An Evaluation Study of Plants for Use on Green Roofs Richard Hawke, Plant Evaluation Manager
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Plant Evaluation Notes Issue 38, 2015 An Evaluation Study of Plants for Use on Green Roofs Richard Hawke, Plant Evaluation Manager The significance of green roofs in urban ar- The environmental benefits include con- While the type of green roof is based on the eas has steadily increased over the past 20 serving water, reducing interior noise pollu- building and user needs, green roofs are years. In the United States, as well as tion, mitigating stormwater runoff, reducing increasingly being developed for their cul- worldwide, green roofs have become vital the urban heat-island effect, improving tural aspects, that is, it is becoming more components of new construction on a urban air quality through carbon diox- common for green roofs to provide interac- broad variety of buildings, from municipal ide-oxygen exchange, and creating habi- tive and amenity space for building occu- to commercial and industrial to residential. tats for a diversity of birds, insects, and pants or the general public. With this usage The benefits of green roofs are measured in animals. Along with extending the life of the comes a desire to have greater plant diver- environmental, economic, and aesthetic or roof by two to three times over a conven- sity so that the green roof functions on an cultural ways. In its simplest form, a green tional roof, economic benefits include re- environmental level as well as provides a roof may contain a sampling of sedums or ducing energy costs—both heating and pleasing landscape. Green roofs are classi- other hardy succulents, but many modern cooling—increasing property values, and cally described by the depth of the growing green roofs have increased the range and meeting requirements for stormwater man- medium—extensive, semi-intensive, and diversity of plants being grown by installing agement. Providing an aesthetic amenity intensive. Extensive green roofs have a deeper growing media and changing the space for building occupants and pleasing growing depth of 3 to 6 inches, and are typ- way green roofs are viewed or utilized. views from within or from a distance, creat- ically lighter in weight with a saturated Whether strictly utilitarian or highly special- ing green space in urban environments, weight of 15 to 30 pounds per square foot. ized and ornamental, green roofs have a attracting wildlife, and growing food crops This fairly shallow growing medium sup- variety of benefits over a traditional roof. are all cultural benefits of green roofs. ports a limited plant palette, often kept to 2 Plant Evaluation Notes drought-tolerant hardy succulents. Exten- When choosing plants for green roofs it is The Chicago Botanic Garden’s sive green roofs are popular with home- important to understand that a green roof is Green Roof Garden owners and large industrial, commercial, not the equivalent of a typical landscape In September 2009, the Chicago Botanic and municipal buildings where the environ- elevated to the top of a building; therefore, Garden opened its Daniel F. and Ada L. mental and economical benefits are most appropriate consideration must be given to Rice Plant Conservation Science Center, a valued. Semi-intensive green roofs are a a plant’s growth habit, native ecosystem, 38,000-square-foot LEED Gold-rated re- heavier system, with a saturated weight of and cultural needs, to name a few factors. search and laboratory facility designed 30 to 50 pounds per square foot, and a Many common garden plants will not sur- by Booth Hansen, a Chicago-based growing medium of 6 to 12 inches. The vive on a green roof. In Green Roof Plants, architectural firm. Atop the Center is a greater growing depth supports a wider va- Ed and Lucie Snodgrass succinctly 16,000-square-foot green roof, divided into riety of perennials, grasses, and shrubs, describe the ideal traits for a good green two 8,000-square-foot gardens that dis- which creates a more gardenlike land- roof plant—“The most successful green play a variety of North American native scape. Depending on the regional climate, roof plants are low-growing, shallow-rooted plants and exotic plants from around the irrigation may be necessary and more perennial plants that are heat, cold, sun, world. The Green Roof Garden of the Chi- maintenance may be required to keep the wind, drought, salt, insect, and disease tol- cago Botanic Garden consists of the Jose- semi-intensive green roof looking good. erant. Green roof plants should also have a phine P. & John J. Louis Foundation Green With a saturated weight greater than 70 long life expectancy or the ability to Roof Garden North and the Ellis Goodman pounds per square foot, intensive green self-propagate, and they should require Family Foundation Green Roof Garden roofs are the heaviest and have growing minimal nutrients and