Water and Plants for Wet, Mesic, or Xeric Green Roofs

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First Principles

In their own unique way, all things live in relation to water and sunlight (Patchett and Wilhelm 1999). This is a “first principle” for life on earth. Plants, living on green roof ecosystems, are no exception to this reality. Resilient green roofs are designed to be maintained with an agreeable pairing of vegetation with green roof substrates, sunlight, and water. Conversely, when there is a mismatch between a green roof’s intended vegetation with the actual performance and movement of water through the green roof system, there can be unexpected and unfavorable outcomes. Vegetation can diminish, perish, and or become displaced or replaced with volunteer plants that prefer those conditions.

Water for Green Roofs

The “art and science” of selecting appropriate vegetation for green roofs can often be distinguished by how well the intended vegetation grows and reproduces, competes with volunteer vegetation, and adapts to the substrate, particularly its water-holding capacity and its ability to drain. Knowledgeable and experienced green roof designers (Peck 2008) have the insight and experience to know how to finesse the design of the substrate and roof deck to support a particular plant community (Snodgrass and Snodgrass 2006). In doing so, there is an acute awareness of the slope of the roof deck, the design of the drainage system and substrate composition, substrate depth, and the delivery and management of supplemental water, nutrients, and other life-sustaining elements of green roof ecosystems (Sutton 2015). If the intended vegetation thrives on a green roof, it is because those plants are naturally adapted to similar growing conditions in their natural environments. If volunteer plants appear and build upon the intended plant community, it is because they also thrive in those conditions (Nagase, Dunnett et al. 2013).

Water can be considered for green roofs by its volume and its source. By design, some green roofs are made to sustain vegetation that is adapted to continually wet conditions while others sustain vegetation that is adapted to mesic (sometimes wet and sometimes dry) conditions. And many green roofs are planted with vegetation that is naturally adapted to xeric or dry environments. The source of water for plants on green roofs can also be a critical factor in the success of plant life. Most green roofs are watered from one of several sources: atmospheric water (rain, snow), municipal water sources, harvested rainwater, or some form of recycled or greywater. This article explores how plants have been selected for green roofs based on their preferences for water.

Precipitation Only Please

In favorable climates, some plants can thrive on green roofs without irrigation. In climates with cool summers, and reliable rainfall during the growing season, it is possible that plants can endure short-term droughts or above-normal temperatures for short periods without permanent automated irrigation systems. Shallow extensive green roof substrates, however, have limited capacity to keep plant roots cool and store enough moisture to be used by plants. Local plant guides and published research can be useful resources to identify substrate depths and substrate designs for green roofs that endure without supplemental watering. It is generally necessary to provide access to water, for the temporary watering of green roofs, even those designed to sustain vegetation without irrigation. Even climates that typically provide reliable rainfall during the growing season can experience drought. A hose bib or some source of water should be provided so that temporary irrigation can be applied when needed.

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*This Rocky Mountain rooftop meadow is watered primarily through precipitation. Photo: Bruce Dvorak*

For example, vegetation living on this Rocky Mountain green roof thrives without a permanent irrigation system. An attentive occupant irrigates the vegetation from a spigot provided on the rooftop, only when necessary. The gently sloped roof deck and variable depth and custom blended substrate (15-17 cm deep) provide sufficient moisture to keep this plant community alive and well since 2009 (Dvorak and Bousselot 2021). The growing medium is a custom blend with some native soil mixed in media. The vegetation on this living roof was inspired by the adjacent mountain meadows. It features nine taxa of grasses including big bluegrass (Poa ampla), blue wildrye (Elymus glaucus), mountain bromegrass (Bromus marginatus Nees), prairie junegrass (Koeleria macrantha), Rocky Mountain fescue (Festuca saximontana), sandberg bluegrass (Poa sandbergii), slender wheatgrass (Elymus trachycaulus), streambank wheatgrass (Elymus lanceolatus), and tufted hairgrass (Deschampsia cespitosa). Herbaceous perennials include Black-eyed Susan (Rudbeckia hirta), blue flax (Linum lewisii), fringed sage (Atemisia frigida), gaillardia (Gaillardia aristata), scarlet gilia (Ipomopsis aggregata).

The green roof vegetation has adapted to the substrate and roof deck, through a cultural relationship established between the owner and the plants. The plant community has survived and regenerated for over a decade, because the owner understands that this living roof periodically needs some additional watering, beyond what falls from the sky during droughts or short-term heat waves.

