A typical ugly commercial roof that provides no ecological, economic, aesthetic, or psychological benefits.	Flowering sedum and Allium schoenoprasum on a roof in Amsterdam.

As our forests and agricultural lands are replaced with impervious surfaces due to urban development, the necessity to recover green space is becoming increasingly critical for the health of our environment as well as our well-being. Vegetated or green roofs are one potential remedy for this problem. Establishing plant material on rooftops provide numerous ecological and economic benefits including stormwater management, energy conservation, mitigation of the urban heat island effect, increased longevity of roofing membranes, as well as providing a more aesthetically pleasing environment to work and live. The mitigation of stormwater runoff is considered by many to be the primary benefit because of the prevalence of impervious surfaces in urban areas. The rapid runoff from roof surfaces can exacerbate flooding, increase erosion, and may result in raw sewage that is discharged directly into our rivers. The larger amount of runoff also results in a greater quantity of water that must be treated before it is potable. A major benefit of green roofs is their ability to absorb stormwater and release it slowly over a period of several hours. Green roof systems have been shown to retain 60-100% of the stormwater they receive. In addition, green roofs have a longer life-span than standard roofs because they are protected from ultraviolet radiation and the extreme fluctuations in temperature that cause roof membranes to deteriorate. 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.

Green Roof Research at Michigan State University
 

An overview of the research platforms at MSU.

The green roof research program at MSU was initiated in collaboration with Ford Motor Company during 2000 in an effort to advise them on the installation of a 10.4 acre extensive green roof on a new assembly plant in Dearborn, Mich.

The objectives of our ongoing research are to evaluate plant species, propagation and establishment methods, plant succession, carbon sequestration potential, water and nutrient requirements, water quality and quantity of stormwater runoff, and energy consumption. Numerous experiments are currently being conducted on the roof of the Plant and Soil Sciences Building (PSSB), on the roof of the Communication Arts Building (CAB), in the Plant Science Greenhouses (PSG), and on 48 roof platforms at the Horticulture Teaching and Research Center (HTRC) at MSU. The roof of the PSSB is equipped with heat flux sensors and thermocouples that are measuring heat flow through the roof envelope and temperatures above the roof, at various depths in the growing substrate, and inside the building. On CAB, long-term plant survival, persistence, and succession are being studied at various substrate depths in the sun and shade. At the HTRC site, electronic tipping buckets record the volume and rate of stormwater runoff from individual platforms. All sites are equipped with a weather station and a datalogger that records measurements every five minutes, 24 hours a day.

Specific research objectives are to:

1. Conduct a performance evaluation of specific plant species for rate of establishment, nutrient requirements, environmental tolerances, plant competition, ability to exclude invasive weeds, and survival and persistence.
2. Evaluate mixed plant communities and succession over time.
3. Examine differences in evapotranspiration rates, substrate moisture levels, and plant performance among species exposed to several substrate depths and various levels of drought.
4. Utilize chlorophyll fluorescence measurements to quantify plant stress before it is evident from visual observations.
5. Determine effect of rooftop microclimate on winter damage and subsequent growth.
6. Determine the carbon sequestration potential of green roofs.
Quantify the differences in water retention among roof vegetation types.
8. Quantify the differences in water retention among combinations of green roof slopes and substrate depths.
9. Evaluate the influence of roof vegetation on roof membrane temperatures, heat flow into and out of the building, and energy consumption.
Public relations: provide visibility regarding green roofs.


Ongoing research studies

Long-term screening of plant species is being conducted on a series of 24 (4’ x 4’) and 24 (8’ x 8’) raised roof platforms at the Horticulture Teaching and Research Center. These test platforms were constructed by ChristenDETROIT Roofing Contractors, Detroit, Mich., and were funded by Ford Motor Company. Each platform is built per the same ASTM International standards that would be required for a commercial building and are equipped with layers of insulation, waterproofing, a green roof drainage system, root barrier, and substrate

Research platforms studying native species.	Urban agriculture with Sedum cucurbita ‘Watermelonii’.
Evaluation of potential green roof species.

Comparison of plant establishment and persistence of Sedum spp. and native taxa.  In 2001, nine roof  platforms containing three commercially available drainage systems were planted with eighteen Michigan native species planted as plugs and nine Sedum spp. planted as either seed or plugs. Each platform received 10 cm of growing substrate composed of 60% heat-expanded slate (PermaTill®; Carolina Stalite; Salisbury, NC) with a particle size ranging from 7.9 mm to 9.5 mm, 25% USGA grade sand, 5% aged compost, and 10% Michigan peat.

