Plant material consists of 13 species and cultivars of Sedum. of all flat roofs are green (Kohler and Keeley, 2005). Modern green roofs can be categorized as intensive or extensive systems depend- parklike areas accessible to the public, they Most green roofs have similar construc- ing on the plant material and the planned are generally limited to flat roofs. tion components (Fig. 3). A root barrier usage for the roof area (Figs. 1 and 2). In contrast, extensive roofs generally re- installed on top of the normal roofing mem- Intensive green roofs are so named because quire minimal maintenance. They are typi- brane protects the roof from root penetration of their intense maintenance needs. They cally not accessible to the public and may not damage. A drainage layer above the root are designed to be similar to landscaping even be visible. Because of their shallower barrier allows excess water to flow away found at natural ground level. They typically media depth (15.2 cm) In addition, extensive green roofs can be built drainage layer. An optional water retention than extensive roofs. Because they are often upon a sloped surface. fabric may be laid on top of this, which HORTSCIENCE VOL. 41(5) AUGUST 2006 1277

3 JOBNAME: horts 41#5 2006 PAGE: 3 OUTPUT: July 12 18:12:28 2006 tsp/horts/118440/01418 Water retention depends on design factors such as substrate depth, composition, and plant species, as well as weather factors such as intensity and duration of rainfall. The effect of slope on rainfall detention is unclear. In Germany, Schade (2000) and Liesecke (1998) found no significant differ- ence in retention amounts across differently sloped roofs. Other studies suggest increas- ing slope increases runoff (VanWoert et al., 2005a; Villarreal and Bengtsson, 2005). The contradicting results may be the result of rainfall patterns in different environments. Many researchers have observed that dry substrate conditions before rainfall results in more stormwater retention compared with initially wet conditions (Connelly and Liu, 2005; Villarreal and Bengtsson, 2005). Because green roofs retain stormwater, they can mitigate the effects of impervious surface runoff. For example, if 6% of the roof surface area in Toronto were green, Peck (2005) estimated that the impact on storm- water retention would be equal to building a $60 million (CDN) storage tunnel. Deutsch Fig. 3. Cross-section of a representative extensive green roof system including typically used layers. The and colleagues (2005) calculated that if 20% drainage layer is place over a root barrier that covers the roofing membrane. The water retention fabric of all buildings in Washington, D.C., that is optional and the media depth and plant material vary depending on design specifications. could support a green roof had one, that they would add more than 71 million L to the citys stormwater storage capacity and store ~958 million L of rainwater in an average year. Delayed stormwater runoff Even though green roof systems retain stormwater, runoff will still occur after the media becomes saturated. However, runoff is delayed because it takes time for the media to become saturated and for the water to drain through the media. This delay can prevent stormwater sewer systems from overflowing, by allowing it to process runoff for a longer time at a lower flow rate. Green roofs can delay runoff between 95 min (Liu, 2003) and 4 h (Moran et al., 2004), compared with the reference roofs for which runoff was nearly instantaneous. After runoff begins on a green roof system, the rate at which the rain leaves the roof is slower than a nongreened roof because of the nature of the green roofing components. Liu (2003) found that when initial rainfall was 2.8 mm/h, runoff from the green roof was reduced to 0.5 mm/h. North Carolina researchers found a 57% to 87% reduction in flow rates on a green roof (Moran et al., 2005). By slowing down the Fig. 4. An extensive green roof native plant community on the convention center of the Church of Jesus rate of runoff and turning it out over a longer Christ of Latter-day Saints in Salt Lake City, Utah. period of time, green roofs can help mitigate the erosional power of runoff that does enter streams, either through direct runoff or storm allows extra water to be retained for the reduction of total amounts of stormwater sewers. benefit of the plants. Finally, a growing sub- runoff. In a green roof system, much of the strate is placed that supports plant growth. precipitation is captured in the media or Increased life span of roofing Design of these components depends on the vegetation and will eventually evaporate membranes purpose of the greening project and building from the soil surface or will be released back Growing media and plant material protect load capabilities. into the atmosphere by transpiration. Kolb roofing membranes from solar exposure and (2004) reported that 45% of all rainfall can be ultraviolet radiation that can damage the Benefits of Green Roofs recycled in this manner. Green roofs may traditional bituminous roof membrane. These reduce runoff by 60% to 100%, depending on materials also reduce day/night temperature Reduced volume of stormwater runoff the type of green roof system (DeNardo et al., fluctuations at the membrane, which reduces Probably the single greatest environmen- 2005; Liescke, 1998; Moran et al., 2004; the stress of daily expansions and con- tal service that green roofs provide is the Rowe et al., 2003; VanWoert et al., 2005a). tractions. One study demonstrated diurnal 1278 HORTSCIENCE VOL. 41(5) AUGUST 2006

4 JOBNAME: horts 41#5 2006 PAGE: 4 OUTPUT: July 12 18:12:34 2006 tsp/horts/118440/01418 fluctuations for a nongreen roof to be 50 C, Even so, they do help to insulate buildings improved health and worker productivity. whereas a green roofs diurnal fluctuation during the winter. On the Plant and Soil Kaplan and colleagues (1988) reported that was only 3 C (Connelly and Liu, 2005). Sciences Building at Michigan State Univer- employees who had a view of nature, such as Research supports the stress differential on sity, we are currently collecting data on heat trees and flowers, were less stressed, experi- roof membranes between a conventional roof flux through portions of a green and conven- enced greater job satisfaction, and reported and a green roof. In Toronto, Canada, the roof tional roof. Although these data are still being fewer headaches and other illnesses than membrane temperature on a nongreened roof collected and analyzed, it is obvious that those who had no natural view. Ulrich reached 70 C in the afternoon, whereas the there is some effect when snow has melted (1984) noted that patients experience faster roof membrane on the green roof was only from the conventional side, but still exists on recoveries from surgery when they have 25 C (Liu and Baskaran, 2003). Peck and the vegetated