Greywater for Wet Plant Communities

Vegetation that is naturally adapted to wet environments is typically found living on level or very poorly drained soils. Low-sloped environments with heavy soils allow precipitation to pool and slowly percolate into the ground over days, weeks, or months. Vegetation that is naturally adapted to these environments forms plant communities such as wetlands, wet meadows, marshes, or other hydric habitats. In addition to water, the roots of plants in wet ecosystems absorb available nutrients in wetlands such as nitrogen, phosphorus, and other elements. This means that wetland plants can be used to clean up water, even on rooftops!

This diagram of the greywater system and wetland green roof is located inside the building and is part of the education and outreach program provided by the developer (Photo: Bruce Dvorak). Photo of wetland green roof Phillip Thompson

More than a few green roofs are installed on flat or very low-sloped roof decks. Since level or very low-sloped roof decks pose a special condition (ASTM E2777 14 2014), special attention is needed to provide a healthy plant community, and more than a few designers have intentionally selected wetland plant communities for them. This includes wetland plants to perform special functions that have been integrated into the daily functions of the building, such as soaking up excess nutrients from effluent greywater sourced from inside the building.

The green roof on the Bullitt Center office building in Seattle, Washington, acquires all its potable water from the sky and stores it in tanks for later use for drinking, showering, and washing. The wastewater system was designed with a wetland green roof to also clean effluent greywater from inside the building, such as from sinks, showers, and dishwashers. The system was designed to collect water in tanks and to be treated by plants grown in a special growing medium. Two species of native evergreen plants were selected to soak up nutrients and clean up wastewater, Equisetum arvense (common horsetail) and Juncus (bulrush). This function, combined with others, allowed the building to be disconnected from the municipal wastewater system (through special permitting and maintenance). The flat roof deck and reservoir function as a constructed rooftop wetland that cycles wastewater through the wetland vegetation until it is ready to be released into a rain garden below, at the street level (Dvorak and Rottle 2021).

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*Anaphalis margaritacea thrives on the Vancouver Convention Center roof. Photo: Bruce Dvorak*

On a much larger scale (2.4 hectares, 6 acres), the green roof on the Vancouver Convention Center in Vancouver, B.C. Canada also treats water from sinks, showers, and other uses inside the building. Oversized cisterns retain wastewater in the lower levels of the building, stored there for pre-treatment and cycling through the rooftop meadow. This enormous green roof has 65 irrigation zones and eight moisture sensors that regulate when the roof needs to be watered. The vegetation typically requires irrigation during the summer. Moisture sensors are set at about a 15 percent volumetric threshold for a minimum level of moisture to trigger the irrigation system to operate. Twenty-four species of native plants were selected to grow across a wide range of roof deck slopes, from 3 to 56 percent. The flatter sections of the roof accept and retain water consistently. The result is a green roof that performs much like a biodiverse meadow with wet, mesic, and dry microclimates (Dvorak and Roehr 2021).

The native Western pearly everlasting (Anaphalis margaritacea) is a welcomed volunteer species on the Vancouver Convention Center roof meadow. The leaves of the Western pearly everlasting attract the American painted lady butterfly (Vanessa virginiensis) and its white puffy flowers attract a host of pollinators that frequent the roof. Thus, the recycled water is routed from inside the building and up onto the roof to become absorbed by the roots of the meadow’s vegetation. With this water, plants grow and serve the many mobile users of the rooftop ecosystem such as pollinators, birds, and insects.

Mesic Plant Communities

*This low-slope roof deck at the Kansas City Public Library is planted with vegetation that is tolerant of wet and dry conditions. Photo: Bruce Dvorak*

This rooftop prairie vegetation on the Kansas City Public Library has thrived since its establishment in 2004 and has over 31 taxa of plants that prefer mesic to dry conditions. Water for irrigation is sourced from potable water lines located inside the building, and they provide moisture to the roots of plants at the bottom of the 15 cm (6 in) deep FLL-compliant substrate. Irrigation runs daily during the growing season. Purple prairie clover (Dalea purpurea) is native to mesic prairies and its deep roots allow it to adapt to frequent drought in prairie landscapes. On this green roof, it needs consistent watering to thrive and survive. The renovated building was originally constructed in 1906. Therefore, potable water was the most efficient path to provide water for plants. Clusters of white heath aster (Aster ericoides) can be seen growing in the background and are native to mesic to dry prairies. These accent perennials grow along with companion grasses blue grama (Bouteloua gracilis), little bluestem (Schizachyrium scoparium), and sideoats grama (Bouteloua curtipendula), which grow in a range of environments from mesic to dry (Dvorak and Skabelund 2021).

