Green Roofs for Native Pollinators using Grasses

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Grasses and Pollinators?

Eighteen species of native grasses make up a Blackland prairie living roof exhibit at the Perot Museum of Science and Nature near downtown Dallas, Texas. Seen from inside the ticket lobby, this tallgrass habitat is part of a multi-habitat exhibit of biodiverse green roofs surrounding the building. Big bluestem (Andropogon gerardi) is planted here, and it is a larval host for Delaware skipper and dusted skipper butterflies. Seeds of big bluestem plants attract the grasshopper sparrow, Henslowe’s sparrow, and other sparrows. When green roofs are designed to attract wildlife, glass facades should be designed to be *bird-friendly* *to prevent bird strikes. Photo: Bruce Dvorak*

Grasses and pollinators go together like summer and hot weather. However, one might not intuitively think that grasses attract pollinators because most grasses lack the bold floral display of wildflowers. Although all grasses flower, many native grasses have small flowers that look like seeds, and many are quite beautiful. Grasses are pollinated through wind dispersion, yet many insects feed on grass pollen and make use of grasses for other needs. For example, male and female fireflies are grass-loving pollinating beetles that like to hang out in meadows, edges of woodlands, and grassy habitats near water (Kalra and Arora 2016). Fireflies play an important role in reducing predator species, however, due to the destruction and degradation of their habitats, firefly populations have been in a slow decline. Fortuitously, fireflies have been observed on rooftop meadows in North America and Europe (MacIvor Lundholm 2011). Additionally, at least 70 species of butterflies and moths make use of some of the 1275 taxa of grasses native to North America to complete their life cycle at different stages of their development (Damude 1995). Thus, grass-based green roofs could play a crucial role in making habitat for pollinators in urban regions, especially where grassland habitat fragmentation has already occurred (Brückmann, Krauss et al. 2010).

Many such complex relationships exist between natural habitats and wildlife that need our attention and creative solutions, on living roofs. In this third and last article in a series about pollinators on living roofs, I focus on how native pollinators use and require native grasses for their survival.

The “Big Four” and Butterflies

Biodiversity in a natural landscape is often realized through a few dominant species that make up an “ecoregion,” while its associated diversity is often found to a lesser degree in the margins or variations of dominant habitats. Native tallgrass prairies, for example, might be dominated by only a few species of grasses across large areas of land, while hundreds of species of other grasses and forbs might be found in a smaller proportion in the various habitats influenced by topography, slope, soils, sunlight, and shade.

Green roofs on the mechanical rooms of the Chicago City Hall building are planted with exotic sedums and native prairie grasses including big bluestem (Andropogon gerardii), shown here. Vegetation was established in a 10-cm-deep substrate (4 inches) without irrigation. The big bluestem continues to persist in this shallow substrate because excess water from precipitation events drains slowly from the roof. The FLL-compliant growing medium has a gravel drainage system and a water retention fabric. Also, the low-sloped roof deck has only one drainage outlet along the parapet wall that acts as a reservoir to retain moisture long after precipitation events. Parent (left) and juvenile (center) big bluestem plants can be seen. When grasses are left to drop their seeds, new plants can generate to build up the next generations of plant-to-pollinator relationships. Photo: Bruce Dvorak

Tallgrass prairies commonly include the “big four” grasses: switchgrass (Panicum virgatum), Indiangrass (Sorghastrum nutans), little bluestem (Schizachyrium scoparium), and big bluestem (Andropogon gerardii). Big bluestem, for example, is a larval host for Delaware skipper and dusted skipper butterflies, beard-grass skippers, and common wood nymph butterflies. Additionally, the thatch leftover from the previous growth cycle of big bluestem provides nesting sites for sedge wrens and western meadowlarks (LBJWFC 2022). Switchgrass (Panicum virgatum) is a butterfly larval host for green skipper butterflies. Indiangrass (Sorghastrum nutans) serves as a larval host for skipper butterflies. Little bluestem (Schizachyrium scoparium) serves as a nesting site for bees and birds, and as a larval host for butterflies and moths including Ottoe skipper (Hesperia ottoe), crossline skipper, dusted skipper, and cobweb skipper (Hesperia metea). Other grasses common to some prairies include prairie dropseed (Sporobolus heterolepis) which provides a habitat as a nesting site for native bees (Vogt 2017). All of the above mentioned grasses have been utilized on deep-extensive, semi-intensive, or intensive green roofs in North America (Dvorak 2021). Supplemental watering, periodic maintenance, and substrate designs are all key factors in making and maintaining suitable habitats for native grasses to be successful on green roofs (Dvorak and Skabelund 2021).

