Re-innovating Green Roofs for Biodiversity: Seven Steps
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A rain garden planted with native vegetation attracts pollinators, butterflies, fish, dragonflies, insects, birds, rabbits, ducks and cleans up runoff from pavements and rooftops at the Capitol Region Watershed District building in St. Paul, Minnesota. This Integrated site design educates Minnesotans about the importance of conserving biodiversity with green infrastructure. As part of this effort, the City of Minneapolis set goals to build 150 green roofs by 2015, which expands the possibilities for integrated designs. Municipal support is necessary to help cities protect watersheds and biodiversity. Photo: Bruce Dvorak, October 2023
ASLA Scales Up to Support Biodiversity
Minneapolis, Minnesota was the host city for the 2023 American Society of Landscape Architecture (ASLA) Annual Conference. This year ASLA asked landscape architects to “scale up”. By 2040, ASLA aims to see all new landscape projects achieve zero greenhouse gas emissions, increase carbon sequestration, serve an equitable distribution of climate investments, restore ecosystems, and increase and protect biodiversity.
The conference offered many education sessions on various themes of sustainability. I was excited to co-present a workshop titled, “Living Surfaces—How to Design Successful Green Roofs that Optimize Biodiversity”. Through hosting this workshop, ASLA helped to advance the re-innovation of green roofs. Re-innovation means there is a hard look at improving something that is already proven (Cheng and Shiu 2008). Biodiverse green roofs (BGR) are a viable component of integrated designs aimed at realizing goals for protecting biodiversity in urban settings (Gedge and Kadas 2005, Brenneisen 2006, Clar and Steurer 2023). The challenge of the workshop was to convey in 2.5 hours the importance of biodiverse green roofs and outline a seven-step design process to design biodiverse green roofs. Co-presenters included David Yocca (Facilitator), Steven Peck, and Angie Durhman. In this issue of LAM, I explore seven steps to designing biodiverse green roofs.
Why Biodiverse Green Roofs?
Steven Peck challenged workshop attendees to answer a simple question, “Why would you want to build a biodiverse green roof? My short answer to this question is, “Anyone involved with land development shares the responsibility to “be” a steward of the earth”. We live in an integral relationship with nature and biodiverse green roofs are one of the viable development approaches that resolves the perceived dilemma between conservation versus development (Francis and Lorimer 2011, Kowarik 2011). To help disseminate knowledge about how to design biodiverse green roofs (BGR), we facilitated an ASLA workshop through a seven-step design process.
Step 1: Understand the rooftop’s potential for a BGR:
With a critical assessment of rooftop conditions, many, if not most new rooftops could be designed to accommodate biodiverse green roofs. The first step in the design process is to assess and analyze a roof deck’s potential for the inclusion of a green roof. Although green roofs may be a good fit with many buildings, a biodiverse green roof may not be a good fit with every rooftop. A roof deck with no direct access, or one that is cluttered with heating and ventilation equipment or has numerous intake or exhaust vents may not be a good fit.
An assessment of a rooftop’s strengths, weaknesses, opportunities, and threats (SWOT) is needed. During the workshop, we assessed the pre-existing condition of Chicago’s Peggy Notebaert Nature Museum roof deck for its potential to support a BGR. This roof deck has several features that could be considered a strength or a weakness. The low slope flat roof deck could be beneficial if mesic or hydric conditions were desired. Precipitation runs off slower on a flat roof deck compared to sloped roofs. However, if a xeric (dry) habitat was desired, additional modifications would be necessary to make the substrate well-drained. In this case, the flat roof deck was seen as an opportunity to make a ponded environment, to provide water to support wetland plants and wildlife (Dvorak 2003). Potential threats to the existing roof include heat gain from reflected solar radiation from west-facing walls and windows. A maintenance path was necessary for building maintenance workers to access the ladder at the end of the long roof (see the red circle in the figure below).
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In this case, the client wanted habitats for a diversity of native plants. The structural deadload for the roof deck ranged from 40 to 90 pounds per square foot deadload. It allows for a gradual increase in substrate depth, which facilitates deeper plant roots and more plant and biological diversity. A major constraint of the project was the long narrow shape of the roof deck. However, the path design could be used to separate segments of different green roof systems.
Step 2: Investigate regional biodiversity to develop design program goals (biodiversity is the client).