portion. Regardless, if energy a natural view. Not only are the green associates (1999) estimated that temperature savings were the only reason for installing surroundings aesthetically pleasing, but land- moderation can extend the membrane life a green roof, it would be much less expensive lords can often increase tenancy rates and two to three times. to install additional insulation when con- hotels are able to charge more for a room structing the building rather than installing with a view compared with traditional Energy conservation and the a green roof. barren roof scenery. urban heat island In addition to increasing roof membrane Increase biodiversity and provide Mitigation of air pollution life, green roofs provide shade and insulation, habitat Plants can filter out particulate matter and resulting in energy savings and mitigation of Because most extensive green roofs are gaseous pollutants in the air. Particles will the urban heat island effect. Media depth, inaccessible to the public, they can provide eventually be washed away into the soil via shade from plant material, and transpiration undisturbed habitat for microorganisms, in- rainwater movement, and some of the pollu- can reduce solar energy gain by up to 90% sects, and birds. In a biodiversity study of 17 tants will be absorbed into plant tissues. A compared with nonshaded buildings. Green green roofs in Basel, Switzerland, 78 spider variety of airborne contaminants can be roofs have reduced indoor temperatures 3 to and 254 beetle species were identified during alleviated by green roofs. A German study 4 C when outdoor temperatures were be- the first 3 years. Eighteen percent of those demonstrated that green roof vegetation can tween 25 C and 30 C (Peck et al., 1999). spiders and 11% of the beetles were listed as significantly reduce diesel engine air pollu- Every decrease in internal building air tem- endangered or rare (Brenneisen, 2003). In tion (Liesecke and Borgwardt, 1997). Yok perature of 0.5 C may reduce electricity use a West Berlin study of 50-y-old green roofs, Tan and Sia (2005) found a 37% and 21% for air-conditioning up to 8% (Dunnett and Darius and Drepper (1984) found grasshop- reduction of sulfur dioxide and nitrous acid Kingsbury, 2004). pers, white grubs, beetles, and a high number respectively directly above a newly installed Air temperatures above the building have of mites. In northeastern Switzerland, nine green roof. Others have estimated that green been shown to be 30 C lower when vege- orchid species and other rare and endangered roofs can remove dust particles on the order tated compared with a conventional roof plant species existed on a 90-y-old green roof of 0.2 kg of particulates per year per square (Wong et al., 2003), resulting in up to 15% (Brenneisen, 2004). In addition, many birds meter of grass roof (Peck and Kuhn, 2001). annual energy consumption savings. But the have been recorded using green roofs in The adverse health effects of particulate amount of shading is highly dependent on the Germany, Switzerland, and England (Bren- air pollution include increased respiratory types of plants chosen, because leaf area neisen, 2003; Gedge, 2003). problems, decreased lung function, and in- index has a significant impact on the shading Even relatively new green roofs can pro- creased hospitalizations and other health care effect (Wong et al., 2003). Over a 30-d warm vide habitat. One of the worlds largest green visits for respiratory and cardiovascular dis- fall period in British Columbia, total heat roofs is in Dearborn, Mich., on top of a Ford ease. Increased respiratory morbidity, as flow through a reference roof and green roof Motor Company assembly plant. The 42,900 measured by absenteeism from work or was 2.634 kW/m2 and 0.7 kW/m2 respecti- m2 greened roof consists of a mix of 13 school or other restrictions in activity, and velya 70% reduction (Connelly and Liu, Sedum species planted in less than 7.6 cm increased cardiopulmonary disease mortality 2005). media. Within 2 years of initial plant estab- are also induced by air pollution (Pope et al., Because buildings consume 36% of total lishment, 29 insect species, seven spider 1995). energy use and 65% total electricity con- species, and two bird species were identified But air pollution as it relates to health is sumption, green roof implementation on (Coffman and Davis, 2005). not the only issue. Some municipalities such a wide scale could significantly impact en- Some researchers are evaluating roofs as as Washington, D.C., are in danger of losing ergy savings (Kula, 2005). Laberge (2003) a potential way to restore native plant species federal funds because they do not meet estimated that for the Chicago city hall, to an area. (Dewey and colleagues 2004) federal air quality standards for particulate energy savings alone could result in $4000 evaluated 35 native grasses and wildflowers matter. If 20% of all existing green roof- annually for heating and cooling combined. If on an irrigated intensive green roof and found ready buildings in Washington, D.C., im- all of Chicago had green roofs, the savings that 21 species were suitable for a meadow plemented the technology, the resulting could be $100,000,000 annually Laberge, mixture with a 1.0-m media depth. Others are plantings would remove the same amount of (2003). Most of the insulation benefits result evaluating natives on nonirrigated extensive air pollution as 17,000 street trees (Deutsch from cooling, as heating reductions range green roofs (Monterusso et al., 2005). One et al., 2005). An air quality model for from 0.12% to 0.2% and cooling reductions would expect that a green roof consisting of greening all rooftops in Chicago predicts a range from 6.2% to 6.4% (SaizAlcazar and a native plant community would provide reduction of 417,309.26 kg of nitrogen Bass, 2005). This is partly the result of how greater biodiversity than a typical Sedum- oxide and 517,100.61 kg of sulfur oxide large air-conditioning systems work: There is based roof. However, depending on the area, emissions per year (Laberge, 2003). At the lower power demand when intake air is cooler. there may not be native vegetation that can University of Michigan, Clark et al. (2005) When intake air exceeds 35 C (95 F), power withstand the normal environmental stresses estimate that if 20% of all industrial and requirements for the air conditioner increase encountered on a rooftop. commercial roof surfaces in Detroit, Mich., and cooling capacity drops (Leonard and were traditional extensive Sedum green roofs, Leonard, 2005). Improved aesthetic value more than 800,000 kg per year of NO2 (or Most energy savings from green roofs will When humans view green plants and 0.5% of that areas emissions) would be occur during the summer months. This is nature, it has beneficial health effects, such removed. because the insulation properties of the sub- as reducing stress, lowering blood pressure, strate are greater when air space exists in the releasing muscle tension, and increasing Noise reduction pores as opposed to when they are saturated, positive feelings (Ulrich and Simmons, Hard surfaces in urban areas are more which is normally the case during winter. 1986). These benefits can be translated into likely to reflect sound, whereas green roofs HORTSCIENCE VOL. 41(5) AUGUST 2006 1279