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Plant a Diversity of Grasses for Greater Biodiversity

This green roof on the Berry Biodiversity Institute building is part of a research roof used to educate students, faculty, and visitors about biodiverse green roofs. The semi-intensive green roof sits over a vertebrate exhibit below. Here, ten species of mid-to-short stature grasses native to the nearby ecoregions grow with a diversity of 100 wildflowers and bulbs that all work together to support native pollinators. The substrate is 25 cm (10 in) deep and is covered with a light-colored native aggregate mulch used to retain moisture by reflecting sunlight and preventing heat gain in the growing medium. This living roof is irrigated during the growing season. Photo: Bruce Dvorak

Aside from the taller forms of grasses that dominate prairies and grasslands, there are hundreds of smaller and perhaps less common native grasses and forbs that compete to diversify and add depth and dimensions to plant communities, including biodiverse living roofs. The green roof on the Berry Biodiversity Institute in Laramie, Wyoming maintains ten species of native grasses and sedges and 100 taxa of herbaceous perennials. Warm-season grasses include Blue grama (Bouteloua gracilis, little bluestem (Schizachyrium scoparium), needleleaf sedge (Carex duriuscula), prairie dropseed (Sporobolus heterolepis), and sideoats grama (Bouteloua curtipedula). Cool-season grasses include Sandberg’s bluegrass (Poa secunda), prairie junegrass (Koeleria macrantha), needle-and-thread grass (Stipa comate) Indian ricegrass (Oryzopsis hymenoides), and Western wheatgrass (Pascopyrum (Elymus) smithii). Inspirations for the selection of plants on this living roof came from the natural shortgrass prairie habitats of the Laramie River Valley ecoregion, grassland habitats found in the foothills of the Rocky Mountains, and sub-alpine habitats of the Rocky Mountains (Dvorak and Bousselot 2021).

*Festuca idahoensis*, *Elymus lanceolatus*, and Poa secunda thrive on this Montana living roof. The 21-cm-deep (8.5 in) substrate is drip-irrigated and is FLL compliant for granularmetric particle distribution and organic content. These important properties ensure that the substrate will be stable over time, retain moisture and provide the nutrients that these grasses need. The grasses here were preestablished in modules that provide instant habitat for birds and some butterflies. Poa secunda is a host to at least 20 species of butterflies in western states including *California. Photo: Bruce Dvorak*

Grasses also thrive on green roofs in the semi-arid regions of the Intermontane West. When supplemental water is available and substrate depths, slopes, and drainage systems are designed to support grasses, these habitats can support biodiversity. There must be a designated person that has experience and knowledge to watch over the green roof system, to make sure it is properly maintained, and that the maintenance plan is periodically updated.

Design Considerations for Grassed Roofs

Several important properties of grassed roofs should be discussed and considered when designing grassed roofs. The depth of substrate needs to be appropriate for the application of grasses, as grasses typically require a deep-extensive or semi-intensive system to accommodate the fibrous root system that allows grasses to survive drought, in their natural conditions. On living roofs, where substrates are shallow, grass roots grow laterally, and while they can tolerate drought in natural landscapes, they may need deeper substrates and irrigation to thrive on a living roof. The substrate depths of projects featured in this article range from 10 cm (4 in) to 30 cm (12 in). Although the additional depth of substrate materials typically requires additional weight loads on roof decks, the cost of a semi-intensive green roof is typically not drastically different from the costs of installing extensive green roofs, on new construction, and the deeper substrate provides a good return on the investment (Silva, Gomes et al. 2016).

Irrigation is typically required for grassed-based green roofs in most climates to keep plants actively growing during dry periods and to address any code requirements. Any green roof with dead or dormant vegetation could potentially become flammable; however, there are ways to prevent fire damage on buildings, including green roofs planted with grasses through fire breaks, setbacks, and irrigation practices. Harvested rainwater and greywater are sustainable sources of water for grassed roofs, especially for new construction (Dvorak and Skabelund 2021).

