Green Roof Plants: Strategies for Success Bruce Dvorak, Assistant Professor Department of Landscape Architecture and Urban Planning, Texas A&M University Langford Architecture Center, 305 A College Station, TX 77843,
[email protected] Astrid Volder, Ph. D., Associate Professor Department of Horticultural Sciences, 2133 TAMU, Horticulture/Forest Science Building Texas A&M University, College Station, TX 77843
[email protected] Introduction Landscape architecture is a profession that has evolved alongside the current trends and movements fashioned in mainstream culture. Today, North American landscape architects are well positioned to bridge their traditional areas of expertise such as planting design to many of the new sustainable technologies that have evolved both at home and abroad over the last decade. Ecological design strategies such as green roofs and bioswales are now beginning to become part of our everyday design vocabulary. A little over ten years ago, Bill Thompson in Landscape Architecture magazine wrote, “An opportunity is emerging to introduce landscape architecture into another realm-the roofs of buildings-in a revolutionary way.” Since then, green roof technology has become a well know phenomenon across some North American cities; however, in many regions there is not sufficient knowledge of how to apply this technology locally. There have been more than a few green roof installations where growth media, plants and/or drainage materials were unsuccessful in their initial application and required reinvestment or redesign. Landscape architects were involved with some of these projects. This brings to light a need for increased knowledge of green roof technology for landscape architects. Since green roof installation is now encouraged through incentives or other bonuses in North America (Chicago, Portland, Washington D. C., New York, Los Angeles) and in some cases required (Toronto), successful selection of vegetation and its supportive substrate design is critical. Plant failure on a green roof can lead to unfavorable conditions where plant parts or growth media can be blown off a roof, drains can become clogged, and many of the economic and ecological benefits attainable with green roofs are not achieved. Since vegetation often plays a critical role in the success of green roofs, there is a need to understand the different factors that can influence successful designs, and make them known. This issue can be complex as plant species and substrate designs for one ecoregion may not be useful in another ecoregion. Although there is much known about green roof design in Europe, there is much less knowledge about successful plant species for green roofs across North America’s diverse ecological regions. One German green roof plant grower has over 3,000 species of plants listed in their catalog, where one of North America’s largest green roof producers on the East coast has investigated several hundred species. This comparison is made to point out the good work already accomplished in Europe and North America, but also brings to
light how far each distinct ecological region in North America may yet still have to go to fully explore its possibilities. There are several investigations concerning the effectiveness of installed green roofs as systems, and a couple of books investigate design criteria and critical analysis of components. Green Roofs for Healthy Cities, a large North American green roof organization has a green roof education program with accreditation. A comprehensive review of North American green roof plants is found in Dvorak and Volder (2010), and several important investigations are cited relative to green roof plants in Cantor (2008). With regards to general guidance from green roof standards, strategies for selecting plant material for green roofs exist in ASTM E 2400-06, Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems, and in the German FLL Guidelines. ASTM E 2400 recommends consideration of design intent, aesthetics, climate, micro-climate, plant characteristics, growing media, and maintenance. The FLL Guidelines covers these same topics but in greater detail. To investigate the potential effectiveness of these guidelines and identify plant species for an untested ecoregion of North America, the guidelines were used as part of the process used to select green roof plants for south-central Texas green roof research. These plants were tested over the 2009 and 2010 growing season to measure their effectiveness during a low input green roof experiment. The outcomes of this research reveal potential plant species for one ecoregion, and also include discussion of how the selected guideline criteria may have played a role in the results. Methods Plant Selection Two main sources of guidance were used to make decisions regarding plant species; the German FLL Green Roof Guidelines and ASTM E 2400, “Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems.” The