5 JOBNAME: horts 41#5 2006 PAGE: 5 OUTPUT: July 12 18:12:34 2006 tsp/horts/118440/01418 absorb sound waves because of the nature of stormwater management. Some extensive ideal substrate is comprised of a balance of the substrate and vegetation. At the airport in roofs may not even be seen except from the lightweight, well-drained material, has ade- Frankfurt, Germany, a 10-cm-deep green air. quate water and nutrient holding capacity, roof reduced noise levels by 5 dB (Dunnett Combinations of evergreens and flower- and will not break down over time. The major and Kingsbury, 2004). Other research shows ing plants with a long blooming season pro- component of green roof substrates should be that 12 cm of green roof substrate alone can vide a visual impact when grown together. mineral-based materials such as expanded diminish noise by 40 dB (Peck and Kuhn, However, summer droughts can turn flower- slate, shale, or clay. Other inorganic compo- 2001). There have been ample scientific ing perennial plants into a mass of browned- nents may include sand, pumice, perlite, studies on the effects of noise exposure to out, dead-looking plants that could be a fire vermiculite, and crushed recycled clay bricks humans, with hearing impairment, hyperten- hazard. Similarly, grasses are difficult to keep or tiles (Beattie and Berghage, 2004; Dunnett sion and ischemic heart disease, annoyance, green throughout the summer, especially on and Kingsbury, 2004). High levels of com- sleep disturbance, and decreased school per- extensive roofs. To grow most annuals, pe- post are not recommended because it will formance being just some of the problems rennial flowering herbaceous plants, and decompose and result in substrate shrinkage found (PasschierVermeer and Passchier, grasses, either irrigation must be present or (Beattie and Berghage, 2004), and it can 2000). substrate depths must be deeper than nor- result in increased N and P in the runoff mally found on extensive roofs. If irrigation (Moran et al., 2005). Also, it is not feasible or Leadership in energy and environmental is not available, then succulent species such practical to replace the substrate on a rooftop design (LEED) standards as Sedum, Sempervivum, and Delosperma are continually. Substrate composition will de- Implementing green roofs can lead to considered good choices because of their pend on what materials are available locally additional credits in the LEED program ability to withstand extended drought con- and can be formulated for the intended plant administered by the US Green Building ditions and other adverse environmental con- selection, climatic zone, and anticipated level Councilan agency that promotes the de- ditions often present on a rooftop (Snodgrass, of maintenance. sign and construction of buildings that are 2005). However, these taxa may not have the Rowe et al. (2006) compared five planting environmentally responsible, profitable, and same aesthetic appeal. Visual appearance substrate compositions containing 60%, healthy places to live and work (US Green may not be a concern if the roof is not 70%, 80%, 90%, and 100% of heat-expanded Building Council, 2005, p.2). The LEED normally visible and was installed primarily slate to evaluate the establishment, growth, program was created to establish standards for its functional attributes such as storm- and survival of two stonecrops (Sedum spp.) and guidelines for designing and constructing water retention. Thin-layer Sedum roofs can and six nonsucculent natives to the midwest- buildings that reduce the negative impact that serve this function, possibly without the ern US prairie. Grown in 10 cm of substrate buildings have on the environment and occu- added costs of structural reinforcement to without any supplemental irrigation, the high- pants. Participation in the program can in- the building. The aesthetic value of the roof er levels of heat-expanded slate in the sub- crease the buildings valuation, decrease will continually change throughout the grow- strate generally resulted in slightly less growth vacancy and improve retention, reduce lia- ing season and over time. Plant competition and lower visual ratings across all species. By bility, and improve occupant performance. If and succession will occur as in any land- the end of 3 years, the majority of the non- at least 50% of the building is covered with scape. Similarly, identical plant palettes will succulents were dead, but both stonecrops a green roof, one LEED point each will be look and behave differently depending on the (Sedum spp.) achieved 100% coverage after awarded for reducing heat islands and for local environmental conditions. one season and maintained this coverage stormwater management (Oberlander et al., throughout the study. Results suggest that 2002). In fact, with quality green roof design, Environmental conditions moderately high levels of heat-expanded slate one can earn as many as 15 LEED credits in Regardless of the desired aesthetic effect, (up to 80%) can be incorporated into a green the categories of sustainable sites, water climate and microclimate have a major im- roof growing substrate when growing succu- efficiency, and energy and atmosphere (Kula, pact on plant selection. In particular, average lents such as stonecrop, without sacrificing 2005). high and low temperatures, extreme hot and plant health, and at the same time reducing the cold temperatures, irradiance levels, wind, load placed on a building. However, the non- and the amount and distribution of rainfall succulents used in this study require deeper Plant Performance on Green Roofs throughout the year will determine what substrates, additional organic matter, or sup- species can survive in a specific area. plemental irrigation. If the benefits of green roofs are to be High levels of organic matter or the Drought tolerance is important because high realized, then green roof plant performance is addition of fertilizer to the substrate can also levels of solar radiation and low media extremely important. Factors that must be result in a substrate that is too fertile. High moisture are usually the norm, especially in considered include aesthetic appeal, environ- fertility will encourage lush growth that is shallow extensive systems. Likewise, micro- mental conditions including macro- and mi- more subject to the inevitable drought stress climates on the roof must be considered: roof croclimate, substrate composition and