Planting Green Roofs with Native Shrubs

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Green roofs grow in almost every climate on earth. While a green roof is a universal concept, green roofs are most beneficial when they are planted with flora that have a long history in the region. After all, it is the native vegetation that the native fauna is in search of to feed, rest, breed, and nest. For these reasons, it makes a lot of ecological sense to make use of the regional vegetation on green roofs.

Steppes and Scrublands

Seacliff buckwheat hybred (Eriogonum parvifolium) (white blooms) and manzanita (in the foreground) are on display at the One Van Ness building demonstration green roof in downtown San Francisco. Low-growing evergreen shrubs like these are mixed in with native forbs and grasses to demonstrate how plants native to coastal California plant communities grow in substrates depths of 15 cm (6 in) as shown here. Photo: Bruce Dvorak

However, what if the native vegetation is dominated by a suite of low-growing shrubs associated with steppe and scrubland habitats, such as those found in semi-arid climates? In temperate climates, a typical extensive green roof has shallow substrates (7.6-12.7 c’, 3-5”). However, in hot and dry regions, the viable depth of substrates for green roofs is typically deeper (15-30 cm; 6-12”) to match the depth of the root systems of native plants (Dvorak and Skabelund 2021). In North America, the coastal regions of central and southern California, the valley basins of the Desert Southwest and the Intermountain semi-arid basins, and the steppes of the Great Plains are all such places. These are vast landscapes with an abundance of natural habitats known as a “scrub,” which can include plants such as sagebrush, rabbitbrush, bunch grasses, annuals, geophytes (bulbs, corms, tubers, etc.), and low-growing succulents. Although some of these plants may appear wild in their form and texture, with a little bit of innovation and intuition, some of these hardy and wild “shrubby” plants have been made to flourish on green roofs. In this article, shrubs and low-growing woody plants are explored for their innovative use on green roofs.

Shrubs are not the typical form of plant one thinks of when considering vegetation for green roofs. However, many species of shrubs are well-adapted to rocky soils, climates with hot summers and cold winters, intermittent precipitation, snow, and constant wind. These are climates where trees typically cannot survive. Shrubs are well-suited to the exposed conditions often found on rooftops as they experience direct or reflected sunlight, wind, rapid heating, or cooling, and or the drying of substrates. These plants possess drought adaptation strategies and are therefore able to take advantage of limited precipitation and hot diurnal temperatures (Dvorak and Bousselot 2021).

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Sagebrush, grasses, and a diversity of wildflowers carpet the valley floor between the mountains in northern Wyoming protected within the boundaries of the Greater Yellowstone Ecosystem. These plants handle tough growing environments where shallow, well-drained and low-nutrient soils are present. Wild environments such as these can teach about potential plant associations, textures, and seasonal floral interests of shrublands. From Phoenix, Arizona to Calgary, Alberta, from Denver, Colorado to Southern California, urban centers continue to expand into semi-arid regions, and green roofs have great potential to serve buildings in such environments by keeping buildings cool inside, reduce energy use, soak up carbon and precipitation, and reduce urban heat islands. Photo: Bruce Dvorak

Interestingly, shrubs are more abundant, found in more kinds of ecosystems than trees. Ecologically, shrubs play a very important role in the functioning of many ecosystems. Shrubs can be in evergreen or deciduous form, can provide important fruits and flowers for wildlife, and shrubs are resilient, often able to recover from extreme climate events, whereas trees may never recover (Götmark, Götmark et al. 2016). For these and many other reasons, shrubs have a role on green roofs and are recognized as important contributors of green roof systems in Europe as they are considered equals with other forms of plants in the FLL Guidelines for Green Roofing (FLL 2018).

Shrub-based Green Roofs

A view to the shrub-based green roof from the conference room of the Moda building in Bend, Oregon. Visual contact with nature is important for human health and productivity. Occupants of this space are surrounded by a shrub-based green roof inspired by high-plains steppe vegetation. Photo: Bruce Dvorak

Bend, Oregon is located at the intersection of the high plains of central Oregon and lies within the rain shadow of the Cascade Mountains. Its urban footprint is one of the fastest growing in the U.S. (Stebbins 2022). Precipitation averages 280 mm (11.2 in) annually, including 60.5 cm (23.8 in) of snowfall. The native plant communities are rich and diverse ranging from the typical basin and range sagebrush communities to higher-elevation pine savannas, rocky fields, and riparian habitats along major drainages. Dr. Richard (Rick) Martinson, co-owner of WinterCreek Restoration and Nursery of Bend, Oregon grew up surrounded by the native vegetation of central Oregon. He studied it, learned about ecological restoration, and now works to protect it through education and restoration work. He grows native plants and educates widely about native landscapes and promotes environmental stewardship. When the opportunity came for his participation in a green roof project for the Moda Health Services building in Bend, Rick had a clear vision for a different kind of green roof (Dvorak and Woodfin 2021). This vision was guided by his years of experiences with and knowledge of native plants growing in the wild, years of propagating native plants, and studying the results of restoration plans taking shape on the land. Although he knew what kind of roof garden could be possible for the region, he had not seen any such built example. It was time to innovate and with the right collaborations and a client willing to take a risk, his persistence and knowledge of wild plants paid off.