FLL Guidelines make use of several knowledge areas to guide the selection of plants for green roofs. Chapter 4, “Functions and Effects,” discusses consideration of the interrelated functions and effects as a means to selecting the type of roof greening such as stormwater retention, modification of micro-climate, or reclamation of green space. Our purposes are first to find species that survive the regional and micro-climatic challenges found at the research site without using supplemental irrigation. We assumed that irrigation would likely produce positive results, since research in nearby ecological regions has proven viable with irrigation. We therefore opted for a conventional extensive green roof experiment with a shallow substrate and no irrigation beyond plant establishment to investigate possible species. Our intent with the plant study was to set a high priority on selecting for plant performance and not necessary stormwater or cooling benefits as successful plant establishment is a prerequisite for providing optimal stormwater and cooling benefits. Other plant selection criteria from the FLL guidelines include guidance for depth of growing media substrates (Chapter 7, “Requirements for the Construction of Vegetated Areas” section 7.2.1). Table 2, (page 43) gives a range of possible depths for extensive green roofs and suitable forms of plants. The modular trays used in this study are 10.6 cm deep and fall within the “Sedum-moss-herbaceous” plants extensive greening category according to German ecological regions. Generally, as substrate depths become thicker, plant forms become more diverse beginning with only succulents and a few herbaceous species enduring the shallowest of substrates, and progressing toward greater diversity by including herbaceous forms of plants and
graminoids. FLL gives no recommendation for particular species of plants, but emphasizes that the use of natives or locally adapted vegetation is recommended (FLL 2008, p 16). FLL Chapter 10 “Vegetation Support Course” covers much detail regarding growing media requirements to support plants. The modular tray systems used in our study makes use of an FLL tested and compliant extensive growth media (Rooflite®, Skyland USA LLC), and therefore we were assured that its composition meets FLL standards regarding organic content, grain size, structural stability, behavior under compression, water permeability, water storage, air content, Ph value, salt content, nutrient content, adsorptive capacity, and other considerations. FLL also has guidance on drainage, but the modular trays used in this study have not been evaluated, and therefore we had to make best guesses about the drainage performance characterizes of the trays. Chapter 12 “Plants and Planting” covers typical maintenance considerations regarding plant establishment such as supplemental watering, periodic weeding and general plant maintenance. Since the growth media design specified in FLL Chapter 10 establishes a low nutrient environment, plants were selected based upon their apparent tolerance of low nutrient soil conditions and minimal watering and maintenance during the establishment period. Initially, all of the ASTM E2400 criteria were considered: design intent, aesthetics, climate, plant characteristics, media, and irrigation. Since plant establishment with a low input approach was our research goal, some of the criteria were deemed more important than others. Aesthetics, for example, is given significant coverage in the ASTM E2400 guideline, and although aesthetics are an important consideration of green roof design, if an attractive plant cannot survive rooftop conditions without irrigation, it was of little value to the goals of this study. Our criteria from ASTM guidelines placed climate, plant characteristics, growth media and drought tolerance as high priorities for consideration of potential species. Our initial list of over 100 species was culled to fourteen of the seventeen species identified in Table 1 below. Three species, Sedum reflexum ‘Blue Spruce,’ Sedum spurium ‘Red Carpet,’ and Sedum tetractinum, were acquired with one donated module (LiveRoofTM) composed of pre-grown plants and we were not involved with the selection of those species. Regular irrigation was recommended by the provider; however, we decided not to irrigate to test plant viability. In summary, of all the potential variables mentioned in the FLL and ASTM guidelines to consider important when selecting plants for extensive green roofs, we prioritized these criteria in order of importance: (1) drought tolerance, (2) adaptability to cold hardiness zone 8b (USDA) and heat zone 9 (American Horticultural Society), (3) tolerance of exposure to full sun and potentially high gusts of wind (4) adaptable to shallow well drained substrates, (5) native to the region, (6) aesthetics, and (7) available in the nursery trade. Using these criteria, the following species were selected