depth, on roofs that are not irrigated. In a fertility slope and orientation may influence the in- plant selection, installation methods, and study, Rowe et al. (2006) found that plants of tensity of the sun and substrate moisture maintenance. smooth aster (Aster laevis L.), junegrass content, surrounding structures may shade Aesthetics. Design intent and available (Koeleria macrantha Regel), and showy a portion of the roof, air vents from heating installation and maintenance budgets are key goldenrod (Solidago speciosa L.) survived and air-conditioning units may dry the sub- factors in determining substrate depth and in greater numbers when they were not strate, and chemical exhaust from industrial plant selection, which in turn influence aes- fertilized. Presumably, these plants could buildings may influence plant growth. Envi- thetics. Expectations of aesthetics must be survive drought conditions for a longer pe- ronmental conditions, especially the amount addressed before selection of plant species, riod of time because they had less biomass to and distribution of rainfall and temperature because many species have dormant periods maintain. Similarly, a German study reported extremes, will eliminate the use of certain during which the green roof may not appear so that stonecrop sprouts did not survive when species or will dictate the need for irrigation. green. For example, many native prairie strongly fertilized (Jauch, 1993), although Although, aesthetic appeal is an important grasses and perennials will normally dry and the author did not indicate what is considered criteria on many roofs, the chosen plants must brown in the summer. Although, a natural strong. Furthermore, substrates with a low first be capable of surviving. occurrence, some may find this to be unaccept- to medium fertility level may encourage able. Visual appeal is generally more im- a more diverse plant community, reducing portant on intensive roofs designed for Substrate composition and depth the likelihood of dominant aggressive species public visitation compared with some shallow Substrate composition has a major impact (Dunnett and Kingsbury, 2004). As with extensive roofs with the main purpose of on plant selection for green roof systems. The compost, applying the minimal amount of 1280 HORTSCIENCE VOL. 41(5) AUGUST 2006

6 JOBNAME: horts 41#5 2006 PAGE: 6 OUTPUT: July 12 18:12:35 2006 tsp/horts/118440/01418 fertilizer to maintain plant health, potential choice among extensive green roofing proj- establishment and overwintering, and visual contaminated discharge of N, P, and other ects as a result of its tolerance for drought and appearance (Monterusso et al., 2005; Rowe et nutrients from green roofs is likely to be shallow substrate adaptability (Dunnett and al., 2005). All nine Sedum spp. tested were reduced. Kingsbury, 2004). Many Sedum spp. have found to be suitable for use on Midwestern Substrate depth also influences the plants been identified as exhibiting some form of green roofs. However, of the 18 native taxa that can be grown (GomezCampo, 1994; Crassulacean acid metabolism (CAM; Grav- studied, Allium cernuum L. (nodding wild GomezCampo and GomezTortosa, 1996). att, 2003; Gravatt and Martin, 1992; Kluge, onion), Coreopsis lanceolata L. (lanceleaf Deeper substrates are necessary for woody 1977; Lee and Kim, 1994; Sayed et al., 1994; coreopsis), Opuntia humifusa Raf. (prickly species, grasses, and many annual or peren- Teeri et al., 1986). CAM is a unique form of pear), and Tradescantia ohiensis L. (spider- nial flowering plants. Shallower substrate photosynthetic carbon fixation. It operates wort) were the only species that still existed depths will dry out faster (Dunnett and Nolan, such that stomata open during the night to after 3 y. If irrigation had been available, then 2004; VanWoert et al., 2005b), but some taxa uptake CO2 and store it in the form of an other native species would have likely sur- are naturally found growing under these organic acid (usually malate) in the cells vived. Even so, the species that did survive conditions. In Madrid, Spain, GomezCampo vacuoles. During the following daylight pe- could be incorporated into a standard Sedum (1994) found S. album growing spontane- riod, stomata remain closed while stored roof that is not irrigated to add diversity and ously on roofs, suggesting minimal needs for organic acid is decarboxylated back into aesthetic appeal. The upright form of both substrate depth. Research at Michigan State CO2 as the source for the normal photosyn- nodding wild onion and spiderwort would University has shown that substrate depth thetic carbon reduction cycle (Cushman, provide contrast to the horizontal spreading influences rate of substrate coverage and 2001). stonecrops. plant growth regardless of species Durhman Drought tolerance of Sedum species has Although the majority of the plants tested et al., (2004). Deeper substrates are beneficial been well tested. Lassalle (1998) found that were considered to be drought tolerant, most for both increased water-holding capacity S. album L. (white stonecrop) could survive native prairie species rely on deep tap roots to and as a buffer for overwintering survival more than 100 d without water. Others have obtain moisturea situation that cannot exist because shallow substrates are more subject confirmed that S. album is a drought-hardy on a shallow extensive roof. All 18 of the to fluctuations in temperature. A shallow species (Kirschstein, 1997), along with above mentioned nonsucculent native species depth will likely make root systems more S. acre L. (biting stonecrop), S. kamtschati- thrived when irrigated during the first season, susceptible to cold damage (Boivin et al., cum ellacombianum Fischer & Meyer (or- but suffered when irrigation was not provided 2001) because roots are generally not as cold ange stonecrop), S. pulchellum Michaux during the second and third season (Monter- tolerant as the tops of plants (Wu et al., 2000). (birds claw sedum), S. reflexum L. (crooked usso et al., 2005). An alternate strategy is to Despite the cultural limitations of shallow stonecrop), S. spurium Bieb. Coccineum consider plants native to the shortgrass prai- substrate depths, they are often desirable (creeping sedum), and S. spurium Bieb. ries found further west in locations such as because the building must be structurally Summer Glory, all of which survived 88 Colorado. These plant communities receive strong enough to support the added weight days without water (VanWoert et al., 2005b). less annual rainfall and many have shallower of the green roof. One species of Sedum, S. rubrotinctum R. T. root systems. An excellent example of an Clausen (stonecrop), was found to survive at extensive green roof native plant community Plant selection least 2 years without water in a greenhouse is the convention center of the Church of Criteria for selecting