Within the platform partitions, three groups of plants were cultivated to evaluate the effect of drainage system on plant establishment, growth, and survival. One group consisted of seven Sedum spp. propagated from seed.  Taxa included S. acre, S. album, S. kamtschaticum, S. ellacombeanum, S. pulchellum, S. reflexum, and S. spurium ‘Coccineum’.  Seed was applied at a rate of 1.0 g×m^-2 and was mixed with 250 ml×m^-2 of dry sand to ensure even distribution.  All seed was obtained from Jelitto Staudensamen, GmbH (Schwarmstedt, Germany). A second group consisted of two Sedum spp. planted from plugs (116.3 cm³, 38/flat): S. middendorffianum ‘Diffusum’ and S. spurium ‘Royal Pink’.  These plugs were supplied by Hortech, Inc. (Spring Lake, MI) and the study contained 108 plugs of each taxa.  The third group consisted exclusively of 18 taxa of Michigan native plants: Agastache foeniculum (lavender hyssop), Allium cernuum (nodding wild onion), Aster laevis (smooth aster), Coreopsis lanceolata (lanceleaf coreopsis), Fragaria virginiana (wild strawberry), Juncus effusus (spikerush), Koeleria macrantha (junegrass), Liatris aspera (rough blazingstar), Monarda fistulosa (bergamot), Monarda punctata (horsemint), Opuntia humifusa (prickly pear), Petalostemum purpureum (purple prairie clover), Potentilla anserina (silver feather), Rudbeckia hirta (black-eyed Susan), Schizachyrium scoparium (little bluestem), Solidago rigida (stiff goldenrod), Sporobolus heterolepis (prairie dropseed), and Tradescantia ohiensis (spiderwort).  All native plants were planted from plugs (150.8 cm³, 38/flat) obtained from Wildtype Nursery Inc. (Mason, MI) except for Potentilla anserina, which was planted from stolons supplied by Hortech, Inc.  There were 27 plugs of each native taxa included in the study. All plugs and seed were planted or sown on the platforms 15 June 2001.  An automated overhead irrigation system (Rainbird; Azusa, CA) was utilized to support seed germination, plant establishment, and plant coverage.

Plants were evaluated over three years (2001-2004) for growth, survival during establishment and overwintering, and visual appearance. During the first season, growth of most native plants peaked in September, and then declined with the onset of dormancy. Optimum growth and appearance during the entire first season was possible because irrigation was provided during the plant establishment phase.  Supplemental irrigation was much reduced during the second season and was terminated completely by 10 July of the second season.  Therefore, after this date plants had to rely on natural rainfall, the likely scenario on most extensive green roofs. The termination of supplemental irrigation was fatal to many of the native taxa. At the end of the 2002 growing season, there were no surviving plants of A. foeniculum, A. laevis, F. virginiana, L. spicata, M. fistulosa, M. punctata, P. purpureum, P. anserina, R. hirta, S, scoparium, and S. rigida.  Likewise, there were high mortality rates for C. lanceolata, J. effusus, K. macrantha, and S. heterolepis.  Allium cernuum, O. humifusa, T. ohiensis, but all of the Sedum proved to be drought tolerant.

Ideal plant selections for extensive green roofs in northern climates such as Michigan that lack irrigation must be heat and cold tolerant, drought resistant, have a high growth index in order to provide quick coverage, and must be self-generating by seed, root systems, or some other means. When the study was terminated in 2004, of the species tested, all nine species of Sedum along with A. cernuum, C. lanceolata, and T. ohiensis were the most suitable for unirrigated roofs. Although, Opuntia humifusa survived, it lacked the ability to provide quick surface coverage. If irrigation is available, then other native species are potential selections. No significant differences were found among the commercial drainage systems.

Complete results are published in Monterusso et al. (2005) HortScience 40(2):391-396.

A camera stand was built for taking digital photographs from above the platforms.  Images are analyzed with SigmaScan Pro software to provide a quantitative assessment of initial plant growth and coverage.	A point frame transect is used to measure plant competition over time.

Evaluation of Crassulacean species on roof platforms.  Two plant establishment, competition, and survival studies are currently be conducted on 24 (4’ x 4’) platforms. In first study was initiated in 2003 when cuttings of 25 succulent plant species were propagated on platforms with substrate depths of 2.5 cm, 5.0 cm, and 7.5 cm (0.98 in, 1.97 in, and 2.96 in) (Table 1). Species included Graptopetalum paraguayense, Phedimus spurius ‘Leningrad White’, Rhodiola pachyclada, Rhodiola trollii, Sedum acre, Sedum album ‘Bella d’Inverno’, Sedum clavatum, Sedum confusum, Sedum dasyphyllum ‘Burnati’, Sedum dasyphyllum ‘Lilac Mound’, Sedum diffusum, Sedum hispanicum, Sedum kamtschaticum, Sedum mexicanum, Sedum middendorffianum, Sedum moranense, Sedum pachyphyllum, Sedum reflexum, Sedum sediforme, Sedum ‘Rockery Challenger’, Sedum ‘Spiral Staircase’, Sedum spurium ‘Summer Glory’, Sedum surculosum, Sedum x luteoviride, and Sedum x rubrontinctum. Succulents such as sedum root easily from leaf and stem cuttings and are often propagated by dropping the cutting material on top of the substrate. Cuttings were propagated on eight-inch centers with 25 plants per platform. The study is a split-plot on a completely random design with substrate depth as the main plot factor and species as the sub-plot factor. Each species is replicated eight times within each substrate depth for a total of 600 plants.