maintenance.” Need- South. The Green Roof Garden is both a depths greater than 12 inches. This type of less to say, selecting the right plants for living laboratory and a beautifully designed roof is commonly designed for access by green roofs is anything but simple. The garden enjoyed by visitors year-round. people, supports shrubs and trees, and Snodgrasses’ words bear out the chal- has higher maintenance needs. lenge of finding plants suited to the harsh growing conditions on green roofs. North green roof Koeleria macrantha and Penstemon digitalis 3 Plant Evaluation Notes Diagram: Design Concept The green roof gardens have a two-fold thetic design of a green roof as well as cre- calamint (Calamintha nepeta ssp. nepeta) purpose: to display the best plants for ating a habitat for a variety of beneficial and Russian sage (Perovskia atriplicifolia) green roofs and to trial untested plants for pollinators and wildlife. are interplanted to contrast and comple- their adaptability to this environment. Fol- ment each other in flower and form; and Our green roof gardens were designed by lowing an initial five-year evaluation phase, goldenrod (Solidago ‘Wichita Mountains’) Oehme, van Sweden Landscape Architec- new trials of uncommon and/or untested and reed grass (Calamagrostis brachytri- ture (OVS), Washington, D.C., and feature plants will continually cycle into the green cha) are paired perfectly so that the gold- their signature combinations of herba- roof gardens. The ultimate goal of the trial is enrod’s bright yellow flowers glow through ceous plants to create a succession of or- to develop a diverse and extensive list of the delicate seedheads of the grass in namental traits throughout the seasons. recommended plants for green roof cul- autumn. For example, generous sweeps of lesser ture. Greater biodiversity supports the aes- 4 Plant Evaluation Notes The two 8,000-square-foot green roofs, lo- cated on the north and south sides of the building’s central clerestory, are differenti- ated by their plant palettes and bed de- signs. Each of the green roofs is divided in half into display and evaluation areas, and then each area is further divided into beds with 4, 6, and 8 inches of growing medium (see Diagram). At these growing depths, the green roof is within the extensive to semi-intensive range. Curvilinear aluminum bands—both decorative and structural— run from north to south across the roofs, delineating the spaces between the three growing depths. The growing medium is a mixture of expanded clay and shale, ver- miculite, perlite, sand, and organic matter. The north green roof features native and nonnative plants, as well as a variety of se- dums (Sedum spp.) planted in three linear ribbons that bisect the roof from east to west. The sedum plantings are meant to draw the eye across the roof using color, texture, and form to create the feeling of movement, and to act as an experimental control, since sedums are the workhorses on extensive green roofs. The intersection North green roof display beds of the curved and straight lines creates a green roof a pleasing patchwork appear- compartmentalized effect, giving the north ance that from the beginning appealed to visitors. North American native plants, at the spe- cies level, inhabit the south green roof. Here, the gently curving beds are not inter- rupted by sedum strips, so this roof is less compartmentalized, allowing for greater sweeps of plants. The overall look is natu- ralistic, resembling a prairie or meadow landscape. Superimposed on the display area are 16 evaluation squares—6- by 6- foot boxes flush with the surface of the growing medium but separated from the display plantings by metal edging. Within each square are nine GreenGrid modules— black plastic trays that contain a variety of plants replicated in each of the three grow- ing depths. The squares are visible from the visitor viewing deck to demonstrate the evaluation process that is taking place in the nonaccessible areas of the roof. The custom-designed roof is composed of a series of layers from the growing medium down to the roof membrane. The base of 5 the green roof starts with a 8-inch gypsum Sedum strip on north green roof 5 Plant Evaluation Notes board overlaid with an Electric Field Vector Mapping (EFVM) system—EFVM ground- ing screens use low-voltage electrical puls- es to detect breaches in the waterproofing membrane. A felt pad covers the EFVM, and the Samafil waterproofing membrane is laid down next. EFVM conductors are then placed along the perimeter of the roof and another felt pad covers them. Continu- ing upward, extruded polystyrene insula- tion, a water retention mat, and a water drainage panel make up the next three lay- ers, respectively. The drainage panel chan- nels water to the central roof drain that empties into a bioswale surrounding the building. Filter fabric covers everything, and the growing medium is the final layer.