and acquired from green roof plant suppliers: Allium senescens ‘Glaucum’, Bulbine frutescens, Delosperma nubigenum ‘Basutoland,’ Delosperma cooperi, Delosperma ‘Psfave’ (Lavender Ice), Lampranthus ‘Red Shift’ (Trailing Ice Plant), Malephora lutea, Portulaca pilosa, Sedum album f. ‘Murale,’ Sedum mexicanum, Sedum moranense, Sedum moranense ssp. Grandiflorum, Sedum kamtchaticum, and Talinum calycinum. None of the plant species selected in this research was found on the USDA invasive plant list for Texas. Research Site Texas is a very climatically and geologically diverse landscape. Precipitation ranges from 127 cm (56 inches) annually along the eastern state boundary to less than 25.4 cm 10 inches along the western boundary. The northwestern panhandle lies in USDA cold hardiness zone 6b, where the southern boundary near Mexico is 9b. Topographic elevations range from mean sea
level at the Gulf of Mexico to a high elevation of 3750 m in the Guadalupe Mountains. The research site is located in College Station, Texas, which lies near the geographic center of its Subtropical Humid climate zone at about 150 m above mean sea level. College Station experiences over 100 days a year averaging high temperatures above 32.2° C (90°). College Station receives about 99 cm (39 inches) of precipitation annually, however, on average only 20.3 cm (7.8 inches) falls between June and the end of August during the hot season. June has a mean precipitation depth of 9.6 cm (3.7 inches), where July averages 4.8 cm (1.9 inches), and August averages 6.67 cm (2.3 inches) per month. Although precipitation appears to be evenly distributed throughout the year, the standard deviations about the means are high, and precipitation during the growing season can be unreliable. The plants were investigated on a research platform located on the roof of a four-story building on the campus of Texas A&M University (Figure 1).
Figure 1, Green roof research site May 7, 2010.
Plant Installation, Measurement and Maintenance The first year (2009) the modules were installed, twenty-seven individual plants were installed for each of the three initial species selected, (Delosperma cooperi, Sedum kamtschaticum, and Talinum calycinum) across nine 0.61 x 0.61 (2’x2’) modules (TectaGreenTM, Tecta America Corp, Skokie, IL). Eleven additional species were installed in eleven new modules during the 2010 growing season (March 5) and three additional species were acquired in June, 2010. In total, seventeen species of plants were investigated in the 11.4-cm -deep (4.5”) modular green roof trays with 8.9 cm (3.5 inches) depth of growth media and a 2.54 cm (1 inch) depth of expanded shale filled inside the drainage cells. A non-degradable landscape fabric was provided by the green roof vendor and was placed between the two layers of substrate materials to maintain their separate functions. Plant measurements and photographs were taken once a
month. A plant growth index (equation 1) was used to monitor monthly growth patterns for five of the selected species (Bulbine frutescens, Delosperma cooperi, Sedum moranense, Sedum kamtschaticum, and Talinum calycinum ): (1) Where: h = mean height of plant canopy a = area of plant canopy p = fraction of live growth occupying (a) Example: one S. kamtschaticum plant measured h = 6.0 cm, a = 19 × 18 cm, and p = 0.6 6.0 × 342 × 0.6 =1231.2 GI = 1231.2 cm3 Monthly GI values were measured and recorded for each plant. If a plant had died in a module, its GI would not be calculated in the average. Averages presented in Figure 2, are from live plants only. All measurements ended on October 19, 2010. Maintenance College Station had been in a drought during the 2008 growing season as it was about 23 cm (9 inches) below long-term precipitation averages. Forecasts for the 2009 growing season predicted the drought to lessen over the summer, but this was not the case. Beginning May 25 to July 19th, no rain fell. This included 55 days without precipitation and 28 days with temperatures over 37.8° C (100° F) during the drought. Since plants were still in their establishment period, a water conserving approach was used. Irrigation was applied by hand at a rate of 3.1 liters of water per 0.31 square meters per module, or 0.53 centimeters (0.25”) depth of water per module per week. Typical irrigation for ornamental landscaping in central Texas consists of about 2.4 cm to 5 cm (1-2”) per week. This project was set up to irrigate only during establishment, and only when natural rainfall subsided to a point where plants began to show signs of stress. During the 2009 growing season, from May 7th through August 24th if less than 0.25 cm of rain fell during the previous week and ambient air temperatures were high; irrigation was applied (Table 1). No supplemental irrigation was applied to any plants after August 24, 2009, including all new species planted in 2010. Table 1 Rainfall and irrigation rates for 2009 growing season in cm.
