Erin Schroll, a student at Oregon State University (OSU), undertook a study of extensive green roofs to determine aspects of their design, plant selection, installation, and maintenance that would enhance the goal of stormwater management in the Pacific Northwest. This article is based upon the results of Erin’s project, which was presented at the Seventh Annual Greening Rooftops for Sustainable Communities, held in Atlanta, Georgia in June 2009.
An extensive green roof is a contained ecosystem with a soil depth of less than six inches (15.25 centimeters); the shallow depth results in a lighter-weight roof than a traditional intensive roof garden, thus reducing the cost of the building’s engineered structure. Such roofs have been shown to provide both aesthetic improvements (beauty and useable space) and economic benefits (reduced energy use and extended roof life). However, researchers, horticulturalists, and commercial developers are increasingly creating extensive green roofs that meet specific ecological goals, such as stormwater management, reduction of the urban heat-island effect, enhanced air filtration, and habitat creation.
Creating an extensive green roof that provides ecological services is not easy. Structural constraints limit soil types, which, in turn, influence plant selection. Extensive green roofs are harsh environments, often subjected to prolonged periods of drought, high temperatures, and intense wind; these conditions are of particular concern here in the West. Green roof plants are expected to serve multiple functions while growing in a thin aggregate planting medium, yet our understanding of these complex and dynamic ecosystems remains limited. In some cases, green roof management practices actually conflict with the goal of the green roof. For instance, in the Central City District of Portland, Oregon, extensive green roof managers are expected to follow an irrigation regime that could only support summer-dormant native bulbs and hardy succulents through the dry summers.
A green roof is constructed in a series of layers: the waterproofing membrane, drainage layer, planting medium, and plants. Each interacts with the other layers and components to influence the performance of the roof. These interactions mimic the functioning of natural ecosystems; for instance, in a natural system, soil structure and type, slope angle, plant composition, and climate all influence the pattern of water movement. In the engineered ecosystem of a green roof, the drainage layer, growing medium, roof pitch, and plants interact to affect a green roof’s ability to mitigate stormwater. Considerable effort and thought has been put into the engineering and architectural considerations for green roofs, but many of the issues in functional design are inherently horticultural in nature.
The Black Arts
Although green roofing has been described as a combination of “black” and “green” arts, issues within the two arts often overlap. The black arts are performed by professional roofing contractors and involve all of the non-living components, the most important of which is the roof structure. Extensive green roofs add a minimum saturated weight of two to seven pounds per square foot (48 to 170 kilograms per square meter). As with traditional roofs, important components of the black arts include a waterproofing membrane and drainage details that facilitate the movement of water off the roof. Certain waterproofing membranes have organic components, such as rubberized asphalt, that require a root barrier, a layer that protects the membrane from breaking down as it comes into contact with plant roots. Root barriers are usually a tightly woven fabric or an inorganic barrier such as sheets of polyvinyl chloride (PVC); some cities have banned root barriers impregnated with chemical herbicides, because the manner in which these chemicals break down is not yet fully understood.
A retention mat is an additional layer that may be included to increase the water-holding capacity of the green roof. Placed below the drainage layer, the mat is intended to increase stormwater retention and slow its release to the drainage layer, not merely to provide the plants with more water.
The Green Arts
The green arts involve the living portion of the green roof. Every green roof requires decisions on these five horticultural issues: drainage, planting media, plant choice, planting method and maintenance.
The slopes of roofs can vary, yet even “flat roofs” have slopes of around one to two percent; the less the slope, the slower water moves off the roof. Placed immediately above the various waterproofing membranes is a drainage layer that may be either a prefabricated plastic layer (resembling an egg carton) or a two-inch (five-centimeter) layer of pure aggregate such as pumice. This layer further facilitates the movement of water away from plant roots, once the planting medium is saturated. Although prefabricated plastic layers usually create an air layer that is uninhabitable for plant roots during dry weather, they can also be designed to retain additional water. An aggregate drainage layer has the potential to provide additional depth for plant roots, which may enhance plant growth.