The Museum staff wanted a place to demonstrate how green roofs could support habitat for different forms of wildlife. This roof deck is assessable through elevators and stairs, which provides a ground connection. The design team at Conservation Design Forum (now Environmental Consulting & Technology, Inc) saw the potential to create habitat for: native plants, native birds, bees, butterflies, dragonflies, insects, spiders, and more.
Tallgrass prairies, wetlands and oak savannas were investigated for their potential to inspire hydric (saturated), mesic (moist), or xeric (dry) habitats on green roofs. Plants native to these habitats were investigated and selected for their potential adaptation to substrates ranging in depths from 4 inches (10 cm) to 12 inches (30.48 cm). Plant forms considered included perennials, annuals, bulbs, grasses, perennial wildflowers, and low-growing woody vegetation native to the region. The final planting plan excluded annuals, because of the small size of the roof and the potential for annuals to become invasive. Annuals can be a great addition to a green roof to attract pollinators, however, the client was not staffed to support a high maintenance green roof. It would be maintained through volunteers.
Step 3: Develop a conceptual sketch to outline potential habitat needs.
Regarding the habitat needs of wildlife, there are generally five essential elements: places to acquire food, water, cover, space, and shelter or the arrangement of space. Food sources can include plant parts such as leaves, seeds, and nectar. Investigating particular needs for species of birds, butterflies, bees and other critters can lead to a list of potential plant species to be included. Water can be provided according to the quantity and quality of water. Spatial components can be important regarding habitat openings or degree of cover for protection. Space can include the properties of air based on its temperature, purity, degree or direction of wind or movement of air, sound, and properties of light and shade. Shelter includes places for resting, nesting, and protection.
Other elements include the structural requirements of habitat such as varied heights of plants and plant species diversity (generally more is better); varied substrate depths and composition; varied forms of plants like perennials, annuals, bulbs, succulents, shrubs & trees (if the building structure allows); varied microclimate habitat diversity (wet, mesic, xeric); and the inclusion of logs/branches, varied sizes of stone, pebbles and ponded water.
At the Peggy Notebaert Nature Museum, there are several qualities that allowed for the creation of wildlife habitat. The roof deck is near level, which allows for the ponding of water and the making of a wetland habitat to serve pollinators, birds, dragonflies, and other insects that need moisture. The concept for the green roof display was to demonstrate how different depths of substrate and different moisture regimes yield different possibilities for plants and animals. Since the structural deck supports 40 pounds per square foot deadload at the public view area, and 90 pounds per square foot at the furthest point, the design concept was to use the maintenance path to delineate the different sections of the green roof with each switchback being a transition in substrate depth.
Image of the concept plan for the Green Roof Demonstration Garden at the Peggy Notebaert Nature Museum, located north of downtown Chicago, Illinois. Photo: CDF
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Wetland with open water at the Peggy Notebaert Nature Museum Green Roof Demonstration Garden. This green roof incorporates hydric, mesic and some xeric habitat for wildlife native to the shorelines of Lake Michigan, near downtown Chicago, IL. Photo: CDF
A wetland habitat was created up front near the public observation area and includes twenty-eight species of native plants including Acorus, Asclepias, Carex, Iris, Juncus, Sparganium, Spartina, Pontederia, Sagittaria, Scirpus, and others. Mesic prairie species occupy the midsection, and an intensive green roof supports a shingle oak (Quercus imbricaria) with savanna vegetation as habitat for birds to rest or nest. The ornamental native grass Sporobolus was planted and supports habitat for pollinators, nesting habitat for bees and birds, and seeds for insects and birds. Three species of Asclepias were planted to attract butterflies including Asclepias incarnata (swamp milkweed) Asclepias tuberosa (butterfly weed), and Asclepias verticillata (whorled milkweed). This biodiverse green roof was planted with 80 species of native plants that have provided ecosystem services for the building since 2002.
Step 4: Work through the details.
The next step includes working through the details of the green roof system design. Plants need to be selected based on the appropriate substrate depth for the plants (ASTM E2777 14 2014). The FLL Guidelines for Green Roofing publish data for selecting plants based on plant form and substrate depth. In the workshop we presented a translated version of the information in FLL Table 2 Standard course depths for different types of roof greening, to the Midwest of North America, however, there are yet to be published formal guidelines for plants in North America (Dvorak 2011). There is research that supports local plant adaptation to extensive and semi-intensive green roofs (Sutton, Harrington et al. 2012, Schneider, Fusco et al. 2014); however, long-term studies of plants on green roofs in the region are ideal sources to assist with plant selection (Rowe 2015).