plant material in- (Teeri et al., 1986). Jesus Christ of Latter-day Saints in Salt Lake clude design intent; aesthetic appeal; local Not only do many Sedum species tolerate City, Utah (Fig. 4). However, to maintain the environmental conditions; plant characteris- drought conditions, they also have strong roofs high visual appeal, the roof requires tics such as rate of establishment, longevity, persistent qualities. In Germany, Liesecke irrigation and intense maintenance. Regard- and disease and pest resistance; and the (1999) tested different types of green roof less, many native species may be suitable and substrate composition and depth available constructions and found that S. album was a need for further testing is needed. for planting. A wide array of taxa are poten- a dominant persistence species for all types Because of the variability of green roof tial choices for intensive roofs because of of construction tested, followed closely by design and climate in the United States, it deeper substrate depths and the likelihood of S. sexangulare L. (tasteless stonecrop). would be impractical to list every possible available supplemental irrigation. In contrast, Monterusso et al. (2005) reported 100% plant candidate for extensive green roofs. drought tolerance is one of the most limiting survival of several Sedum spp. during the However, as a general rule, potentially suit- factors on extensive green roof systems given course of 3 years when grown on roof plat- able species can be found by looking at the their shallow substrate depths (

7 JOBNAME: horts 41#5 2006 PAGE: 7 OUTPUT: July 12 18:12:35 2006 tsp/horts/118440/01418 of water are all factors in determining the trients leaching into stormwater runoff. In opers, as well as the other ecological, eco- appropriate planting density of each species addition, fungicides, herbicides, and insecti- nomic, aesthetic, and psychological benefits to achieve optimal green roof coverage in the cides should be used sparingly or not at all to society as a whole. Although the benefits to desired time frame. Another option is spon- because of the potential for premature deg- the building owner and the community at taneous colonization, during which growing radation of roof membranes and runoff con- large (less waste to go to a landfill when the substrate is installed and one waits for plants cerns (ASTM, 2006). roof is replaced) are obvious, a roofing con- to colonize the roof. Although, this method is Also, maintaining vigorous, healthy green tractor that replaces roofs may not look at this less expensive, sustainable, and may ensure growth throughout a drought period will as a positive. that local species will result, it does not maintain a roofs aesthetic appeal and does Third, there is still a lack of quantifiable guarantee that these species are actually not necessarily influence the functional qual- data pertaining to the benefits that green roofs native to the area. Also, the visual appeal ities of the roof. VanWoert et al. (2005a) can provide to the building owner, its occu- may be questionable to some. compared stormwater retention on vegetated, pants, and the community. Data of this nature Regardless of installation method, the substrate only, and conventional roofs with exist for some areas (primarily Germany), but ideal time to plant is in the spring after the a gravel ballast. There were no significant most is not transferable to specific climatic last frost, or during the autumn before the first differences in stormwater retention between conditions found in North America and is not frost. Depending on location, installation the roofs that were vegetated with stonecrop written in English. Also, much of the current during the summer may require supplemental and those that were covered with substrate information is anecdotal in nature, proprie- irrigation (especially for cuttings that may only. This suggests that the main factor for tary, or the experiments were not performed dry out before they root) and possibly shade water retention is the physical properties of in a replicated study. In the United States, cloth (to protect seeds and cuttings from the media. academic green roof research programs have scorching). Initial irrigation will be required However, plant species can influence been established at Penn State, North Caro- immediately after planting, and the fre- stormwater retention of green roofs as a result lina State, and Michigan State universities to quency of watering during the first few weeks of differences in shading and rates of tran- address these problems. Researchers are ad- of establishment will depend on the amount spiration (Dunnett et al., 2005). VanWoert dressing questions of plant species, sub- of rainfall. Thereafter, vegetation can be et al. (2005b) showed that even though strates, water retention and water quality slowly weaned off supplemental watering. several Sedum spp. survived for 88 d without issues, energy conservation, and cost/benefit The need to provide long-term irrigation water, their evapotranspiration rates dropped analysis. depends on climate, plant selection, substrate to nearly zero by day 4. Such low rates of Fourth, there is a lack of technical in- composition and depth, and desired aesthetic evapotranspiration would likely diminish formation on how to build them. The Ger- quality. the potential of a green roof system to pro- mans have had written green roof standards When vegetation has been established, vide cooling to the building underneath. If for construction for more than a decade. On a roof inspection is recommended once or sufficient substrate moisture was available, the other hand, standards and building codes twice per year for optimal roof and plant either naturally or through supplemental pertaining to green roof design and construc- performance (ASTME, 2006; Dunnett and irrigation, then taxa exhibiting higher tran- tion do not exist in the United States. How do Kingsbury, 2004; FLL, 1995). This involves spiration rates may result in decreased water roofing contractors know if they are doing the determining the need for fertilization, weed- runoff, as well as greater evaporative cooling job correctly when there are no directions or ing of undesirable species, infilling bare spots potential. experience to rely on? How do they prepare (with cuttings, plugs, or seeds), replacing an accurate bid? An American Society for eroded substrate, pruning vegetation back The Future of Green Roofs in Testing of Materials (www.astm.org) inter- from building structures, and clearing plant North America national committee of professionals from debris away from roof drains. Keeping drains industry and academia are currently writing clear is of particular importance, because Will green roofs ever catch on in the green roof standards for the United States. a clogged drain could result in standing water United States