ASTM and FLL Guidelines suggest a plant establishment period of two years for extensive green roofs. First year plant growth should result in at least 80 percent of the planted area be occupied with live plants, with all initial species present. For our purposes, we
considered an 80 percent survival rate of individual plants representing each species to be present to be considered favorable. Results and Discussion Initially three species of plants were installed and monitored monthly in 2009, but were also included in the 2010 measurements for a total of 19 months. During the 2010 growing season, two additional species were measured monthly for 8 months, and twelve additional species were monitored for survival, but not incremental growth. Table 2 reports plant survival per numbers of individual plants per plant species where PI= plants installed, PS= plants survived, SR= survival rate. Seven of the seventeen species selected were found to be effective during the evaluation period including: T. calycinum, M. lutea, Lampranthus ‘Red Shift’ (Trailing Ice Plant), P. pilosa, S. moranense ssp. Grandiflorum, and S. album f. ‘Murale.’ Marginally successful were D. cooperi, B. frutescens, S. kamtschaticum and S. mexicanum. M. lutea, and Lampranthus ‘Red Shift’ (Trailing Ice Plant) only had 9 individual plants representing their species. The positive results from their heat and drought endurance are encouraging. Portulaca pilosa, S. moranense ssp. Grandiflorum, and S. album f. ‘Murale’ had only a couple individuals representing their species, and may warrant more investigation to prove their endurance. More research is needed on these species to see if they are valid candidates. Table 2 Species survival rates for unirrigated 11.4-cm –deep extensive modular trays. Species (botanical name) PI PS SR Comments Planted April 3, 2009 27 0 0% All plants survived the first Delosperma cooperi growing season including 2 unirrigated individuals in test plots. All plants were damaged from cold temperatures during January of 2010 (-7.2° C, 19° F). During the 2010 growing season all individuals were beginning to recover, however, plants began to fade during the summer heat and drought, and none survived. 27 7 Very drought and heat tolerant Sedum kamtschaticum 26% once established. May need more than 8.9 cm of growth media. 27 27 Talinum calycinum 100% Rapidly spreading. Very drought and heat tolerant. Winter dormant. Planted March 10, 2010 27 4 Possible species for Bulbine frutescens 15% unirrigated green roof once establishment, or may need deeper media. 9 0 0% Faded early July. Delosperma cooperi 36 0 0% Faded quickly beginning in Sedum moranense June. 9 9 Talinum calycinum 100% Established and reproduced
Origin Africa
IA, MN, PA, NY, NH Texas
Africa
Africa Texas Texas
Species (botanical name)
PI
PS
SR
Comments without irrigation. Planted March 10, 2010 (growth rates not monitored) Allium senescens ‘Glaucum’ 3 0 0% Faded early July. Delosperma ‘Psfave’ (Lavender 6 0 0% Faded late June. Ice) 6 0 0% Faded late June. Delosperma nubigenum ‘Basutoland’ Lampranthos ‘Red Swift’ 9 9 100% Performed extremely well. 9 9 Malephlora lutea 100% Performed extremely well. Sedum album ‘Murale’
3
3
100%
Sedum mexicanum
9
1
11%
Sedum moranense ssp. Grandiflorum
2
2
100%
Portulaca pilosa
3
3
100%
Origin
Exotic Africa Africa Exotic Exotic
Growth was stunted by drought. Remaining individual set seeds. Performed well, some signs of stress during drought.
North America Florida
Stunted growth, but reseeded.