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The green roof on the Moda Building in Bend, Oregon is innovative as it is a first of its kind prototype for making semi-intensive green roofs with native shrubs, grasses, and wildflowers (Dvorak and Woodfin 2021). Built in 2007 the LEED Gold-certified building makes use of the green roof to reduce energy use, minimize urban heat islands, and soak up stormwater runoff and much more. A majority of the 884 m2 (9500 ft2) green roof is planted in substrates that range between 20 cm (8 in) in the grass habitats, and 40 cm (16 in) in herb/shrub areas and up to 76 cm (30 in) in isolated locations to support a few trees. The growing media was locally blended at WinterCreek Nursery and was made from locally sourced materials. Soil composition accounted for plants that adapt to similar soil moisture regimes in the high desert plains soils. To supercharge the plant growth, the substrate includes soil fungal to establish bacterial communities and make a self-sustaining system by recycling the soil nutrients. A subgrade irrigation system supplies water to the roots of plants, and irrigation zones are designed to deliver water at a rate and frequency that is based on the needs of the plants. The roof garden receives monthly maintenance, to keep aggressive invasive plants off the garden, and to maintain vegetation.

*An ADA accessible path cuts through the scrub habitat which sits over office space underneath the green roof. A skylight penetrates the planting to allow natural daylight below. Photo: Bruce Dvorak*

The overall visual aesthetic of the green roof when walking through it is an amazing blend between a wild landscape and a xeriscape garden. Martinson used inspiration from the natural juxtaposition and association of plants in the wild to arrange plants on the green roof by their texture, color, form, and size. From a small paved outdoor gathering space near the garden entrance, a single path leads one to explore the different ecological niches of the shrub-based green roof. The vegetation is the show here, featuring plants that some people may write off as “gangly” “awkward” or “unsophisticated”. When I visited the roof in September of 2018, I had traveled across thousands of miles of semi-arid landscapes of the western US. Before I visited the Moda building, I had seen all of the plants used on this roof garden in the wild, but I never imagined how they could be brought together in such a dynamic and beautiful assembly.

*Pockets of diverse herbs and grasses dot the scrub habitat and provide hotspots where native fauna is served. Photo: Bruce Dvorak*

Shrubs on the roof include big sagebrush (Artemesia tridentata), cliff spiraea (Holodiscus microphyllus), desert sweet (Chamaebatiaria millifolium), gray rabbitbrush (Ericameria nauseosus), green rabbitbrush (Chrysothamnus viscidiflorus), manzanita (Arctostaphylos patula), and prostrate Oregon grape (Mahonia repens). Native grasses include Idaho fescue (Festuca idahoensis), and prairie junegrass (Koeleria cristata) and groundcovers include kinnikinnick (Arctostaphylos uva-ursi), and strawberry (Fragaria sp.). The floral colors and textures are provided to serve wildlife, these are clustered between major shrub groupings and include pussytoes (Antennaria microphylla), linear-leaf fleabane (Erigeron linearis), naked buckwheat (Eriogonum nudum), sulphur buckwheat (Eriogonum umbellatum), cushion buckwheat (Eroigonum caespitosum), California poppy (Eschscholtzia californica), scarlet gilia (Ipomopsis aggregate), blue flax (Linum lewisii), sickle keeled lupine (Lupinus albicaulis), lowly penstemon (Penstemon humulis), pine mat penstemon (Penstemon pinnifolius), Rocky Mountain penstemon (Penstemon strictus).

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During a workday, these habitats provide a restful scene from inside the building for occupants and visitors to view. These habitats also serve bees, insects, chipmunks, monarch butterflies, hummingbirds, chickadees, finches, blackbirds, pine siskins, and robins. The habitats illustrate the value of selecting the right plants and in the right conditions for them to grow. The success of the roof’s vegetation demonstrates how knowing the conditions, forms, textures and living environments of the vegetation greatly informs the making of a suitable substrate and maintenance practices. The approach to the planting design for this green roof ignored the traditional route which would favor a mixture of ornamental exotics, or mass assemblages of cultivars of native plants that may no longer attract pollinators, if too far removed from the characteristics of local species. This green roof is ecologically useful, because it was designed from a universal ideal, adapted with knowledge of local ecosystems and a great sense of creativity and innovation.