Foreground, the native perennial Geranium sanguineum in bloom at the Peggy Notebaert Nature Museum Green Roof Demonstration site. During the spring wild geraniums attract bees, flies, beetles and butterflies. In the background, a bird can be seen scoping out a landing site on the extensive green roofs above. Photo: CDF
Other important details include the calculation of substrate depths/weights, drainage system depth/weights, irrigation placement and design and moisture management on the roof deck, including a wetland and small pond. These activities should be accompanied with the input of an experienced green roof designer. Additional knowledge and support can be learned through other sources such as the Green Roof Professional (GRP) training and the Living Architecture Academy (LAA). Other online and print versions of the green roof design process may need to be consulted.
Step 5: Develop a maintenance plan for the biodiverse green roof.
A good approach to prepare for normal maintenance activities of a biodiverse green roof is to design the substrate moisture management to match the moisture needs of the selected vegetation. This helps prevent unnecessary maintenance activities. For example, placing plants that naturally grow in xeric or dry soil conditions on a green roof that has mesic or moist conditions may result in high maintenance. Invasive weeds that thrive in moist conditions may establish on the green roof and may outcompete the xeric vegetation. Additionally, the xeric vegetation could develop root rot and perish. So, the best approach to minimize maintenance activities is to match the water needs of plants with appropriate substrate designs and moisture regimes. Mixing xeric and mesic plants in the same water moisture zone should be avoided as neither plant type will be happy. Another preventative strategy is to design the irrigation system through zoning. Each green roof moisture habitat should have its own irrigation zone with a separate control value and drainage design. Other important preventative measures include placing plants in the substrate depth that they require. Placing deep-rooted plants in substrates that are too shallow may result in the need for overwatering and pose a threat to plant health when weather conditions exceed norms.
With a good design in place, plants should thrive and may require only typical maintenance procedures. For biodiverse green roofs in the public view this could include deadheading plant parts, checking the irrigation system for appropriate watering times, and clipping or pulling weeds. If pulling weeds, then knock the growth media off the roots to keep media on the roof. A composter can be used to aid the replacement of soil mulch and nutrients. However, invasive plants with seed heads should not be composted. If the biodiverse green roof is not in public view, then some of these activities may be omitted.
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When designing green roofs for biodiversity, it is expected that wildlife will use the roof. Sometimes, wildlife can cause issues. Some bird species are known to pull up plants, peck at the soil to dig up insects, can spread invasive seeds, bring in pests, or cause unexpected effects. Nuisance species can show up when a green roof system is under stress. With quarterly visits by maintenance crews, a watchful eye will help keep maintenance issues under control and keep green roofs thriving.
This biodiverse green roof in Kansas City has a designated steward. The steward leads monthly maintenance visits during the spring and early summer to reduce aggressive invasive plants. In addition to a lead steward, staff are needed who understand the maintenance plan and know which plants to keep, remove, or relocate. Photo: Bruce Dvorak
A maintenance manual is necessary for upkeep and care of the green roof when transfer of ownership takes place. Manuals should include seasonal maintenance activities, names and photos of installed plants and common invasive plants. Manuals should be laminated and printed in bilingual languages. Maintenance activities should be reviewed each year in a meeting between the building owner and all invested in the upkeep and aesthetic appearance of the green roof.
Step 6: Estimate a budget for the implementation of the biodiverse green roof.
Green roofs in the design phase need a budget of probable construction costs. The design team should search for local examples of constructed green roofs to estimate pricing. Biodiverse green roofs could involve some elements that are not familiar to green roof contractors. The inclusion of water, branches, stones, or pebbles on a BGR may take some creative sourcing. Local sources are best, even better if the existing site has the potential to salvage materials. The more mature the green roof market in your area, the more accurate cost estimating can be. If there are no local projects or experts in your region, then clients should be prepared to pay to bring in experienced installers. Sourcing to lower-priced but inexperienced installers may be tempting if a budget is limited, but many costly mistakes can take place if installers are not familiar with installing green roof systems.