Texas
Texas
Green roof tray with multiple individuals pre-established in green house set on rooftop June 5th, 2010. Sedum reflexum ‘Blue Spruce”, Sedum 0% None of the multiple Exotic spurium “Red Carpet”, Sedum individuals of species growing kamtschaticum, Sedum tetractinum in one tray survived. PI= plants installed, PS= plants survived, SR= survival rate
Sedum kamtschaticum achieved the most growth as measured above the substrate, with a maximum GI of 2614 for individual plant totals measured in July, 2010 (Figure 2). This includes all sixteen S. kamtschaticum individuals that survived the previous growing season, however, only seven of those sixteen plants survived through September, 2010 (Figure 3). Plants from all species except S. moranense were performing well until late July, when drought and heat persisted. From July 2nd through September 7th (2010), only 2.2 cm (0.87”) of precipitation fell over seven separate rain events ranging from 0.02 to 0.27 cm per individual rain event. Long term precipitation averages for July and August total 11.5 cm (4.2 inches). During August, there were 15 days with high temperatures above 37.8° C (100° F), and a maximum temperature recorded of 40.5° C (105 F°). Long term daily high temperatures for College Station in August range from 36.1° C (97° F) to 34.4° C (94° F). Minimum temperatures were as low as 24° C (75° F) for a few hours, but minimum temperatures for most days were at or just above 27° C (80° F) for August, 2010. Long term daily low temperatures for College Station in August range from 22.7° C (74° F) to 23.3° C (74° F). The drought and heat stress proved excessive for D. cooperi and S. moranense which had no surviving individuals, even though their GI values were competitive compared to other species during the previous month (Figure 2). Talinum calycinum performed very well throughout the hottest and driest periods, however, it began to drop leaves by late September. Its GI of zero in November 2009 through March 2010 and in October 2010 is due to the plant going dormant for winter. Its form is upright so it is not an effective ground cover but should be considered an accent. It also re-seeds itself readily beyond its initial location throughout the growing season even during dry and hot conditions. By the end of August it stopped blooming and showed some signs of stress. It rapidly rejuvenated, however, with the next rainfall in September and its GI slightly increased from 1061 to 1099 (Figure 2). S. moranense began to show signs of stress early in the growing season
during a short dry and warm period in June, but the plants rejuvenated when temperatures cooled off and precipitation fell. Once the July heat and drought persisted S. moranense quickly began to fade. Perhaps with normal or above normal precipitation the species could adapt, but during a year that was warmer and dryer than normal, it did not establish. All of the individual plants of B. frutescens were thriving through early June, but began to show signs of stress in late June, and most individuals were in decline by late July. Only four of twenty-seven individuals survived through October. All these individuals were healthy at the time of planting in March, however, as pre-grown plugs they appeared to be top heavy on growth, and may have had difficulty establishing because of lack of proportional root development to support its rapid top growth once the drought established.
Figure 2, Growth Index results for April 2009 to October, 2010
In an offsite trial garden setting (8 km east), 35 individual B. frutescens, 35 D. cooperi, and 17 S. kamtschaticum from the same batch of plants used in this study were grown in a residential garden in sandy-loam soils in a partly-sunny setting. Plants were watered weekly to bi-weekly and all individuals from all species survived. Further investigation of B. frutescens, D. cooperi, and S. kamtschaticum is warranted to see if they perform better on extensive green roofs during a normal or wetter than normal establishment period or if they would perform better when planted in the fall rather than the spring which would give the plants an additional 4-5 months to establish before summer heat and drought would stress the plants. Species grown in the pre-planted module including Sedum reflexum ‘Blue Spruce”, Sedum spurium “Red Carpet,” and Sedum tetractinum were not monitored for growth rates, but survival only. All of these species were cold hardy species and are reliably grown on green roofs in the Mid-Atlantic ecoregion (greenroofplants.com). Since no plants from these species survived with natural rainfall in our study, this may mean that these species are not adaptable to extensive green roofs in the Blackland Prairie ecoregion without irrigation; however, their poor performance could also be due to the method of plant establishment. These plants were
established in a greenhouse, with irrigation and fertilizer treatments to achieve rapid growth. These species should be further investigated to understand how they perform while establishing themselves on the roof environment, and not in a greenhouse. Perhaps these plants were not hardened or prepared for the transition to natural rainfall only. Further investigation will better determine if they are adaptable to central Texas without irrigation, or irrigation only during their first growing season. Concerning ecoregion cross adaptability, Talinum calycinum and Sedum album f. ‘Murale’ also reliably grow in the Mid-Atlantic region without irrigation and also performed well in the Backland Prairie ecoregion in south-central Texas without irrigation.