As with a traditional flat roof, water on a green roof may pool in certain low spots, often around drains or at the bottom of slopes; these areas are micro-environments for plants that prefer more water. In the same vein, any dry areas, as might be found at the top of slopes, are good sites for the toughest and most drought-tolerant plants. It is important to understand each plant’s drainage needs and suit either the drainage to the plants or the plants to the drainage.
Extensive green roof planting medium compositions vary widely across regions depending upon materials available locally and the specific green roof goals. The medium typically contains little organic matter (five to twenty percent) and has a high mineral content (up to ninety percent), consisting of a lightweight aggregate such as pumice, expanded slate, or expanded clay.
Standards for planting media were developed in 1982 by the German Landscaping and Landscape Development and Research Society to provide guidelines for professionals. Known as the FLL, they have heavily influenced planting media selection for North American green roofs. FLL-compliant media are designed to be lightweight and to reliably support species of Sedum and other succulents, which are among the most commonly used plants for extensive roof gardens.
Some practitioners have been experimenting with native soils and with a higher percentage of organic matter in engineered lightweight media, to better retain water and to permit a greater diversity in plant palettes. Although these media have been used successfully on extensive green roofs, such alternatives are still a topic of debate. A planting medium high in organic matter is likely to decompose and compact, unless it is periodically amended, which can, in turn, change the medium composition; this is not recommended for roofs with serious weight constraints.
An installer of green roofs since 1995, Vermont-based Rick Buist advocates using a planting medium higher in organic matter, and planting green roofs with grasses to enhance stormwater management. He has not seen any loss in soil depth on existing green roofs; when grasses are mowed once or twice a year, the clippings, left in place, contribute organic matter back into the system.
Soil-based media have also been successful. Originally covered in native soil to help moderate temperatures within the building, the Zurich, Switzerland, MOOS Water Filtration Plant is a nine-acre roof meadow with 175 different plant taxa, including nine orchid species. Constructed in 1914, the original medium depth was two inches (five centimeters) of sand and six to eight inches (fifteen to twenty centimeters) of topsoil.[1. Werthmann, Greener Green Roofs.] It has become common practice in Switzerland to green rooftops with native soils and call upon an ecologist to assist in designing the rooftop habitat.[2. Brenneisen, Space for Urban Wildlife.]
Professional laboratory testing of organic percentages, soil pH and nutrients, weight, porosity, drainage capacity, and water retention is always recommended prior to installing soils and components for any green roof.
Extensive green roofs can be planted in a variety of ways, including (in order of increasing cost): seeds, cuttings, plugs, containers (4″ pots or larger), sedum mats, sod, and preplanted modules (such as those used on the roof of the California Academy of Sciences in San Francisco’s Golden Gate Park). Seeds and cuttings take more time to establish, whereas modules are often fully established at the time of installation.[3. Snodgrass and Snodgrass, Green Roof Plants]
The planting season plays an important role in extensive green roof establishment. Depending upon regional differences, supplemental irrigation and regular weeding may be needed to ensure the healthy establishment of the plants.
Extensive green roofs require minimal maintenance, if designed properly; in many cases, they are only accessible for maintenance, and, in some cases, they are not even visible. As with any roof, a yearly inspection is recommended: clean gutters, remove debris from drainage areas, scout out and remove any competitive weeds, and monitor overall plant survival.
Irrigation requirements on a green roof depend upon several variables: the planting method, the design (substrate depth and composition, plant selection), the local climate, and the goals of the green roof. Irrigation can be critical in the establishment phase of a green roof, which can take six to eighteen months; it will be most frequent initially, tapering off as plants acclimate to the medium and the climate. The amount and frequency of each watering varies according to the climate, rainfall events, planting season, and planting method—just as in a garden in the ground.