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Xeric (Dry) Plant Communities

Some green roofs are designed for semi-arid climates, in locations that experience infrequent rainfall and extremes in temperatures. The green roof (shown here) on the Museum of Northern Arizona, in Flagstaff, Arizona was built in 2009. It is a LEED-platinum-certified building that retains collections of artifacts from indigenous cultures. The diverse planting on the green roof includes over 32 native species, including annuals, perennials, grasses, and succulents.

*This moderately-sloped roof deck on the Museum of Northern Arizona is planted with drought-adapted vegetation. Photo Bruce Dvorak*

The ponderosa pine ecoregion remains dry most of the year, except for winter to early spring and the monsoon season which is July to August. A large cistern that is located at the ground level (inset) provides the only source of supplemental water when the weather turns dry and hot. A monsoon shower is shown here in the background. The occurrence of monsoon precipitation is erratic and unpredictable. Although rain fell less than 1 kilometer away from the building, no rain fell on this roof that day. However, rain did fall the previous week on this roof, thus some plants had begun to emerge from dormancy. The museum staff that maintains the roof observes that plant species and population dynamics change over the years due to variations in precipitation, including winter snowfall.

With a 15-cm deep substrate, designers included a wide range of plants that are naturally adapted to a variety of conditions, and that diversity benefits the green roof (Dvorak and Coseo 2021). During drought years, species that prefer those conditions thrive, while during wet years, those species also thrive and spread on the roof. The vegetation was plugged and seeded into biodegradable trays, with a substrate made from pumice, sand, and local sources of compost (pine bark). A shallow layer of the same substrate was laid below the trays to support root growth and retain moisture. Over time, the species population dynamics have changed, based on the available moisture and their position on the roof deck such as the top of the slope versus the bottom of the slope where there is more residual moisture in the substrate.

*These succulents prefer dry substrates. The moderate slope of this roof deck at the Multnomah County Library evacuates moisture and prevents root rot. Photo: Bruce Dvorak*

In 2008, this sedum-based green roof was planted on the Multnomah County Library green roof with vegetation that is native to well-drained habitats in the mountains of the Pacific Northwest. These sedums thrive here, in part because the roof deck slope is well-drained. These sedums don’t naturally grow in flat or low-slope environments; thus, they prefer to not live in saturated conditions. This retrofit roof is on a building that was constructed in 1913, originally without a green roof. Even though Portland receives an abundance of rainfall during the fall and winter months, this green roof vegetation does not experience standing water, as the aggressive slopes drain water away quickly, which is important for these succulents to survive. Sedums that are native to the Pacific Northwest include broadleaf stonecrop (Sedum spathulifolium), Oregon stonecrop (Sedum oreganum), and Pacific stonecrop (Sedum divergens). Introduced species include Sedum hispanicum, Sedum album 'Coral Carpet', Sedum kamptschaticum, Sedum spurium ‘Dragons Blood, and Sedum rupestre 'Angelina' (Dvorak and Starry 2021).

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Managing Extremes

Sometimes roof decks present special challenges for managing water and vegetation on green roofs. When roof decks are flat (level), very steep, or combine a variety of contrasting conditions, the design of the substrate, drainage system, water retention, and supplemental watering must align with the water and drainage needs of the plants selected for any roof deck environment.

This building has a flat upper roof deck (not shown here), combined with a moderately sloped section, and a roof saddle, where all the water from the upper sections of the roof drains (shown here). When I visited this roof, the green roof had large areas on the very flat and aggressively sloped roof decks that had only a few of the original plants remaining. The green roof also has areas with ponded water due to level slopes, however, it was planted with vegetation that thrives in well-drained habitats. There were also areas where not enough water was being retained on steep slopes for the species of plants selected. Part of the issue is that there was a low diverse mix of drought-adapted sedums that were planted across multiple roof deck slopes ranging from level to very steep, without adjustments to the substrate, drainage, and irrigation design. There was a single substrate design combined with large irrigation zones that covered different watering needs. Some of the plants were overwatered and some were underwatered. These sedums do not go dormant during drought or recover from extreme weather events. The “art and science” of the management of water on green roofs are a significant issue for the success of vegetation. In climates where green roofs don’t have a long trajectory of research and development, it should be anticipated that some redesign or replanting may be needed to help build up a knowledge base in regions where green roofs are an emerging technology (Dvorak and Skabelund 2021).