as they have in Europe? Several Five standard documents pertaining to green over the vegetation, possibly leading to plant barriers to widespread acceptance exist; how- roofs have been published within the last 6 fungal diseases and dieback (Dunnett and ever, the same barriers have been overcome months, including a standard guide for selec- Kingsbury, 2004). in Germany. First, there is a lack of aware- tion, installation, and maintenance of plants Fertilizers and pesticides should be used ness regarding green roofs, but education can for green roofs; a standard practice for de- with caution, especially if stormwater runoff help alleviate this problem. Green Roofs for termination of dead loads and live loads quality is of particular concern. If needed, Healthy Cities (GRHC), a nonprofit organi- associated with green roof systems; and a slow-release NPK fertilizer for extensive zation based in Toronto, has increased public standard test methods for maximum media green roofs is recommended at the rate of awareness by offering classes in green roof density for dead load analysis, for water 5 gm2 of N (FLL, 1995). As mentioned implementation; encouraging interaction capture and media retention of geocomposite earlier, Rowe et al. (2006) reported that among industry professionals, public policy- drain layers, and for saturated water perme- three herbaceous perennials that received makers, and academic researchers; and orga- ability of granular drainage media. In the near no fertilizer produced the least amount of nizing an annual green roof conference. The future, additional standards will be available growth, but exhibited higher survival rates first GRHC conference in North America and the lack of technical information and than those that received fertilizer. In contrast took place in Chicago during 2003. Public experienced contractors will be a thing of the to containerized nursery stock in a production awareness of green roofs has increased dra- past. In addition, Penn State University is schedule, maximum growth is not always matically during the past few years. When- now offering a service for testing and certi- desirable on a green roof. Ideally, a green ever a green roof is constructed, it usually fying green roof substrates and roofing mem- roof fertilization schedule would use enough draws attention. During Ford Motor Com- branes. fertilizer to maintain acceptable plant health panys centennial celebration in 2003, their Finally, Germany has experienced a 10% and aesthetics while minimizing the amount 42,900 m2 green roof was the subject of much to 15% growth per year in the green roof of runoff contamination. Emilsson (2004) media coverage. industry during the past 10 y. Many cities in studied three fertilization levels (low and Second, green roofs may cost up to twice Germany have incentives of one kind or medium rates of controlled-release fertilizer as much to install. However, the lifetime of another to encourage the building of green [CRF] and CRF in combination with water- a conventional roof is about 20 years, roofs, because the widespread adoption of soluble applications) on two types of green whereas a green roof should last 40 years or this technology saves the cities from building roof vegetation in Sweden (old prevegetated longer. Plus, there are all the added benefits additional expensive detention mechanisms. mats and fertilization of newly established such as energy savings to the building owner, For example, the city of Esslingen in Ger- green roofs). All had the potential for nu- stormwater management benefits for devel- many will pay up to 50% of the cost of 1282 HORTSCIENCE VOL. 41(5) AUGUST 2006

8 JOBNAME: horts 41#5 2006 PAGE: 8 OUTPUT: July 12 18:12:36 2006 tsp/horts/118440/01418 installing a new green roof, and the city of 2005), they provide a unique opportunity. ington, DC. 46 May 2005. The Cardinal Darmstadt will pay up to 5000 Euros toward These typically unused spaces can become Group, Toronto. a new green roof. The city of Bonn reduces a way to reclaim habitat that was lost as Connelly, M., and K. Liu. 2005. Green roof re- the landowners monthly stormwater fees by a result of construction while also aiding in search in British Columbia: An overview, p. 416432. In Proc. of 3rd North American 0.75 Euro/m2, and the cities of Cologne and the protection of our environment through Green Roof Conference: Greening rooftops Mannheim will slice the stormwater fee in more sustainable practices. for sustainable communities, Washington, half (Herman, 2003). Green roofs can provide economic benefits DC. 46 May 2005. The Cardinal Group, Other municipalities throughout the globe to the green industry. Nurseries that are Toronto. are following suit with these incentives. In growing ground covers, perennials, or grasses Cushman, J.C. 2001. Crassulacean acid metabo- Tokyo there is an ordinance that new build- will have the most to gain. Although woody lism. A plastic photosynthetic adaptation to arid ings with more than 100 m2 of rooftop must plants can be used as green roof plants, they environments. Plant Physiol. 127:14391448. have a vegetated roof (Liu and Baskaran, require a much deeper substrate, additional Darius, F., and J. Drepper. 1984. Rasendacher in 2003). In Canada, Quebecs Energy Board maintenance, and the need for a more struc- West-Berlin. Das Gartenamt 33:309315. approved a $10.76/m2 incentive for green turally sound building. For landscape contrac- DeNardo, J.C., A.R. Jarrett, H.B. Manbeck, D.J. Beattie, and R.D. Berghage. 2005. Stormwater roof implementation in 2003, as long as the tors, the potential exists for installations and mitigation and surface temperature reduction roof meets certain design criteria (Mishra, maintenance contracts. In Germany, many by green roofs. Trans. ASAE 48:14911496. 2004). In Basel, Switzerland, homeowners landscape contractors may have construction, Department of Health and Human Services. 2005. can claim 20% of green roof investment costs maintenance, and green roof divisions, and About Extreme Heat. Centers for Disease Control for converting unused rooftops to vegetative others may specialize in green roofs alone. In and Prevention. 08 Nov. 2005. http://www. rooftops. This policy was so successful that in North America, the concept of green roofs is bt.cdc.gov/disasters/extremeheat/about.asp/. 18 months an area the size of seven football just now being introduced and will likely Deutsch, B., H. Whitlow, M. Sullivan, and A. fields was greened. Now, there is a new law in become more common in the future. They Savineau. 