Although permanent irrigation may be unnecessary in all but arid and semi-arid climates, it is often recommended to achieve certain green roof goals. Maintaining specific aesthetic values or high plant biodiversity, reducing ambient air temperatures, and storing rainwater all require actively growing and transpiring plants. An irrigation system may also need to remain in place in case of periodic drought, which can cause costly plant dieback.
During the years subsequent to establishment, irrigation requirements for extensive green roofs are low. With a depth of less than six inches (15.25 centimeters), the medium becomes quickly saturated and runoff occurs, so there is no need to water beyond that point. In the irrigation trials at OSU, green roof plants growing in only five inches (12.7 centimeters) of aggregate medium reliably survived with less than seven inches (174 millimeters) of combined irrigation and summer precipitation over the ninety-day experimental period of a typical dry summer in Oregon’s Willamette Valley; that is less than .6 inches (14.5 millimeters) each week.
Weeds move into bare places in the medium and increase in biomass with increasing irrigation amounts, just as in ground-level gardens. During establishment, it is important to keep plant competition down by removing any aggressive weeds so that the intended plants can increase in size and establish good coverage. In some situations, the aesthetic value of an extensive green roof is important and regular weeding of non-aggressive weeds may be necessary depending upon what the horticulturist and the client decide is a weed. Aggressive weed species and trees should always be removed throughout the life of the green roof.
The plants best suited to extensive green roofs are mat-forming groundcovers that can withstand heat, sun, wind, and cold, in addition to being low maintenance and relatively pest resistant. Fitting that description are succulents (often Sedum species), because they are drought tolerant, low growing, fibrous rooted, and usually evergreen. To date, most extensive green roofs in North America and Germany have been planted with succulents. Accent plants such as bulbs might offer seasonal interest, but they do not provide the same type of coverage as succulent groundcovers.[4. Ibid.]
Succulents such as Sedum, Sempervivum, and Delosperma have proven successful in aggregate planting media in many regions and often survive in non-irrigated extensive green roofs. According to Getter and Rowe, Sedum album can survive more than one hundred days without water; Sedum acre, S. kamtschaticum var. ellacombianum, S. pulchellum, S. reflexum, S. spurium ‘Coccineum’ and S. spurium ‘Summer Glory’ all survived eighty-eight days without water. Sedum rubrotinctum has survived two years without water in a greenhouse.[5. Getter and Rowe, The Role of Extensive Green Roofs in Sustainable Development.]Cape Blanco stonecrop (Sedum spathulifolium ‘Cape Blanco’), a West Coast native, and Delosperma cooperi, from the mountains of eastern South Africa, survived a dry Pacific Northwest summer without any supplemental irrigation.
The green roof industry is now stressing plant choices that increase the ecological function of a green roof. In some cases, this may mean expanding the plant palette to include regionally native species, driven by a desire for green roofs to serve a habitat function, either for specific plant species or for the animals that are associated with them.
In our study at OSU, we looked at the suitability of native Pacific Northwest plants for use on extensive green roofs, and found that the natives we chose survived reliably when provided with enough summer irrigation. The optimal survival and growth was observed under an irrigation regime in which plants received a total of 6.75 inches (171.45 millimeters) of combined irrigation and summer precipitation over the ninetyday trial period. Included in this study were common camas (Camassia quamash), common woolly sunflower (Eriophyllum lanatum var. lanatum), Roemer’s fescue (Festuca idahoensis var. roemeri), yellow-leaf iris (Iris chrysophylla), Cape Blanco stonecrop, and Idaho blue-eyed grass (Sisyrinchium idahoense). Of these, the camas and stone-crop survived without supplemental irrigation, receiving only 1.5 inches (38.1 millimeters) of natural summer precipitation, although the appearance of the sedum declined with decreasing irrigation levels.
Selecting for Stormwater Management
There is an increasing emphasis on the selection of plants that are suited for both regional climate and rooftop microclimates. Plant selection becomes more complicated when conflicts arise between different functional goals. For instance, plants that improve stormwater management through high transpiration rates may actually require more supplemental irrigation (eg, during dry summers) to thrive in the green roof environment. A green roof designed for stormwater management must include plants that can tolerate fluctuations between quick drainage and complete saturation of the soil. In the Pacific Northwest, as in other parts of North America, soils will typically remain wet for several consecutive days through an average rainy season.