Summary

A single-zoned irrigation system can provide too little or too much water and make it difficult for site managers to adjust for the watering needs of different forms of vegetation in the same zone. Zoned irrigation allows for variation in watering frequency and duration for vegetation that has different needs. Photo: Bruce Dvorak

The summer of 2022 has been marked by an extreme lack of water or extreme abundance of water across much of North America and parts of Europe. This means that green roofs that survive these extreme conditions of water abundance or absence will be tested for their resilience. To best attain ecosystem services from green roofs, the plant communities need to be selected to thrive on the roof deck environments through periods of drought or deluge. This article explores several mature green roofs where the vegetation is well-suited and proven to thrive on the designed roof environment based on their relationships with water and sunlight, and one example of a green roof that requires some refinement in the design of the water delivery and retention.

An integrated design process was used for many of the successful green roofs in this article. This means that input from architects, landscape architects, and building maintenance managers was considered well before the completion of the building and green roof design. This means that green roofs can be adapted to almost any climate, given that there is a resource base for water and plants, and a knowledgeable industry that is willing to expand and explore possibilities.

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All green roofs need water, some more than others. From the examples explored in this article, several conclusions can be made:

Flat or low-slope roof decks are special conditions. Flat roofs are slow draining, and plants from wetlands or wet meadows can be used and can perform vital ecosystem services for buildings and their multiple users, including wildlife. When drought-adapted vegetation (i.e., sedums) is placed on flat, or very low-sloped roof decks, special measures may be needed to keep the roots dry and prevent grasses and other moisture-loving weeds from invading the roof.
Plants from mesic plant communities are well-suited for green roofs because these plants are adapted to environments that are sometimes wet and sometimes dry. Roof decks with at least a 2 percent slope provide opportunities for mesic plant communities, along with 10 cm to 15-cm-deep substrates and a reliable source of supplemental irrigation.
Roof decks with moderate or aggressive slopes may need more than a simple monoculture to keep an active vegetative cover. Different forms of plants such as annuals, bulbs, perennials, and succulents should be used in their preferred microclimates.
Green roofs can be designed to provide supplemental water from municipal sources, harvested water, or reclaimed or greywater sources. Each method has its unique requirements, limitations, and maintenance requirements.
In some climates, green roofs can be designed to sustain vegetation without permanent irrigation systems. In such installations, a reliable water source should be located within reach of the roof for temporary supplemental watering during extended droughts or extreme heat events.
The substrate depth in most of the examples in this article is considered semi-intensive. As there is no single substrate design to address the needs of these three distinctive designs, experts and local substrate providers should be consulted for the best application.
Local green roof professionals should be consulted to make wise choices about which plants to consider for buildings or structures where green roofs are part of the site and building development goals. No two green roofs are identical, and local expertise may be necessary to provide guidance and recommendations to make appropriate plant recommendations for the design goals of the project.

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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. Come to Cities Alive in Philadelphia for the book signing of Ecoregional Green Roofs October 17 during Lunch on the Trade Show Floor. Several co-authors will be present, and a limited supply of books will be available on site, but a 20% discount and free shipping will also be provided by Springer for book orders.

Acknowledgments: I would like to thank Andy Creath for arranging access and information about the green roof in Colorado. I would like to thank Elizabeth Morris and Alan Proffitt for a site visit and for sharing information on the Multnomah County Library eco roofs. I would like to thank Dr. Jan Busco with the Museum of Northern Arizona, for sharing information and the site visit. Thank you also to Bruce Hemstock (PWL), who provided valuable information and access to green roofs in Vancouver, B.C. Special thanks to Phillip Thompson, who shared information about the Bullitt Center green roof.

Water and Plants for Wet, Mesic, or Xeric Green Roofs

First Principles

Water for Green Roofs

Precipitation Only Please

Greywater for Wet Plant Communities

Mesic Plant Communities

Xeric (Dry) Plant Communities

Managing Extremes

Summary

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