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Step 7: Present your biodiverse green roof for Client Feedback
The final step is acquiring approvals from clients, local authorities, and financial institutions. Although even when everyone is on board with the development of a green roof, it is not uncommon that a building owner may sell off the building within a couple years of construction. When this takes place, the new building owner will need to understand the needs of the green roof system including maintenance and requirements to keep plants healthy. All involved with the approval and maintenance of the green roof will need some education and agreement to the requirements of the green roof, including what it will look like at different times of the year. Clients need to know that some plants may have dormancy periods, and the green roof may change in its appearance throughout the year. It is the responsibility of the design team to communicate these important steps before signoff for bidding and installation. The building owner needs to set aside funds for regular maintenance and a small fund to potentially replace some plants each year.
Biodiverse green roof at the EcoCenter, Heron's Head Park, Port of San Francisco. This biodiverse roof includes salvaged materials from the site. Design Team: Habitat Gardens with Eco Catalyst with Lisa Lee Benjamin. Photo: Bruce Dvorak
Summary
Conventional roofing offers little in terms of the replacement of the ecosystem services of the landscape prior to development. Biodiverse green roofs can help mitigate those ecosystem services.
Selecting vegetation local to the region can offer native wildlife something that they need to survive.
The design of the biodiverse green roof should include multiple moisture regimes such as xeric, mesic or hydric, as well as different heights of plants and substrate depths.
Including a source of water on the roof will attract wildlife.
Working through the details of the design is perhaps the most critical step. Designers need to match the green roof system moisture and substrate design to the needs of the vegetation.
Budgeting and pricing green roof systems is dependent upon accessibility of experienced installers.
New Green Roof Biodiversity Courses
To learn about how green roofs can support heat-tolerant plants and biodiversity, see the new Living Architecture Academy Course: Case Studies of Biodiverse Green Roofs, by Bruce Dvorak.
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Bruce Dvorak, FASLA is a Professor at Texas A&M University in the Department of Landscape Architecture and Urban Planning, where he has been conducting green roof and living wall research since 2009. Bruce is a member of the GRHC Research Committee and founded a new Regional Academic Center of Excellence in 2022, the Southern Plains Living Architecture Center. Bruce received the GRHC Research Award of Excellence in 2017 and teaches green roofs and living walls in his courses in landscape architecture programs at Texas A&M University. His edited book, Ecoregional Green Roofs: Theory and Application in the Western USA and Canada (2021) provided inspiration and content for this article.
Acknowledgments
Thank you to the co-presenters of the ASLA Workshop: David Yocca, FASLA, Steven Peck, GRP, Hon. ASLA and Angie Durman, GRP for their input, creativity, and inspiration. Thank you also to Ted Lee, FASLA of HGA Minneapolis for offering me a long list of outstanding green infrastructure projects for me to visit while in Minneapolis.
References
ASTM E2777 14 (2014). Standard Guide for Vegetative (Green) Roof Systems: 14.
Brenneisen, S. (2006). "Space for Urban Wildlife: Designing Green Roofs as Habitats in Switzerland." Urban Habitats 4(1): 27-36.
Cheng, C. J. and E. C. Shiu (2008). "Re-innovation: The construct, measurement, and validation." Technovation 28(10): 658-666.
Clar, C. and R. Steurer (2023). "Climate change adaptation with green roofs: Instrument choice and facilitating factors in urban areas." Journal of Urban Affairs 45(4): 797-814.
Dvorak, B. (2003). "The greening of a Nature Museum: a demonstration project."
Dvorak, B. (2011). "Comparative analysis of green roof guidelines and standards in Europe and North America." Journal of Green building 6(2): 170-191.
Francis, R. A. and J. Lorimer (2011). "Urban reconciliation ecology: The potential of living roofs and walls." Journal of Environmental Management 92(6): 1429-1437.
Gedge, D. and G. Kadas (2005). "Green roofs and biodiversity." Biologist 52(3): 161-169.
Kowarik, I. (2011). "Novel urban ecosystems, biodiversity, and conservation." Environmental Pollution 159(8–9): 1974-1983.
Rowe, B. (2015). Long-term Rooftop Plant Commuinites. Green Roof Ecosystems. R. K. Sutton. Switzerland, Springer. 223: 311-332.
Schneider, A., M. Fusco and J. Bousselot (2014). "Observations on the survival of 112 plant taxa on a green roof in a semi-arid climate." Journal of Living Architecture 1(5): 10-30.
Sutton, R. K., J. A. Harrington, L. Skabelund, P. MacDonagh, R. R. Coffman and G. Koch (2012). "Prairie-based green roofs: literature, templates, and analogs." Journal of Green Building 7(1): 143-172.