2005. Re-greening Washington, that city that all new flat roofs must be represent an entirely new market for nursery DC: A green roof vision based on environmen- greened (Brenneisen, 2004). stock and landscape contractors, and the po- tal benefits for air quality and storm water management, p. 379384. In Proc. of 3rd North In the United States, there are promising tential market consists of all existing and American Green Roof Conference: Greening strides toward encouraging green roofs as future roofs in the country. rooftops for sustainable communities, Wash- well. In 2003, the city of Atlanta imple- ington, DC. 46 May 2005. The Cardinal mented a credit system for the detention or Literature Cited Group, Toronto. retention of stormwater in which green roofs Dewey, D., P. Johnson, and R. Kjelgren. 2004. qualify for a discount on your monthly ASTM E 2400. 2006. Standard guide for selection, Species composition changes in a rooftop grass installation, and maintenance of plants for stormwater bills (Taube, 2003). The cities green roof systems. ASTM International, West and wildflower meadow. Native Plants. 5:5665. of Portland, Ore., and Chicago both provide Dunnett, N., and N. Kingsbury. 2004. Planting Conshohocken, Pa. incentives for the creation of green roofs and green roofs and living walls. Timber Press, Barnes, K., J. Morgan, and M. Roberge. 2001. in some cases the city of Chicago requires Inc., Portland, Ore. Impervious surfaces and the quality of natural Dunnett, N., A. Nagase, R. Booth, and P. Grime. green roofs (Liu and Baskaran, 2003). For built environments. Baltimore: Department of 2005. Vegetation composition and structure Chicago, these incentives appear to be work- Geography and Environmental Planning, Towson University. significantly influence green roof performance, ing. As of summer 2004, the city has seen p. 287296. In Proc. of 3rd North American more than 92,903 m2 in green roof commit- Beattie, D.J., and R. Berghage. 2004. Green roof media characteristics: The basic, p. 411416. In Green Roof Conference: Greening rooftops for ments or implementation (City of Chicago, sustainable communities, Washington, DC. 4 Proc. of 2nd North American Green Roof Department of Environment, 2004). Conference: Greening Rooftops for Sustain- 6 May 2005. The Cardinal Group, Toronto. Beyond tax incentives, some building able Communities, Portland, OR. 24 June Dunnett, N., and A. Nolan. 2004. The effect of owners are providing green roofs for other 2004. The Cardinal Group, Toronto. substrate depth and supplementary watering on reasons as well. In New York City, the Earth Boivin, M., M. Lamy, A. Gosselin, and B. Danser- the growth of nine herbaceous perennials in Pledge office constructed a rooftop garden eau. 2001. Effect of artificial substrate depth on a semi-extensive green roof. Acta Hort. that provides food for all its workers (Loder freezing injury of six herbaceous perennials 643:305309. grown in a green roof system. Horttechnology Durhman, A., N.D. VanWoert, D.B. Rowe, C.L. and Peck, 2004). In fact, some businesses are Rugh, and D. EbertMay. 2004. Evaluation of reclaiming the unused roof space to save 11:409412. Brenneisen, S. 2003. The benefits of biodiversity Crassulacean species on extensive green roofs, money on food costs. The Fairmont hotel in p. 504517. In Proc. of 2nd North American from green roofs: Key design consequences, p. Vancouver uses its roof to grow food for their 323329. In Proc. of 1st North American Green Green Roof Conference: Greening rooftops for restaurant, resulting in a savings of $30,000 Roof Conference: Greening rooftops for sus- sustainable communities, Portland, OR. 24 (CDN) per year (Peck, 2005). Other devel- tainable communities, Chicago. 2930 May June 2004. The Cardinal Group, Toronto. opers are using green roofs as a means to 2003. The Cardinal Group, Toronto. Dwight, R.H., D. Baker, J. Semenza, and B. Olson. overcome community resistance to infill de- Brenneisen, S. 2004. Green roofs: How nature 2004. Health effects associated with recrea- velopment, by giving back green space to the returns to the city. Acta Hort. 643:289293. tional coastal water use: urban versus rural community when an unpopular building pro- Cheney, C. 2005. New York City: Greening Goth- California. Amer. J. Public Health 94:565567. ams rooftops, p. 130133. In EarthPledge. Emilsson, T. 2004. Impact of fertilisation on ject is proposed (Loder and Peck, 2004). vegetation development and water quality, p. Green roofs: Ecological design and construc- tion. Schiffer Books, Atglen, Pa. 541548. In Proc. of 2nd North American Conclusions City of Chicago, Department of Environment. 2004. Green Roof Conference: Greening rooftops Green roofs open to the public. 1 Nov. 2005. for sustainable communities, Portland, OR. 2 Green roofs are one potential method to http://www.cityofchicago.org/environment/. 4 June 2004. The Cardinal Group, Toronto. counteract the destruction of natural habitats Clark, C., B. Talbot, J. Bulkley, and P. Adriaens. Ferguson, B.K. 1998. Introduction to stormwater: as we further our built environment. Today, 2005. Optimization of green roofs for air Concept, purpose, design. John Wiley and we have identified environmental benefits pollution mitigation, p. 482597. In Proc. of Sons, New York. that green roofs can provide such as storm- 3rd North American Green Roof Conference: FLL (Forschungsgesellschaft Landschaftsent- water retention, stormwater runoff delay and Greening rooftops for sustainable communi- wicklung Landschaftsbau). 1995. Guidelines for ties, Washington, DC. 46 May 2005. The the planning, execution and upkeep of green-roof rate reduction, increase roofing membrane Cardinal Group, Toronto. sites. Forschungsgesellschaft Landschaftsent- life, shading and insulation benefits, biodi- Coffman, R.R., and G. Davis. 2005. Insect and wicklung Landschaftsbau. Bonn, Germany. versity and habitat, aesthetics, and control of avian fauna presence on the Ford assembly Frith, M., and D. Gedge. 2005. London: The wild noise and air pollution. Because roofs repre- plant ecoroof, p. 457468. In Proc. of 3rd North roof renaissance, p. 117120. In EarthPledge. sent 21% to 26% of urban areas, both American Green Roof Conference: Greening Green roofs: Ecological design and construc- residential and nonresidential (Wong, rooftops for sustainable communities, Wash- tion. Schiffer Books, Atglen, Pa. HORTSCIENCE VOL. 41(5) AUGUST 2006 1283