Table 1.  Representative growth (coverage) at three substrate depths (2.5, 5.0, and 7.5 cm) over three years.
Substrate Depth	15 July 2003	1 July 2005	7 July 2006
2.5 cm
5.0 cm
7.5 cm

A second study was initiated in May 2005 with an additional 12 species that were planted as plugs on 8’ x 8’ platforms. Species in this study include Sedum ‘Angelina’, Sedum cauticola ‘Lidakense’, Sedum ewersii, Sedum floriferum, Sedum hispanicum, Sedum ochroleucum, Sedum reflexum, Sedum sarmentosum, Sedum sediforme, Sedum sexangulare, Sedum spurium ‘John Creech’, and Sedum stefco. In this study, each species is replicated 12 times within each of three substrate depths for a total of 432 individual plants.

In both experiments, species are being evaluated at the three substrate depths for such characteristics as propagation success, rate of establishment, growth and survival, groundcover density, ability to exclude invasive weeds, competition among species, persistence over several years, tolerance to low winter temperatures, and drought resistance. Rate of establishment and initial plant growth are being determined by utilizing SigmaScan Pro 5.0 image analysis software (SPSS Science, Chicago, Ill.). A camera stand was constructed to suspend a digital camera equipped with a 0.8X wide conversion lens approximately 163 cm (5’4”) above the platforms. These pictures are taken weekly until the plants enter dormancy in late fall and then picture taking is resumed the following Spring. The SigmaScan program analyzes these digital photographs to determine the percentage of plant canopy that can be attributed to each individual species.

Point frame placed over platform.	At each of the 100 intersection points, leaf area index is measured by recording all species that come in contact with a vertical needle at three canopy levels.

After initial growth rates are established and plants begin to fill in the platforms, a point-frame is utilized to measure community composition and change over time. The point-frame is a stainless steal frame with ten strings spread horizontally and vertically to create a fish line grid.  At each of the 100 intersection points, leaf area index is measured by recording all species that come in contact with a vertical needle.  Every species that intersects is recorded based on their origin in the canopy layer. It is essential to determine the tendency of neighboring plantings to displace and possibly eradicate adjacent species. Susceptibility to plant competition will be important to predict the long-term stability of the planted communities as well as preservation of the aesthetic goals of the roof design. Equally critical is determination of the resilience and seasonal fluctuation of each of the selected species over the growth season and after winter dormancy. Therefore, the platform installations in these studies must be evaluated over the course of several seasons for overwintering plant survival, recovery, and plant community development.

Initial results are published in Durhman et al. (2007) HortScience 42(3):588-595. 

Effect of planting season on establishment. The objective of was to quantify the effect of substrate depth and planting season on successful establishment of plugs of Sedum species on green roofs.  Plugs of nine species of Sedum were planted at depths of 4.0, 7.0, and 10.0 cm on green roof platforms in autumn (September 20, 2004) and spring (June 8, 2005) and then evaluated for survival on June 1, 2005 and June 1, 2006 respectively.  Overall, spring planting exhibited superior survival rates (81%) compared to autumn (23%) across substrate depths.  Sedum cauticola ‘Lidakense’, S. floriferum, and S. sexangulare were not affected by season of planting.  Sedum cauticola barely survived at any substrate depth or planting season, whereas the latter two exhibited nearly 100% survival regardless of planting season.  All other species had superior survival percentages when planted during spring. 

Complete results are published in Getter and Rowe (2007)  J. Environ. Hort 25(2):95-99.

Greenhouse evaluation of succulents.	Sedum kamtshaticum can live for at least 88 days without water.

Greenhouse studies on drought tolerance. The first greenhouse study consisted of seven species combinations subjected to five different watering regimes. Species included individual trays of a native perennial (Coreopsis lanceolata); a native grass (Schizachyrium scoparium); Sedum album; Sedum reflexum; Sedum kamtschaticum; a mixture of seven sedum species; and a control with no plants. Coreopsis lanceolata and S. scoparium were chosen to include a native forb and grass. These species were two of the most drought tolerant plants of the 18 Michigan native species that we have tested on roof platforms over the past two years. Sedum album, S. reflexum, and S. kamtschaticum were chosen because they possess small, medium, and large sized leaves, respectively, traits that may influence evapotranspiration. Irrigation regimes consisted of watering every other day, once a week, once every two weeks, once every four weeks, and never. Each treatment was replicated eight times for a total of 280 trays arranged in a completely random design.

The second greenhouse study consisted of two species combinations (a mixture of seven sedum species and a control as described above), three substrate depths, and five watering regimes (same as above). The three substrate depths included 2.5 cm of media with 1 cm of water retention fabric, 2.5 cm of media with 2 cm of fabric, and 5 cm of media with 1 cm of fabric. Each treatment was replicated eight times for a total of 240 trays arranged in a completely random design. All trays in both studies included the XeroFlor green roof drainage system and substrate carrier (Behrens Systementwicklung, Groß Ippener, Germany). The growing substrate was composed of expanded slate, USGA specification sand; peat, and aged compost.

During both greenhouse studies, evapotranspiration was measured by obtaining tray weights. Initially tray weights were obtained daily and thereafter on a periodic basis. Substrate moisture content was recorded with a Theta Probe Soil Moisture Sensor ML2X (Delta-T Devices, Ltd., Cambridge, U.K.). In addition, plant stress was monitored with chlorophyll fluorescence (F_v/F_m) measurements taken on randomly selected leaves with a Plant Efficiency Analyzer (PEA) fluorometer (Hansatech Instruments Ltd., Norfolk, England, UK). Since chlorophyll fluorescence levels are tied to the maximum dark-adapted photochemical efficiency of photosystem II (PSII), they can serve as a general measurement of plant photosynthetic potential. At the conclusion of each experiment, dry mass accumulation was calculated from shoot and root dry weights.