There has been only limited research evaluating plant species or groups to determine which best mitigate stormwater. Evaluating the ability of specific plant groups to improve stormwater management on an extensive green roof, England’s Nigel Dunnett and his colleagues found that grasses performed better than forbs, which, in turn, performed better than succulents.[6. Dunnett et al, Influence of Vegetation Composition on Runoff in Two Similar Green Roof Experiments.]
The plants with the greatest potential for stormwater management include grasses, herbaceous perennials, and mosses. Those that can tolerate seasonal wetlands, such as vernal pools, may prove to be particularly strong candidates for stormwater management on green roofs; for instance, Roemer’s fescue is a seasonal wetland grass suitable for extensive green roofs in the Pacific Northwest; it requires only minimal irrigation for survival through dry summers. Idaho blue-eyed grass is another suitable native. Carex and Juncus are genera with many species native to seasonal wetlands in the West, yet few have been tested for use on green roofs. In our research at OSU, common camas, a native, summerdormant bulb often found in locations that are wet throughout the winter and spring rainy season, had a one hundred percent survival in a typical Northwest summer under three different irrigation regimes, including non-irrigated.
Studying their stormwater retention capabilities, OSU’s Malcolm Anderson has found that native mosses retain more water than vascular plants, and can withstand the seasonally dry summers of the Pacific Northwest; his research will soon be published by Urban Ecosystems.
Succulents are often recommended for extensive green roofs where stormwater management is a stated goal, and they are well suited for non-irrigated green roofs with a highly aggregate planting medium. They have the ability to take up and store a great deal of water, but they also have a small, fibrous root system that may allow more runoff compared to grasses and herbaceous perennials.[7. Ibid.]
Successful extensive green roofs result from a holistic design approach, influenced by regional climates, rooftop microclimates, plant needs, and, most importantly, the green roof goals. The composition of a green roof varies within and across regions, reflecting local considerations that may include building codes and neighborhood character, but certainly bear upon the prioritizing of these ecological goals. In the Pacific Northwest, for instance, stormwater management ranks high among those goals. Whether they arise from structural constraints, seasonal dry periods, or client needs, there are many issues to be resolved in designing any extensive green roof to maximize a specific ecological goal. We still have a lot to learn, but increasing numbers of researchers and practitioners are busy focusing on the effort to make our roofs serve us in better and more effective ways.
Brenneisen, Stephan. 2006. Space for Urban Wildlife: Designing Green Roofs as Habitats in Switzerland. Urban Habitats 5:27-36.
Dunnett, Nigel, and Noel Kingsbury. 2003. Planting Green Roofs and Living Walls. Portland, OR: Timber Press.
———, Ayako Nagase, Rosemary Booth, and Philip Grime. 2008. Influence of Vegetation Composition on Runoff in Two Similar Green Roof Experiments. Urban Ecosystems 11:385-398.
Getter, Kristin L, and D Bradley Rowe. 2006. The Role of Extensive Green Roofs in Sustainable Development. HortScience 41(5): 1276-1285.
Green Roofs for Healthy Cities, a non-profit industry association to promote the intelligent use of green roofs, 406 King Street East, Toronto, ON M5A 1L4 Canada
Schroll, Erin, John Lambrinos, and David Sandrock. 2009. Irrigation Requirements and Plant Survival on Northwest Green Roofs. Proceedings of the Seventh Annual Greening Rooftops for Sustainable Communities, Atlanta, Ga.
Snodgrass, Edmund C, and Lucy L Snodgrass. 2006. Green Roof Plants: A Resource and Planting Guide. Portland, OR: Timber Press.
Werthmann, Christian. 2007. Greener Green Roofs. Architecture Week. https://www.architectureweek.com/2007/1107/environment_1-1.html