5/4/2009
1‐Sep‐09
28‐May‐10
2‐Sep‐10
Figure 3, Time laps photos of S. kamschaticum survival from May 2009 to September, 2010
The following species did not yield any remaining survivors and are therefore considered not likely effective on green roofs in south-central Texas without irrigation: Allium senescens ‘Glaucum,’ Delosperma nubigenum ‘Basutoland,’ and Delosperma ‘Psfave’ (Lavender Ice). The two Delosperma species began to show signs of stress such as wilting early on during periods
where heat and drought were combined. These two were the first species to fade, and one of the three Alliums survived until late July. Conclusions Seven criteria from the German FLL Green Roof Guidelines and the ASTM E 2400, “Standard Guide for Selection, Installation, and Maintenance of Plants for Green Roof Systems” were selected; (1) drought tolerance, (2) adaptability to cold hardiness zone 8b (USDA) and heat zone 9 (American Horticultural Society), (3) tolerance of exposure to full sun and potentially high guests of wind (4) adaptable to shallow well-drained substrates, (5) native to the region, (6) aesthetics, (7) available in the nursery trade. Seven of seventeen investigated species including Talinum calycinum, Malephora lutea, Lampranthus ‘Red Shift’ (Trailing Ice Plant), Portulaca pilosa, Sedum moranense ssp. Grandiflorum, and Sedum album f. ‘Murale’ were selected and planted in modular green roof trays. These species endured the trial period without any loses. Delosperma cooperi, Bulbine frutescens, Sedum kamtschaticum, and Sedum mexicanum, however; all had individual plants surviving, but also lost significant numbers of plants. These four species perhaps warrant further investigation to see if they can perform better during normal or wetter and or cooler than normal establishment periods. Perhaps once they are well established during a favorable growing season, they may fare drought and heat better. It is also possible that these species need some form of irrigation to be maintained on extensive green roofs in this ecoregion. The following species did not yield any remaining survivors and are therefore considered not likely effective on green roofs in south-central Texas without irrigation: Allium senescens ‘Glaucum,’ Delosperma nubigenum ‘Basutoland,’ and Delosperma ‘Psfave’ (Lavender Ice). The outcomes of this study provide example that more than several species were found promising for unirrigated extensive green roofs in an untested ecoregion through the aid of the content found in FLL and ASTM guidelines. The top three criteria (drought, heat, sun exposure) seemed to be important indicators of how well plants fared the rooftop conditions. Since there were no other guidelines or a control design (with no guidance), it is difficult to say how the guidelines could be improved. Perhaps a future study could investigate if a growing media with more moisture holding capacity would be more effective. More research is necessary to further investigate the application of these guidelines under different conditions be attain a more comprehensive understanding of their application in Texas. Landscape architects venturing into green roof technology in untested regions need not attempt plant species selection blindly. FLL and ASTM guidelines were found effective in guiding the selection of plants for extensive green roofs in the south-central Texas Blackland Prairie ecoregion. Some ecoregions may require different selection criteria or different priority of similar criteria. There are many ecoregions lacking plant research, and there is much inconsistency in methods reporting existing plants studies. The methods outlined in this study may provide useful in other ecoregions as a method for selecting plants for green roofs in untested ecoregions. We recommend reporting of selection criteria in new research to aid in the interpretation of research outcomes.
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