Results of the first study indicate that even after the four-month period, Sedum spp. survived and maintained active photosynthetic metabolism, relative to Schizachyrium and Coreopsis.  Furthermore, when Sedum was watered after 28 days of drought, chlorophyll fluorescence (F_v/F_m) values recovered to values characteristic of the 2 days between watering (DBW) treatment.   In contrast, the non-CAM plants required watering frequency every other day in order to survive and maintain active growth and development.  Regardless of species, the greatest increase in total biomass accumulation and fastest growth occurred under the 2 DBW regimes. 

In the second study, substrate volumetric moisture content could be reduced to 0 m³ • m^-3 within one day after watering depending on substrate depth and composition.  Deeper substrates provided additional growth with sufficient water, but also required additional irrigation because of the higher evapotranspiration rates resulting from the greater biomass.  Over the 88 day study, water was required at least once every 14 days to support growth in green roof substrates with a 2 cm media depth.  However, substrates with a 6 cm media depth could do so with a watering only once every 28 days.  Although vegetation was still viable after 88 days of drought, water should be applied at least once every 28 days for typical green roof substrates and more frequently for shallower substrates to sustain growth. The ability of Sedum to withstand extended drought conditions makes it ideal for shallow green roof systems.

Complete results are published in VanWoert et al. (2005) HortScience 40(3):659-664 and Durhman et al. (2006) HortScience 41(7):1623-1628.

Substrate composition and fertility.   In the green roof industry, there is a need to define growing substrates that are lightweight, permanent, and can sustain plant health without leaching nutrients that may harm the environment. High levels of substrate organic matter is not recommended because it will decompose resulting in substrate shrinkage and can leach nutrients such as nitrogen (N) and phosphorus (P) in the runoff.  The same runoff problems can occur when fertilizer is applied. Also, in the midwestern U.S., there is a great deal of interest in utilizing native species and recreating natural prairies on rooftops.  Since most of these native species are not succulents, it is not known if they can survive on shallow extensive green roofs without irrigation. 

Five planting substrate compositions containing 60, 70, 80, 90, and 100% of heat-expanded slate (PermaTill; Carolina Stalite; Salisbury, N.C.) were used to evaluate the establishment, growth, and survival of two stonecrops (Sedum spp.) and six non-succulent natives to the midwestern U.S. prairie over a period of 3 years.  A second study evaluated these same plant types that were supplied with four levels of controlled-release fertilizer. Both studies were conducted in interlocking modular units (36 x 36 inches) designed for green roof applications containing 10 cm of substrate. 

Higher levels of heat-expanded slate in the substrate generally resulted in slightly less growth and lower visual ratings across all species. By May 2004, all plants of smooth aster (Aster laevis), horsemint (Monarda punctata), black-eyed susan (Rudbeckia hirta), and showy goldenrod (Solidago speciosa) were dead. To a lesser degree, half of the lanceleaf coreopsis (Coreopsis lanceolata) survived in 60 and 70% heat-expanded slate, but only a third of the plants survived in 80, 90, or 100%.  Regardless of substrate composition, both ‘Diffusum’ stonecrop (Sedum middendorffianum ‘Diffusum’) and ‘Royal Pink’ stonecrop (Sedum spurium ‘Royal Pink’) achieved 100% coverage by June 2002 and maintained this coverage into 2004. In the fertility study, plants that received low fertilizer rates generally produced the least amount of growth. However, water availability was a key factor. A greater number of smooth aster, junegrass (Koeleria macrantha), and showy goldenrod survived when they were not fertilized.  Presumably, these plants could survive drought conditions for a longer period of time since they had less biomass to maintain.  However, by the end of three growing seasons, all three non-succulent natives also were dead.

Overall results suggest that a moderately high level of heat-expanded slate (approximately 80%) and a relatively low level of controlled-release fertilizer (50 g×m^-2 per year) and can be utilized for green roof applications when growing succulents such as stonecrop.  However, the non-succulents used in this study require deeper substrates, additional organic matter, or supplemental irrigation.  By reducing the amount of organic matter in the substrate and by applying the minimal amount of fertilizer to maintain plant health, potential contaminated discharge of N, P, and other nutrients from green roofs is likely to be reduced considerably while still maintaining plant health.

Complete results are published in Rowe et al. (2006) HortTechnology  16(3):471-477. 

Carbon sequestration potential. According to the Intergovernmental Panel on Climate Change, carbon dioxide (CO2) concentrations in the atmosphere have increased 32% since 1750.  Moreover, the IPCC states that human activity is the cause of such increases, primarily through the burning of fossil fuels which release CO2.  Because CO2 is one of the atmospheric gases that reduce the amount of outgoing terrestrial energy from escaping into space, increased levels of this gas will ultimately lead to an increase in the earth’s temperatures.  Many scientists are seeking methods to remove atmospheric carbon and store it elsewhere in a stable state, thereby offsetting the human addition of carbon to the atmosphere.  Using plants to do this is referred to as terrestrial carbon sequestration.  Through the process of photosynthesis, carbon dioxide is removed from the atmosphere and stored as carbon in biomass.  If net primary production of these ecosystems exceeds decomposition, then these systems become at the very least a short term sink for carbon. 

