Designing Green Roofs for Dragonflies
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Ponded water, stones and wetland vegetation were incorporated into Chicago’s first wetland green roof at the Peggy Notebaert Nature Museum green roof display in Chicago, Illinois. The wetland has twenty-eight species including plants from the genera: Acorus, Asclepias, Carex, Iris, Juncus, Sparganium, Spartina, Pontederia, Sagittaria, Scirpus, and others. Photo: Conservation Design Forum (2004)
When Biodiversity is the Client
One of the surest signs that summer has arrived is the emergence of dragonflies. Although one may not typically think of dragonflies and green roofs, the presence of dragonflies on green roofs means that there is perhaps a freshwater source nearby and a diverse population of pollinating plants. Researchers have long acknowledged the viability of green roofs in support of dragonflies (Grant 2007, MacIvor and Lundholm 2011), and citizen scientists are beginning to confirm it. For example, dragonflies have been observed on green roofs in Calgary, Alberta, Canada (Struck, Ross et al. 2014), and by citizen scientists on iNaturalist. Volunteers maintaining the California Academy of Sciences green roof have observed an Eight-spotted Skimmer (Libellula forensic) “sawtelle (09-08-2018)”, and at the Peggy Notebaert Nature Museum green roof exhibit “Joelle (07-08-2016)” observed a Blue Dasher (Pachydiplax longipennis).
Dragonflies fall within the order of flying insects called Odonata. Worldwide there are at least 3,000 species of dragonflies, with the greatest diversity located in tropical regions. Dragonflies have been around for over 300 million years, and this makes them one of the oldest and most adaptable insects on Earth. Some of North America’s over 500 species of dragonflies are local to specific habitats, whereas others are generalists, and some species of dragonflies even migrate (May 2013). Dragonflies are predatory carnivorous insects that prey on small flying insects such as mosquitoes, gnats, midges, other dragonflies even sometimes small birds such as sparrows or hummingbirds. Dragonflies are agile creatures that can adapt to human habitats quite well, especially where still water and aquatic vegetation are present. In this Summer 2023 issue of Living Architecture Monitor, I will focus on how you can make green roofs habitable for dragonflies.
A male Blue Skimmer (left) and male Neon Skimmer (right) perch on a remnant stem of the native Maximilian sunflower (Helianthus maximiliani) at the Forster Ndubisi Prairie at the Schob Nature Preserve in College Station, Texas. An adjacent wooded stream is a perfect habitat for dragonfly nymphs, however; the prairie installed by landscape architecture students at Texas A&M University completed the habitat requirements for dragonflies in-part by seeding the half-acre prairie in 2015. During the daytime, dragonflies scan the prairie in search of a meal. Sunflowers are tall, attract insects and grow on green roofs in the central and western United States on deep-extensive and semi-intensive green roofs (Dvorak and Skabelund 2021). Prairie species, including Maximillian sunflower, have also been seeded on an experimental green roof on the Langford building at Texas A&M University in hopes that it will attract a diversity of insects, including dragonflies. Photos: Dwight Bohlmeyer (2021-2020)
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Since 1998, a wetland and pond has occupied space above a city street at Potsdamer Platz in Berlin, Germany. This blue green roof includes water, upright vegetation, and stones. Three elements that provide the ideal habitat for dragonflies. Photo: Bruce Dvorak (2013)
It’s all About the Habitat
Dragonflies can be attracted to green roofs when they include water, diverse forms of vegetation, and small stones or pebbles. Green roofs can attract insects that dragonflies feed on if the previous year’s vegetative growth is allowed to remain on the roof to decompose and recycle its nutrients into the substrate. Ponded water and aquatic or wetland plants will also help attract dragonflies. Dragonflies can complete their life cycle on green roofs when there is a permanent water body built into a green roof system and it includes stones, for females to lay eggs in the water and for emerging larvae to hide during their growth stage. A green roof would also need upright forms of aquatic or emergent wetland plants with stems that allow dragonflies a place to perch. What is planted in and around a green roof is critical to making and keeping dragonflies around. Dragonflies in their youth stage can live from several months to several years submerged in water. Once nymphs are ready to molt their nymph skin, they need an upright stone, or dry plant stem to climb up out of the water and complete their transformation to an adult. Once their wings, legs and exoskeleton emerge and are hardened, they prepare for flight, and begin to scan the environment for a meal. Most adult dragonflies live for at least one week, some live up to a few months and some species can live up to a year.