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This stand of low-stature and fine-textured native grasses grows on the Flight Building (office building) in the River North District of Denver. These grasses are transitioning from their bright green colors of summer to a golden bronze as they begin to prepare for winter dormancy. The semi-intensive irrigated and variable depth substrate (20-30 cm/8-12 in deep) supports big bluestem (Andropogon gerardii), blue grama (Bouteloua gracilis), Indiangrass (Sorghastrum nutans), slender wheatgrass (Elymus trachycaulus ssp. trachycaulus), switchgrass (Panicum virgatum) and little bluestem (Schizachyrium scoparium ‘The Blues’). During a spring hailstorm, *the three grassed roofs protected the roof membrane from costly damage, which was experienced elsewhere on the rooftops where green roofs were not planned. Photo: Wenk Associates*

One of the aesthetic considerations when deciding to use grasses on green roofs is plant dormancy and its maintenance and appearance at different times of the year. Some grasses grow only during the cool season and others won’t grow green until the warm season. All green roofs require maintenance. Grassed roofs need trimming or mowing of dormant vegetation at the appropriate time to prevent thatch build-up and keep the green roof in code compliance. Dormant grasses can also provide critical habitats such as structural support to serve as a larval host for some butterflies and nesting habitat for some native bees and birds. Habitat goals should be discussed during the design and development phase of living roofs so that an appropriate substrate, maintenance plan, and building façade considerations can be addressed.

Planting Roof Meadows to Recover Lost Pollinator Habitat

Eleven species of native grasses compete with native wildflowers on the largest green roof in Canada (2.5 hectares / 6.2 acres). The roof meadow includes grasses, sedges, wildflowers, bulbs, and groundcovers. The convention center also maintains beehives to keep pollinators thriving. This green roof was modeled after a coastal margin habitat, similar in structure to coastal meadows once dominant throughout the Fraser Lowlands and the Olympic Peninsula. The meadow roof is trimmed and mowed once annually to maintain the vegetation. Photo: Bruce Dvorak

Aside from the human-centric benefits that green roofs can provide (e.g., energy conservation in buildings, stormwater attenuation, aesthetics), one of the most important ecological functions of biodiverse green roofs is the potential recovery of lost habitat (Brenneisen 2006), and sometimes with remarkable results. For example, the Fraser Lowlands where the Vancouver B.C. metro region now lies was once vegetated with prairies, meadows, bogs, and shrublands. Today, the Vancouver Convention Center’s living roof is the only and largest coastal meadow downtown. The meadow roof was built over thirteen years ago (2009) and since then 250 taxa of insects have been observed on the roof, including two species of pollinating insects that were thought to be extinct in the Vancouver metro area and have now appeared on the roof meadow (two-spotted lady beetle, and a parasitic wasp) (Ratzlaff, Needham et al. 2016, Dvorak and Roehr 2021). The Vancouver Convention Center roof meadow has a 15 cm-deep (6 in) irrigated substrate (from graywater sources) and a nutrient management plan which supports the growth of the grass-based habitat that these insects are looking for.

The green roof was planted with wildflowers, bulbs, and nine taxa of grasses and sedges native to the region including bentgrass (Agrostis pallens), slimstem reed grass (Calamagrostis stricta), dense sedge (Carex densa), chamiso sedge (Carex pachystachya), Pacific dune sedge (Carex pansa), Berkeley sedge (Carex tumulicola), Idaho fescue (Festuca idahoensis), creeping red fescue (Festuca rubra), prairie junegrass (Koeleria macrantha), and Pacific meadow sedge (Carex pansa) (Dvorak and Roehr 2021). Some of the butterflies and moths that make use of these grasses include Common Ringlet (Coenonympha tullia), Sandhill Skipper (Polites sabuleti), Common Roadside-Skipper (Amblyscirtes vialis), and the Armyworm Moth (Mythimna unipuncta).

At the time the green roof was being designed, there were no coastal meadows remaining in the Vancouver area for the design team to visit and learn from. Coastal meadows had all been built over or converted to agricultural uses. However, coastal meadows are being restored on Vancouver Island and parts of Olympic National Park (Ozette Prairie) (Anderson 2009), which served as a habitat template for the design team. Because there is an acute awareness of the potential habitat benefits by the developers, the city, and design teams, there are now more than a few outstanding grass-based living roofs in the Vancouver metro area.