The objective of this study is to quantify the carbon storage potential of extensive green roofs and to evaluate the effect that species has on carbon flux.  To accomplish this research objective, a study was initiated on the roof of the Plant and Soil Sciences Building in April 2007.  The study area measures 2.84 m x 4.6 m with 20 plots, each measuring 0.71 m x 0.92 m.  Each plot is covered with a single species of Sedum (S. acre, S. album, S. kamtshaticum var ellacombianum, or S. spurium ‘Summer Glory’) germinated from seed or a ‘substrate only’ treatment.  Each row is blocked, with four replicates and five treatments at a depth of 6.0 cm (2.5 in).  Carbon analysis will be performed by sampling aboveground biomass, below ground biomass (roots), and soil carbon content.  Sampling will occur periodically over the course of several years.

View of tipping buckets used to measure the volume and rate of runoff from roof platforms.	Measuring differences in runoff at various substrate depths and roof slopes.


Stormwater management on roof platforms. Two studies were recently completed that quantified the differences in water retention among (1) roof vegetation types and (2) among combinations of green roof slopes and substrate depths. Fifteen simulated roof platforms (ChristenDetroit, Detroit, Mich.) with overall dimensions of 2.44 m x 2.44 m (8 ft x 8 ft) were  utilized at the Michigan State University Horticulture Teaching and Research Center (East Lansing, Mich.). Each platform simulated an actual commercial extensive green roof and included an insulation layer, protective layers, and waterproofing membrane. Three of the platforms were divided into three sections measuring 0.67 m x 2.44 m (2.2 ft x 8 ft) and were  used for the vegetation study. The other twelve platforms were utilized for the slope/depth study. Aluminum sheet metal troughs were attached on the low end of the platforms to capture stormwater exiting the platform. For the vegetation type study the troughs are divided into three separate sections corresponding to the three divided sections. For the vegetation type study, platforms were set at a 2% slope with the top edge of the high end 0.9 m (3.0 ft) above ground level. Platforms used in the slope/depth study were placed at 2% or 6.5% slope. All platforms were oriented with the low end of the slope facing south to maximize sun exposure.

All platforms except for one of the three self-contained sections on each of the platforms utilized for the vegetation type study contained XeroFlor drainage layers, water retention fabric, and substrate carriers. The growing substrate consisted of heat-expanded slate (PermaTill^®, Carolina Stalite Company, Salisbury, N.C.), USGA (United States Golf Association) grade sand, Michigan Peat, 5% Dolomite, composted yard waste, and composted turkey litter. For the vegetation type study, each platform section with the  XeroFlor drainage system installed received planting substrate to a depth of 2.5 cm (1.0 in). The twelve platforms utilized in the slope/depth study received 2.5, 4.0, or 6.0 cm (1.0, 1.6, or 2.4 in) of substrate on top of the substrate carrier. The four treatments were a 2% slope with either 2.5 or 4.0 cm of substrate and the 6.5% slope with 4.0 or 6.0 cm of substrate.

Three roof types were tested in the roof vegetation type study: an extensive green roof with vegetation, an extensive green roof with substrate only, and a conventional commercial roof with a 2 cm (0.8 in) deep gravel ballast. Treatments were arranged in a randomized complete block design with each platform corresponding to one block. Vegetation, substrate only, or gravel ballast were randomly assigned within sections of each platform. For the slope/depth study, all platforms were covered with vegetation. For both studies, 100% coverage was achieved on the vegetated section prior to the initiation of data collection. Taxa included several species of Sedum planted from seed (Jelitto Staudensamen, GmbH, Schwarmstedt, Germany) at a rate of 1.3 g•m^-2 (0.004 oz•ft^-2) for each species.

Model TE525WS tipping bucket rain gauges (Campbell Scientific, Inc., Logan, Utah) were mounted under the drain of each platform section to quantify stormwater runoff. An additional tipping bucket was mounted in each gravel section to record actual rainfall on the vegetation type platforms. Also, a Campbell Scientific’s CM6 tripod weather station was installed on the research site to aid in determining meteorological variables. The weather station included a temperature and relative humidity probe covered by a 6-plate gill radiation shield. In addition, instruments to measure wind speed and direction were installed as well as a pyranometer. Data from the tipping bucket rain gauges and tripod weather station were collected every five minutes, 24 hours a day using a Campbell Scientific CR10X datalogger with peripheral multiplexers, switch closure modules, and storage module. A rainfall event was defined by two rainfalls separated by a period of six hours or more, with an easily distinguishable flow hydrograph.

Over the 14-month period the study was conducted, the vegetated roof treatments retained 60.6% of  rainfall from 83 measured rain compared to 50.4% and 27.2% for the media-only and conventional gravel ballast roofs, respectively.  In the slope/depth study, platforms at 2% slope with a 4 cm media depth had the greatest mean retention, although the difference from the other treatments was minimal.   The combination of reduced slope and deeper media clearly reduced the total quantity of runoff.  For both studies, vegetated green roof systems not only reduced the amount of stormwater runoff, they also extended its duration over a period of time beyond the actual rain event. 