One of several blue-green roofs at the Gates Foundation Headquarters in Seattle, Washington. Previously, the site was a 12-acre (4.5 hectare) parking lot. Now vehicular parking lies below this wetland, and wetlands once again return. The site, in its previously undeveloped condition, included a natural spring, bog and wetland vegetation. Through green roof technology, historic habitats have been returned, including multiple habitats for dragonflies and much more. Photo: Bruce Dvorak (2018)
Water is Not Required
Dragonflies can travel some distance away from water sources in search of insects. So, if there is water in the vicinity of a green roof, some species of dragonflies may find a green roof regardless of if there is visible water on the roof or not. The habitat structure of vegetation surrounding a building can be important to support dragonflies, as dragonflies can be sensitive to the habitat changes when invasive vegetation can reduce open habitat structure by shading out understory vegetation that dragonflies can use. Developers and designers should keep native trees, shrubs, grasses and wildflowers of existing sites, especially those near water (Samways and Sharratt 2010).
The 5.4 acres of rooftop botanical gardens on the Transbay (Salesforce) Transit Center in San Francisco features native and climate adaptable non-invasive exotic plants. There is a small subsurface wetland planting that is used to filter greywater from inside the building. There is no visible or standing water at the surface. In the absence of any freshwater at the surface of the green roof, Blue-eyed Darners (Rhionaeschna multicolor) have been observed on the green roofs (iNaturalist June 26, 2021, and July 13, 2022). This means that no visible water is necessary to attract dragonflies to green roofs, even in dense urban cores, if there is dragonfly habitat nearby. However, a green roof would need pollinating plants to attract a food source for dragonflies.
Diverse plantings make up the Fog and Wind Garden at the Transbay rooftop botanical gardens. The gardens display plants native to the San Francisco bay area, and parts of New Zealand and Australia share similar climates and grow together here in a semi-intensive green roof (Dvorak and Drennan 2021). Sea lavender (Limonium perezii) is shown blooming in the foreground, and Pride of Madeira (Echium candicans) is the bluish-purple spikes. One of the dragonfly observations was not far from this garden (May 2022). Photo: Bruce Dvorak (2019)
Plant for Color and Contrast
Planting for dragonflies means planting for color and contrast. Colorful flowers and upright grasses can attract flying insects. In well-drained locations on a green roof, plant sunflowers (Helianthus ssp.), yarrow (Achillea ssp.), black-eyed Susan’s (Rudbeckia hirta), sweet coneflower (Rudbeckia subtomentosa), and Joe-Pye weed (Eupatorium purpureum). Contrast in habitat means including some water, with aquatic plants. In submerged aquatic habitats plant fanwort (Cabomba caroliniana), arrowhead (Sagittaria latifolia), native water lilies, water horsetail (Equisetum fluviatile) as well as native rushes (Juncus ssp.) and sedges (Carex ssp.). Use a gravel mulch on top of substrates made for aquatic conditions to help keep water clear. Plant swamp milkweed (Asclepias incarnata) at the margins of open water. These plants support dragonflies during their nymph, emergent and adult stage (Malkin 2021). While many of these plants are native to much of North America, check for local forms of similar plants if these are not hardy to your region.