The native bunchgrass prairie junegrass (Koeleria cristata) thrives on the Moda building green roof with plenty of substrate (20.23 cm/8 inches) and open space to reach its full height and breadth (upper left). The yellow blooms of rabbitbrush (Ericameria nauseosa) put on a cheerful show during the late summer and into the fall (upper right). A lone native tree snag is strategically placed to support biodiversity (lower left). Birds visit the branches to take a rest or to scope out insects. Spiders settle in to build webs, and moss and fungi grow on branches and return nutrients and micronutrients to the green roof substrate. The silver blooms of sagebrush (Artimesia tridentata) stand in contrast to the dark green foliage of manzana shrubs seen in the background (lower right). Photo: Bruce Dvorak

Shrub Awareness

Like many forms of plants (vines, forbs, grasses), some native shrubs can be toxic or flammable. One toxic shrub common to the western U.S. is horsebrush (Tetradymia glabrata, T. canescens). It has attractive foliage and bright yellow flowers, but the plant is toxic to livestock. This means that the context of the roof and its proximity to wild or domesticated animals should be considered. Like many forms of plants, some shrubs are flammable. These include chamis/greasewood (Adenostoma fasciculatum), california sagebrush (Artemesia californica), coyote brush (Baccharis pilularis consanguinea), buckwheat (Erigonum spp.) and others. Knowledge of the toxicity and flammability of any plant is important to consider when selecting vegetation for green roofs.

Plants with these characteristics can still be very appropriate and valuable on green roofs, however, the environmental conditions and context of the green roof must be considered by the design team, prior to their use. For example, many species of Asclepias (milkweeds) are planted in butterfly gardens to attract monarch butterflies and are planted on green roofs across North America. The white sap contains cardiac glycosides, which is toxic to many animals and humans. Regardless, when used and maintained appropriately, such plants may not pose a risk to the public.

Nature Awareness and Innovation

While planting shrubs on green roofs is not a new concept, knowledge of the potential benefits of including shrubs on green roofs is sparse in the literature. Green roof research is needed to examine and publish outcomes of green roofs, including the performance of shrubs on green roofs. One of the major observations summed up in Ecoregional Green Roofs is that there are many amazing projects where native vegetation is used on green roofs, including shrubs. However, there are few people studying the multiple dynamics of shrubs on green roofs, their ecosystem services and biophilic benefits for people (Dvorak and Skabelund 2021).

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Oregon grape (Mahonia aquifolium) (upper left) is planted into a semi-intensive green roof in Bellevue, Washington. (upper right) White sage (Salvia apiana) grows on a green roof at the NOAA Southwest Fisheries center in La Jola, California. When in bloom, it feeds hummingbirds and fills the air with an herbaceous scent. (lower-left) A lone hairy manzanita (Arctostaphylos columbiana) was planted at the Heron’s Head Park visitor’s center green roof. The substrate was mounded at this location to support the shrub’s root depth and mass. And at the California Academy of Sciences green roof (lower right), a collection of woody perennials and woody shrubs grow together. These shrubs were added after the initial planting, with some additional substrate added at strategic locations that could support the additional structural load and weight of the deeper substrate.

In his seminal book, Last Child in The Woods, Author Richard Louv said, “The future will belong to the nature smart - those individuals, families, businesses, and political leaders who develop a deeper understanding of the transformative power of the natural world and who balance the virtual with the real. The more high-tech we become, the more nature we need (2005).” Perhaps, with the making of more green roofs with the regional vegetation, including shrubs, green roofs with native plants will come to influence the next generation to be more curious about plants. To do this we need more people today willing to innovate on green roofs and grow a greater appreciation for nature, in its regional forms.

Summary

Although shrubs are not appropriate for all forms of green roofs, green roofs with sustainable sources of irrigation and appropriate substrate depths should consider the inclusion of shrubs. Through the examples provided here, it is clear that when appropriately designed, shrubs are viable on green roofs, especially in climates where shrubs are ubiquitous in nature.

In some climates, the native and regionally adapted shrubs may be a best choice to include on green roofs.
Shrubs typically require semi-intensive substrates and supplemental watering to sustain them.
Shrubs can be added to extensive green roofs in strategic locations, where structural support is sufficient. An appropriately placed shrub can add valuable habitat diversity and complexity to expand the habitat value of a green roof.
Most shrubs cast shade and keep substrates cool. Shrubs can be used to define the edges or create protective edges.
Many forms of shrubs are resilient and their multi-stem habits allow them to recover from extreme weather events.
Many shrubs offer outstanding visual interests including, color, texture, and form.

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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.

Acknowledgments

Thank you to Dr. Rick Martinson of WinterCreek Nursery for providing a tour, reviewing materials for the Moda Health Building case study in Ecoregional Green Roofs. And thank you to Karen Theodore of WinterCreek Nursery for providing information about wildlife use of the Moda green roof.

Project Team

Building Owner/Client: Moda Health Services

Green Roof Design Team Lead: WinterCreek Restoration & Nursery

Architect: GBD, Russ Hale, AIA

Structural Engineer: W&H Pacific

Landscape Architect: W&H Pacific, Chelsea Schneider

Installation Contractor: WinterCreek Restoration & Nursery (Rick Martinson, Ph.D.)

Planting Green Roofs with Native Shrubs

Steppes and Scrublands

Shrub-based Green Roofs

Shrub Awareness

Nature Awareness and Innovation

Summary

A New Blue-Green Roof Solution: LiveRoof RoofBlue® RETAIN™

Ed Snodgrass, 5th Generation Farmer and Green Roof Pioneer