Complete results are published in VanWoert et al. (2005) Journal of Environmental Quality  34(3):1036-1044.

A third stormwater runoff study was initiated in May 2005 to quantify the effect that roof slope has on green roof stormwater retention.  Runoff was analyzed from twelve extensive green roof platforms constructed at four slopes (2%, 7%, 15%, and 25%).  Rain events were categorized as light (<2.0 mm) (0.08 in), medium (2.0 – 10.0 mm) (0.08 in – 0.39 in), or heavy (>10.0 mm) (>0.39 in).  Data demonstrated an average retention value of 80.8%.  Mean retention was least at the 25% slope (76.4%) and greatest at the 2% slope (85.6%).  In addition, runoff that did occur was delayed and distributed over a long period of time for all slopes.  Curve numbers, a common method used by engineers to estimate stormwater runoff for an area, ranged from 84 to 90, and are all lower than a conventional roof curve number of 98, indicating that these greened slopes reduced runoff compared to traditional roofs.

Roof over the headhouse of the Plant and Soil Sciences Building prior to installation of green roof in May 2004.	The roof is instrumented with heat flux sensors, thermocouples, and soil moisture probes at various locations inside the building and in the roof profile.
Green roof on Plant and Soil Sciences Building (May 2006).	Green roof flowering on Plant and Soil Sciences Building (July 2006).

Monitoring of Green Roof on Plant & Soil Science Building.  An extensive green roof was installed in May 2004 on a portion of the roof (3500 square feet) over the headhouse of the Plant & Soil Sciences Building. The green portion covers the natural drain field around the center drain, whereas, the drain field surrounding the southern drain remains as a conventional roof. Thus, we have two experimental roof sections where we can obtain quantifiable data comparing heat flux, roof membrane temperatures, and stormwater issues. In addition, new plant species will be tested, the site will be used to raise public awareness, and the project will help promote MSU’s efforts as a sustainable campus.

On the green portion of the roof, thermocouples are located inside of the building; on top of the roof membrane, insulation layer, and water retention fabric; under the plant canopy on the growing substrate surface, and at a height of one meter above the foliage. Heat flow sensors are placed above the insulation layer and soil moisture probes have been installed in the moisture retention fabric and in the growing substrate. On the conventional portion of the roof, thermocouples are located inside the building, on top of the roof membrane, above the insulation layer, on top of the gravel ballast, and one meter above the roof surface. Heat flow and moisture sensors are also present on the conventional side. Sensors are replicated three times at separate locations in the ballasted and vegetated roof sections for a total of 36 thermocouples, 6 heat flow sensors, and 9 moisture probes. Ambient temperature, irradiance levels, wind velocity, relative humidity, and precipitation are continuously recorded by a weather station located on the roof. Data is recorded every five minutes, 24 hours a day using a Campbell Scientific CR10X datalogger with peripheral multiplexers, switch closure modules, and storage module.

Research plots on the Communication Arts Building are comparing plant responses at various substrate depths in full sun and shade.

Plant evaluations on the Communications Arts Building.  Small green roof research plots (400 square feet) were installed on the Communication Arts Building in May 2005. One factor being studied is the effect of irradiation levels on plant community development. Because of the design of the building, we can compare a plot that is always in full sun to another that is continuously in the shade. One experiment compares irradiance levels on a typical green roof sedum mix of Sedum acre, Sedum album, Sedum kamschaticum, Sedum reflexum, and Sedum spurium in 10 cm (4 in) of media. As second study consists of North American native species Allium cernuum, Carex flacca, Sedum stenopetalum, Sedum divergens, Sedum urvillei, Talinum calycinum, and Talinum parviflorum, as well as Sedum ‘Coral Carpet’ and Sedum acre ‘Oktoberfest’ growing in the sun or shade in 8 or 12 cm (3 or 5 in) of media.

The plots are instrumented with soil moisture sensors, thermocouples, pyranometers to measure irradiance levels, and a weather station to measure wind speed, ambient air temperatures, and rainfall. A data logger powered  by solar energy records data continuously.

The Ford Motor Company assembly plant in Dearborn, Michigan.  The green roof covers 10.4 acres.	An extensive green roof native plant community on the convention center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City, Utah.

The Future of Green Roofs in the United States

In Germany, it is estimated 12% of all flat-roofed buildings are covered with vegetation, a number that is increasing as the German green roof industry continues to grow 10 to 15% per year. In Michigan and the rest of North America the concept of green roofs is just now being introduced. A few installations exist on this continent, the largest extensive roof (10.4 acres) being the new assembly plant at the Rouge facility of Ford Motor Co. in Dearborn, Mich.

Will green roofs ever catch on in the United States like they have in Europe? Several barriers to widespread acceptance exist such as a lack of awareness regarding green roofs, potentially higher installation costs, limited quantifiable data pertaining to the benefits they provide, no technical information on how to build them, and a lack of government incentives or tax breaks. However, all of these problems are currently being addressed. These barriers are not insurmountable, as the same barriers have been overcome in Germany. In the U.S. the concept of green roofs is just now being introduced and will likely become more common in the future. They represent an entirely new market for landscape designers/architects, nursery operations, and landscape contractors; and the potential market includes all existing and future roofs in the country.