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Rathtrevor Beach Provincial Park is located in Parksville, British Columbia, Canada, on Vancouver Island, directly across from Vancouver, B.C. Ecologists are working to restore dunes and meadow habitats once dominant across the coastal margins. A concrete seawall that was built in the early 1970s was removed and the native coastal meadow habitat is now being restored. Educational signage is used to help inform visitors about restoration values and wildlife that is dependent upon this habitat. Several living roofs in the City of Vancouver, B.C., and on Vancouver Island have successfully used these protected and restored coastal habitats as habitat analogs for living roofs. Protected and restored natural areas help inform how living roofs can be designed to better contribute to sustaining natural habitats in cities. Photo: Bruce Dvorak

The native Idaho fescue (Festuca idahoensis) and two native sedums were added to a mixture of introduced grasses on the VanDusen Botanical Garden living roof. Idaho fescue serves as a larval food plant for Lindsey's Skipper (Hesperia lindseyi), *Sandhill Skipper (Polites sabuleti), Sonora Skipper (Polites Sonora), and many other insects. Photo: Connect Landscape Architecture*

While grasses can be very attractive for humans to look at and are very supportive of wildlife, they may need to be supplemented with other forms of plants when applied to extreme slopes or south or west-facing slopes. The dramatically sloping roof of the VanDusen Botanical Garden, for example, was initially completely covered with grasses, including its iconic cone. Over the years, the native sedums that were also planted on the roof outcompeted the grasses in these locations that tended to form drier and warmer micro-climates. Like other grassed roofs in the region, beneficial native annuals and forbs have volunteered on the roof to add to its diversity and ability to recover from extreme weather events.

Summary

When designing a living roof for a particular kind of habitat for wildlife, it is important to know that some forms of wildlife (i.e., specialists) are dependent upon a specific species of grass for its development while generalists can be served by multiple species of grasses. Local experts should be consulted to ensure that the right plants are being planted in the right place and that local codes and ordinances are met. Combining grasses with wildflowers provides habitat functions that some bees, many species of butterflies, and beetles require during different stages of their development. When planning a biodiverse green roof with grasses:

Consider that grass-based ecosystems were once the dominant vegetative cover across much of North America. Many species of birds, bees, butterflies, beetles, and other beneficial insects require grass-based habitats to survive. Grassed-based living roofs can be designed to support their needs.
With the continued advancement of lightweight substrates, rooftop meadows can now be considered a viable and affordable way for cities to recover lost habitats for pollinators and other forms of biodiversity.
Using grasses of similar heights helps to create an attractive living roof.
Planting monocultures with grasses is risky. Successful projects make use of at least 6-10 species of grasses to support a stable and resilient living roof.
Building owners should be aware of the dormancy period of grasses on their living roofs.
Building facades should be appropriately designed to reduce bird collisions. Glass facades can be designed to prevent collisions with birds.
Consult local ecologists, landscape architects, and Green Roof Professionals (GRP) to assist with the selection and design of living roofs planted with grasses.

Native vegetation, when appropriately used on living roofs, is perhaps one of the most affordable and effective ways to keep cities cooler, less prone to flash flooding, reduce CO2 emissions, and create a home for mobile forms of life that are counting on us to survive!

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Bruce Dvorak is an Associate 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

I would like to thank Charissa Wagner for arranging access and information about the living roof in Montana. I would like to thank Coy Talley for sharing information on the Perot Museum living roofs. I would like to thank Bill Wenk and Wenk Associates for sharing their photos and project information on the Flight Building living roof. Thank you also to Bruce Hemstock (PWL), who provided valuable information and access to green roofs in Vancouver, B.C., and Ken Larsson with Connect Landscape Architecture for sharing photographs, information, and access to green roofs.

Grasses: Living Roofs for Native Pollinators Part III

Grasses and Pollinators?

The “Big Four” and Butterflies

Plant a Diversity of Grasses for Greater Biodiversity

Summary

Urban Biodiversity Targets Lay the Ground for Holistic Exploitation of Rooftops in European Cities

The Biodiversity of This New York Coastal Cottage