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Summary
The presence of dragonflies is an indicator of a healthy ecosystem. If water is nearby or is included on a green roof ecosystem, then dragonflies may set up residence. Dragonflies are beneficial consumers in ecosystems and help keep green roof insect populations under control. To attract dragonflies to green roofs:
Arrowhead (Sagittaria spp.) plants thrive at the water’s edge. Arrowhead can make good habitat for dragonfly nymphs to mature, and its upright stems provide a place for nymphs to molt and perch. Dragonflies help keep mosquito populations under control, as a single dragonfly can eat up to 100 mosquitoes a day. Photo: Bruce Dvorak (2023)
Plant colorful pollinating plants that attract other insects from summer to fall. Dragonflies can eat insects as small as gnats and as large as other dragonflies. Some dragonflies have been known to pursue small birds.
On a green roof, shallow water and wetland or aquatic plants can attract dragonflies. Small puddles, ponds and even large aquatic habitats can be supported on structures. The inclusion of still water will attract other insects and dragonflies.
To make habitat for dragonflies on a structure above grade, ponded freshwater sources must be included along with stones or pebbles in a variety of sizes and shapes, contrasting forms of vegetation, and depths of water that won’t freeze during winter. Nymphs can survive in ponds under ice if there are sufficient sources of food for nymphs.
Upland features on green roofs should include a mixture of grasses and wildflowers native to the region. Additions of stones or branches or logs provide a place for dragonflies to roost and rest overnight.
A small pool of water was designed into a green roof at the EcoCenter at Heron’s Head Park in San Francisco, California. Pebbles, upright stones with a flat surface, branches, and diverse native plantings were included to serve wildlife. Yarrow (Achillea millefolium) shown here in the foreground. Dragonflies have been observed at grade, near this green roof, however, this green roof is not accessible for direct observation. Photo: Bruce Dvorak
New Green Roof Biodiversity Courses
To learn more about how green roofs can support biodiversity, see the new Living Architecture Academy Course: Case Studies of Biodiverse Green Roofs, by Bruce Dvorak. In this course, Dvorak explores some of the green roofs covered here in this article and many more.
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Bruce Dvorak, FASLA 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.
Acknowledgements
I would like to thank Dwight Bohlmeyer, president of the Texas Master Naturalist Brazos Valley Chapter for sharing photographs and information on dragonflies. Thank you to Ron Alameida, architect for the Transbay Transit Center, City of San Francisco, for information and a tour of the roof garden. Thank you to landscape architect Berni Alonzo for information and a tour of the Bill & Melinda Gates Foundation green roofs. Thank you also to Annemarie Dompe for information and a tour of the EcoCenter at Heron's Head Park.
References
Dvorak, B. and P. Drennan (2021). Green Roofs in California Coastal Ecoregions. Ecoregional Green Roofs: Theory and Application in the Western USA and Canada. B. Dvorak. Cham, Springer International Publishing: 315-389.
Dvorak, B. and L. R. Skabelund (2021). Green Roofs in Tallgrass Prairie Ecoregions. Ecoregional Green Roofs: Theory and Application in the Western USA and Canada. B. Dvorak. Cham, Springer International Publishing: 83-142.
Grant, G. (2007). "Extensive Green Roofs in London." Urban Habitats 4(1): 1541-7115.
MacIvor, J. S. and J. Lundholm (2011). "Insect species composition and diversity on intensive green roofs and adjacent level-ground habitats." Urban Ecosystems 14(2): 225-241.
Malkin, N. (2021) "15 Plants That Attract Dragonflies." Lawn and Garden, 6.
May, M. L. (2013). "A critical overview of progress in studies of migration of dragonflies (Odonata: Anisoptera), with emphasis on North America." Journal of Insect Conservation 17: 1-15.
Samways, M. J. and N. J. Sharratt (2010). "Recovery of endemic dragonflies after removal of invasive alien trees." Conservation Biology 24(1): 267-277.
Struck, S., K. Ross and J. Sagi (2014). Low Impact Development Guidelines: Module 3-Green Roofs, Calgary.