Former Graduate Students in MSU Green Roof Research Program

Michael Monterusso (MS, Spring 2003)
Horticulture Manager, Daniel Stowe Botanical Garden, Belmont, NC

Nicholaus VanWoert (MS 2004) 
Resource Analyst, Michigan Department of Transportation, Kalamazoo, MI

Angela Durhman (MS 2005)
Green Roof Specialist, Tecta America Corp and Magco, Inc. Jessup, MD.

Kristin Getter (MS 2006)
Currently PhD student at MSU
 


Publications From MSU Research Program
To obtain pdf files of the peer-reviewed scientific articles, please send requests to Brad Rowe at rowed@msu.edu.

Articles in peer reviewed scientific journals 

Getter, K.L., D.B.Rowe, G.P. Robertson, B.M. Cregg, and J.A.Andresen.  2009.  Carbon sequestration potential of extensive green roofs.  Environmental Science and Technology  43(19):7564-7570.
 
Getter, K.L., D.B.Rowe, and B.M. Cregg.  2009.  Solar radiation intensity influences extensive green roof plant communities.  Urban Forestry and Urban Greening  8(4):269-281.

Getter, K.L. and D.B.Rowe.  2009. Substrate depth influences sedum plant community on a green roof.  HortScience 44(2):401-407.

Getter, K.L. and D.B. Rowe.  2008.  Media depth influences Sedum green roof establishment.  Urban Ecosystems 11:361-372.

Getter, K.L., D.B. Rowe, and J.A. Andresen.  2007.  Quantifying the effect of slope on extensive green roof stormwater retention.  Ecological Engineering 31:225-231.

Durhman, A.K., D.B. Rowe, and C.L. Rugh.  2007.  Effect of substrate depth on initial growth, coverage, and survival of 25 succulent green roof plant taxa.  HortScience 42(3):588-595.  

Getter, K.L. and D.B. Rowe.  2007.  Effect of substrate depth and planting season on Sedum plug establishment for green roofs.  J. Environ. Hort 25(2):95-99. 

Oberndorfer, E., J. Lundholm, B. Bass, M. Connelly, R. Coffman, H. Doshi, N. Dunnett, S. Gaffin, M. Köhler, K. Lui, and B. Rowe.  2007.  Green roofs as urban ecosystems:  ecological structures, functions, and services.  BioScience 57(10):823-833.

Durhman, A.K., D.B. Rowe, and C.L. Rugh.  2006.  Effect of watering regimen on chlorophyll fluorescence and growth of selected green roof plant taxa.  HortScience 41(7):1623-1628.    

Getter, K.L. and D.B. Rowe.  2006.  The role of green roofs in sustainable development.  Hort Science  41(5):1276-1285. 

Rowe, D.B., M.A. Monterusso, and C.L. Rugh.  2006. Assessment of heat-expanded slate and fertility requirements in green roof substrates.  HortTechnology  16(3):471-477. 

Monterusso, M.A., D.B. Rowe, and C.L. Rugh. 2005. Establishment and persistence of Sedum spp. and native taxa for green roof applications.  HortScience 40(2):391-396. 

VanWoert, N.D, D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao.  2005. Green roof stormwater retention: Effects of roof surface, slope, and media depth.  J. Environ. Quality  34(3):1036-1044. 

VanWoert, N.D, D.B. Rowe, J.A. Andresen, C.L. Rugh, and L. Xiao. 2005. Watering regime and green roof substrate design affect Sedum plant growth.  HortScience 40(3):659-664. 

Extension Bulletins

Getter, K.L. and D.B. Rowe.  2008.  Selecting plants for extensive green roofs in the U.S.  Extension Bulletin E-3047, Michigan State University.

Papers from proceedings of professional meetings 

Getter, K.L. and D.B.Rowe.  2009.  Carbon sequestration potential of extensive green roofs.  Proc. of 7th North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Atlanta, GA. 3-5 June 2009. The Cardinal Group, Toronto.
 
Getter, K.L. and D.B. Rowe.  2008.  Carbon sequestration potential of extensive green roofs.  Proc. of World Green Roof Conference, London, U.K.  17-18 Sept, 2008.
 
Getter, K.L. and D.B. Rowe.  2008. Effect of solar radiation levels on native and non-native species.  Proc. of 6th North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Baltimore, MD.  30 April -2 May, 2008.  The Cardinal Group, Toronto.

Getter, K.L. and D.B. Rowe.  2007. Effect of substrate depth and planting season on Sedum plug establishment for extensive green roofs.  Proc. of 5th North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Minneapolis, MN.  29 April -2 May, 2007.  The Cardinal Group, Toronto

Rowe, D.B., C.L. Rugh, and A.K. Durhman.  2006. Assessment of of substrate depth and composition on green roof plant performance. Proceedings of 4th North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Boston, MA.  10-12 May, 2006.  The Cardinal Group. 