9 JOBNAME: horts 41#5 2006 PAGE: 9 OUTPUT: July 12 18:12:36 2006 tsp/horts/118440/01418 Gaffin, S., C. Rosenzweig, L. Parshall, D. Beattie, rooftops for sustainable communities, Wash- Peck, S.W., C. Callaghan, M.E. Kuhn, and B. Bass. R. Berghage, G. OKeefe, and D. Braman. ington, DC. 46 May 2005. The Cardinal 1999. Greenbacks from green roofs: Forging 2005. Energy balance modeling applied to Group, Toronto. a new industry in Canada. Canada Mortgage a comparison of white and green roof cooling Liesecke, H.J. 1998. Das retentionsvermogen von and Housing Corporation. Ottawa, Canada. efficiency, p. 583597. In Proc. of 3rd North dachbegrunungen. (Water retention capacity of Peck, S.W. 2005. Toronto: A model for North American Green Roof Conference: Greening vegetated roofs). Stadt und Grun 47:4653. American infrastructure development, p. 127 rooftops for sustainable communities, Wash- Liesecke, H.J. 1999. Langzeitentwicklung einer 129. In EarthPledge. Green roofs: Ecological ington, DC. 46 May 2005. The Cardinal weiteren extensiven dachbegrunung [Long- design and construction. Schiffer Books, Group, Toronto. term development of another extensive roof Atglen, Pa. Gedge, D. 2003. From rubble to redstarts, p. 233 vegetation]. Stadt und Grun 48:769776. Peck, S., and M. Kuhn. 2001. Design guidelines for 241. In Proc. of 1st North American Green Liesecke, H.J. 2001. Zwiebel-und Knollenpflanzen green roofs. Canada Mortgage and Housing Roof Conference: Greening rooftops for sus- fur extensive dachbegrunungen. [Tuber plants Corporation, Ottawa, Ontario. 16 Nov. 2005. tainable communities, Chicago. 2930 May for extensive green roofs]. Stadt und Grun http://www.cmhc-schl.gc.ca/. 2003. The Cardinal Group, Toronto. 50(2):133139. Pope, C.A., D.V. Bates, and M.E. Raizenne. 1995. GomezCampo, C. 1994. Plantas para la natura- Liesecke, H.J., and H. Borgwardt. 1997. Abbau von Health effects of particulate air pollution: time cion de azoteas: El genero Sedum L. Agricul- luftschadstoffen durch extensive dachbegrunun- for reassessment? Environ. Health Perspect. tura (Espana) 749:10411042. gen (Degradation of air pollutants by extensive 103:472480. GomezCampo, C., and L. GomezTortosa. 1996. green roofs). Stadt und Grun. 46:245251. Rowe, B., M. Monterusso, and C. Rugh. 2005. Especies vegetales en las azoteas verdes. Agri- Liu, K. 2003. Engineering performance of rooftop Evaluation of Sedum species and Michigan cultura (Espana) 773:10291031. gardens through field evaluation. Proc. of the native taxa for green roof applications, p. Gravatt, D.A. 2003. Crassulacean acid metabolism 18th International Convention of the Roof 469481. In Proc. of 3rd North American and survival of asexual propagules of Sedum Consultants Institute. 93103. Green Roof Conference: Greening rooftops wrightii. Photosynthetica 41:449452. Liu, K., and B. Baskaran. 2003. Thermal perfor- for sustainable communities, Washington, Gravatt, D.A., and C.E. Martin. 1992. Comparative mance of green roofs through field evaluation, DC. 46 May 2005. The Cardinal Group, ecophysiology of five species of Sedum (Cras- p. 273282. In Proc. of 1st North American Toronto. sulaceae) under well-watered and drought- Green Roof Conference: Greening rooftops for Rowe, D.B., M.A. Monterusso, and C.L. Rugh. stressed conditions. Oecologia 92:532541. sustainable communities, Chicago. 2930 May 2006. Assessment of heat-expanded slate and Herman, R. 2003. Green roofs in Germany: Yes- 2003. The Cardinal Group, Toronto. fertility requirements in green roof substrates. terday, today, and tomorrow, p. 4145. In Proc. Loder, M.A., and S.W. Peck. 2004. Green roofs Horttechnology 16:471477. of 1st North American Green Roof Conference: contribution to smart growth implementation, Rowe, D.B., C.L. Rugh, N. VanWoert, M.A. Greening rooftops for sustainable communi- p. 824. In Proc. of 2nd North American Green Monterusso, and D.K. Russell. 2003. Green ties, Chicago. 2930 May 2003. The Cardinal Roof Conference: Greening rooftops for sus- roof slope, substrate depth, and vegetation Group, Toronto. tainable communities, Portland, OR. 24 June influence runoff, p. 354362. In Proc. of 1st Jauch, M. 1993. Extensive Dachbegruenung. Die 2004. The Cardinal Group, Toronto. North American Green Roof Conference: Fetten sind nicht fit genug. Deutscher-Garten- Mishra, A. 2004. Canadian Public Policy and Greening rooftops for sustainable communi- bau 47:3637. Green Roofs: Moving from policy to practice, ties, Chicago. 2930 May 2003. The Cardinal Kaplan, S., J.F. Talbot, and R. Kaplan. 1988. p. 4965. In Proc. of 2nd North American Group, Toronto. Coping with daily hassles: The impact of the Green Roof Conference: Greening rooftops SaizAlcazar, S., and B. Bass. 2005. Energy nearby natural environment. Project report. for sustainable communities, Portland, OR. 2 performance of green roofs in a multi storey USDA Forest Service, North Central Forest 4 June 2004. The Cardinal Group, Toronto. residential building in Madrid. Greening roof- Experiment Station, Urban Forestry Unit Co- Monterusso, M.A., D.B. Rowe, and C.L. Rugh. tops for sustainable communities, p. 569582. operative. Agreement 23-85-08. 2005. Establishment and persistence of Sedum In Proc. of 3rd North American Green Roof Kirschstein, C. 1997. Die durreresistenz einiger spp. and native taxa for green roof applications. Conference: Greening rooftops for sustainable Sedum-arten. Abgeleitet aus der Bedeutung der HortScience 40:391396. communities, Washington, DC. 46 May 2005. Wurzelsaugspannung-Teil 1. Stadt und Grun Moran, A., B. Hunt, and G. Jennings. 2004. A The Cardinal Group, Toronto. 46:252256. North Carolina field study to evaluate green Sayed, O.H., M.J. Earnshaw, and M. Cooper. 1994. Kluge, M. 1977. Is Sedum acre L. a CAM plant? roof runoff quantity, runoff quality, and plant Growth, water relations, and CAM induction in Oceologia 29:7783. growth, p. 446460. In Proc. of 2nd North Sedum album in response to water stress. Biol. Kohler, M., and M. Keeley. 2005. Berlin: Green American Green Roof Conference: Greening Plant. 36:383388. roof technology and development, p. 108112. rooftops for sustainable communities, Portland, Schade, C. 2000. Wasserruckhaltung und Ab- In EarthPledge. Green roofs: Ecological design OR. 24 June 2004. The Cardinal Group, flubeiwerte bei dunnschichtigen extensivbe- and construction. Schiffer Books, Atglen, Pa. Toronto. grunungen. Stadt und Grun 49:95100. Kolb, W. 2004. Good reasons for roof planting: Moran, A., B. Hunt, and J. Smith. 2005. Hydrologic ScholzBarth, K. 2001. Green roofs: Stormwater Green roofs and rainwater. Acta Hortic. and water quality performance from green management from the top down. Environmen- 643:295300. roofs in Goldsboro and Raleigh, North Caro- tal Design & Construction 4:6370. Kula, R. 2005. Green roofs and the LEED green lina, p. 512525. In Proc. of 3rd North Amer- Snodgrass, E. 2005. 100 Extensive green roofs: building rating system, p. 141153. In Proc. of ican Green Roof Conference: Greening Lessons learned, p. 209214. In Proc. of 3rd 3rd North American Green Roof Conference: rooftops for sustainable communities, Wash- North American Green Roof Conference: Greening rooftops for sustainable communi- ington, DC. 46 May 2005. The Cardinal Greening rooftops for sustainable communi- ties, Washington, DC. 46 May 2005. The Group, Toronto. ties, Washington, DC. 46 May 2005. The Cardinal Group, Toronto. Novotny, V., and G. Chesters. 1981. Handbook of Cardinal Group, Toronto. Laberge, K.M. 2003. Urban oasis: Chicagos City urban nonpoint pollution: Sources and man- Taube, B. 2003. City of Atlanta GreenRoof dem- Hall green roof, p. 194203. In Proc. of 1st agement. 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