Rowe, D.B., M.A. Monterusso, and C.L. Rugh.  2005. Evaluation of Sedum spp. and Michigan native taxa for green roof applications. p. 469-481. In Proceedings of 3rd North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Washington, DC. 4-6 May 2005.  The Cardinal Group, Toronto. 

Durhman, A., N.D. VanWoert, D.B. Rowe, C.L. Rugh, and D. Ebert-May. 2004. Evaluation of Crassulacean species on extensive green roofs. p. 504-517. In Proc. of 2^nd North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Portland, OR. 2-4 June 2004. The Cardinal Group, Toronto. 

Monterusso, M.A., D.B. Rowe, C.L. Rugh, and D.K. Russell. 2004. Runoff water quantity and quality from green roof systems. Acta Hort 639:369-376. 

Rowe, D.B. and C.L. Rugh. 2003. The green roof research program at Michigan State University. Proceedings International Plant Propagators Society  53:104-106. 

Rowe, D.B.  2003.  Green roofs – A new market. Proceedings Southern Nursery Assoc. Research Conf. 48:363-365. 

Rowe, D.B., C.L. Rugh, N. VanWoert, M.A. Monterusso, and D.K. Russell.  2003. Green roof slope, substrate depth, and vegetation influence runoff.  p. 354-362.  In Proceedings of 1^st North American Green Roof Conference: Greening Rooftops for Sustainable Communities, Chicago.   29-30 May 2003.  The Cardinal Group, Toronto. 

Rowe, D.B., C.L. Rugh, and D.K. Russell. 2002. Green roof installation at Ford Motor Company. Proceedings International Plant Propagators Society  52:127-130. 

 

Funding

The following companies or professional organizations have contributed direct support or in-kind support to our research program:

American Hydrotech, Chicago, IL (www.hydrotechusa.com)
Behrens Systementwicklung, Groß Ippener, Germany
Richard Brown, Meridian Township Community Planning and Development (www.meridian.mi.us)
Carolina Stalite, Salisbury, NC (www.permatill.com)
ChristenDETROIT Roofing Contractors, Detroit, MI
Green Roof Plants, Street, MD (www.greenroofplants.com)
Green Roofs for Healthy Cities, Toronto (www.greenroofs.org)
Ford Motor Company, Dearborn, MI    (www.ford.com/en/goodWorks/environment/cleanerManufacturing/rougeRenovation.htm)
Hortech, Inc., Spring Lake, MI  (www.premiumplants.net/)
Intrinsic Perennial Gardens, Hebron, IL  (www.intrinsicperennialgardens.com)
LiveRoof, LLC, Spring Lake, MI  (www.liveroof.com)
McDonough Braungart Design Chemistry, Charlottesville, VA (www.mbdc.com)
Michigan Agricultural Experiment Station  (www.maes.org)
Michigan Department of Agriculture
Michigan Nursery and Landscape Association  (www.mnla.org)
MSU Office of Vice President of Finance and Operations
MSU Office of Vice President of Graduate Research
Osburn Industries, Taylor, MI
Ray Stephenson, International Sedum Society, Northumberland, United Kingdom
Renewed Earth, Kalamazoo, MI (www.renewedearth.com)
Sarnafil, Inc., Canton, MA  (www.sarnafilus.com)
Siplast, Inc., Irving, TX  (www.siplast.com)
University of Georgia, Athens, GA
Walters Gardens, Zeeland, MI  (www.waltersgardens.com)
Wildtype Nursery, Mason, MI  (www.wildtypeplants.com)
Xeroflor America, Durham, NC.  (www.xeroflora.com)


Useful Green Roof Links

Earth Pledge (www.earthpledge.org)
Green Roofs for Healthy Cities (www.greenroofs.org)
GreenRoofs.com (www.greenroofs.com)
U.S. Green Building Council Leadership in Energy & Environmental Design (www.usgbc.org/LEED/LEED_main.asp)
Penn State University (http://hortweb.cas.psu.edu/research/greenroofcenter/index.html)
North Carolina State University (www.bae.ncsu.edu/greenroofs)
Southern Illinois University, Edwardsville, Green Roof Environmental Evaluation Network (www.green-siue.com)
William McDonough & Partners (www.mcdonough.com)
Sustainable Design Web Resources (www.fpm.wisc.edu/campusecology/Docs/Sust%20Web%20Resources.htm)
Greening Links (www.snre.umich.edu/greendana/links/links.html#)
Green Building Products by McGraw-Hill Sweets
 

Photos of Green Roofs

View of an extensive green roof on the assembly plant at Ford Motor Company in Dearborn, Michigan (June 2003).	Ford assembly plant (July 2003).
Aerial view of Chicago City Hall.  The building is eleven stories tall, 220 feet above street level, and covers 38,000 square feet.  (Photo courtesy of Roofscapes, Inc.).	An intensive green roof on a restaurant in Lansing, Michigan.
Sedum cover the Schipol International Airport in Amsterdam.	A meadow-like roof on a commercial building in Kassel, Germany.
The convention center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City, Utah.	Garden shed at Quail Botanic Garden near San Diego, California.
Rowe doghouse for Finnegan and Cooper in Mason, Michigan (August 2003).	Rowe doghouse (July 2006).
Sedum album in bloom on doghouse (July 2004).	Talinum calycinum in flower on doghouse (July 2005).