Four Strategies to Feed Wildlife with Green Roofs

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*Eastern Tiger Swallowtail butterfly (Papilio glaucus) on the University of Hartford green roof feeding on a Wild Bergamot (Monarda fistulosa) plant. Photo: Recover Green Roofs*

Introduction

In wild places, food webs exhibit a clear hierarchy. Insects for example, are critical facilitators in the pollination process, but they are also a major food source for many insect eating wildlife and by far outnumber any other living things on earth by kind (taxa) and number (individuals). There are at least 1 million documented taxa of insects on earth, but potentially 10-30 million taxa, which means that most taxa of insects have yet to be discovered! Worldwide, there are at least 20,000 taxa of bees and 14,000 taxa of ants. Beetles, butterflies, moths, and flies contribute the majority of the number of insects worldwide, with a great diversity of others occurring in smaller populations (Sabrosky, 1953).

Birds, mammals, fish, bats, and other insect-eating wildlife play important roles in ecosystem functioning. Largely, because they feed on insects which helps to prevent insect populations from wreaking havoc on food crops and ecosystems. Many kinds of native insects have co-evolutionary relationships with native plants. Consider that bees, butterflies, moths and some flies have co-evolved with flowers over millennia to achieve an interdependence to complete their life cycles (Briscoe & Chittka, 2001).

One way to help keep green roofs healthy is to plant and maintain green roofs with diverse forms of vegetation in order to sustain a healthy population of plants and desirable insects (Ndayambaje et al., 2024). If a green roof has bees, moths, beetles, flies, it will likely also attract birds, dragonflies, and other important mobile wildlife that rely on insects as a major food source. These ecological relationships are important to keep in balance, to maintain a healthy green roof ecosystem (Sutton, 2015). Green roofs can support insects (invertebrates) through plant selection by selecting specific plants for specific insects (MacIvor & Ksiazek, 2015). Substrates can be designed to support insects by designing appropriate substrate ingredients, depths, moisture levels, nutrient particle sizes, organic contents and macro and microflora (Best et al., 2015).

Consider how these elements of green roofs can be arranged to mimic those natural environments that wildlife is looking for. Most cities were settled along rivers and savanna-like habitats. Today, savannas are one of the most endangered ecosystems world-wide; however, by making green roofs to include key functional elements of savannas, we can reintroduce and support wildlife in urban areas (Coburn et al., 2017).

In this article, four strategies are discussed that can be used to attract, support, and sustain populations of advantageous insects and wildlife on green roofs and include: 1) incorporate grasses, 2) include wildflowers, 3) provide structural support and 4) make provisions for water. These elements are common features once found in natural environments where humans settled and can be provided for on green roofs.

This savanna, at Meadowbrook Nature Preserve in Urbana, Illinois preserves native trees, grasses, wildflowers, structural habitat elements and open sources of water, which attract wildlife. Over 700 taxa of wildlife are observed (iNaturalist, March 2025) at the park including four threatened species. Photo: Bruce Dvorak

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1. Incorporate Grasses on Green Roofs

Grasses make up a large part of the ground-level biomass in savannas and prairies. Forms of grasses include sod forming grasses and bunch grasses. Bunch grasses are a form of perennial grass that grows in tufts, or clumps. Because of their upright form, many bunch grasses make great host plants for insects. Insects use bunch grasses to lay eggs, rest, hide, and find a source of food. There are at least 164 taxa of bunch grasses native to North America and they can be found in tallgrass prairies, shortgrass prairies and oak and pine savannas. The most common bunch grasses found in prairies and savannas that have been used on green roofs include big bluestem (Andropogon gerardii), Indiangrass (Sorghastrum nutans), switchgrass (Panicum virgatum), and little bluestem (Schizachyrium scoparium). These grasses are critical to many beneficial insects including butterflies. Indiangrass attracts 7 species of butterflies, little bluestem 8, big bluestem 22, and switchgrass attracts 30 species of butterflies.

*Big bluestem grass (up front) thrives on a semi-intensive green roof at the Chicago Botanic Gardens. Five other species of native grasses contribute to this wildlife hotspot. Photo: Bruce Dvorak*

Bunch grasses adapt well to green roofs when their habitat needs are provided for (substrate depth, drainage and moisture). One caution about including grasses on green roofs is to understand the green roof environment, and how aggressive or competitive the species may be. Indiangrass, for example, has a reputation for spreading and expanding its domain. So, it is important to select species knowing their growth habits and requirements and what maintenance regimes or design interventions will be needed.

An excellent example of the use of bunch grasses on a green roof can be found at the demonstration green roofs at the Chicago Botanic Garden on the second floor of the Daniel F. and Ada L. Rice Plant Conservation Science Center. Plant surveys reveal that 156 plant taxa survived on the green roofs in 2009 when first planted (Hawke 2015). Native bunch grasses were established in 6 and 8-inch-deep substrates. These include Andropogon gerardii, Bouteloua curtipendula, Koeleria Glauca, Koeleria macrantha, Schizachyrium scoparium and Sporobolus heterolepis. The iNaturalist web page (accessed March 2025) identifies at least 56 taxa of wildlife visiting the green roofs with fifteen species of bees and wasps on this green roof, twelve species of spiders, ten species of butterflies and moths, two species of dragonflies and five species of birds have been observed. Not all of these were identified on grasses, but the integrated approach of mixing grasses and wildflowers attracts beneficial invertebrates to the roof.

2. Include Wildflowers

One of the surest ways to attract beneficial insects to green roofs is to plant them with native wildflowers. They should be selected to offer what insects and wildlife are looking for during different stages of their life cycle and different seasons (Ksiazek-Mikenas et al., 2018). Insects continually scope out plants for fresh sources of pollen, leaves, seeds, fruits, stems, roots, or tubers. Some of the most versatile plants to attract a range of beneficial insects to green roofs include many species of aster, species in the daisy family (coneflowers), solidago (goldenrod), asclepias (milkweeds), helianthus (sunflowers) and salvia (sage). These can be planted in a range of substrate depths, with different requirements in different climates, but most adapting to semi-intensive substrates and some can adapt to deep extensive (6 inches) if moisture levels are managed appropriately.

From left to right: buckeye butterfly (Junonia coenia) feeding on lantana; Gulf fritillary butterfly (Dione vanilla) feeding on Frogfruit; Monarch caterpillar (center); viceroy butterfly (Limenitis Archippus); and a yellowjacket feeding on a goldenrod plant. Photo: Bruce Dvorak

A meadow-based green roof at the University of Hartford. This green roof is heavily planted with 34 taxa of wildflowers. These were selected to produce blooms throughout the growing season, from early spring through summer and fall. Photo: Bruce Dvorak

An outstanding example of a green roof designed to maximize wildflowers is the Green Roofs for Healthy Cities award-winning roof meadow on top of the University of Hartford Hursey Center. This semi-intensive green roof has at least 32 taxa of wildflowers and one species of grass (see photo). The wildflowers include mostly natives, with some non-invasive exotic plants. Plant life cycles include annuals and perennials, each blooming during different seasons of the year. This approach maximizes opportunities to attract insects, birds, and other mobile wildlife to the roof. Wildlife observed on the meadow roof includes dragonflies, bees, honeybee sweet wasps, monarch butterflies, sparrows, Eastern tiger swallowtail and black swallowtail butterflies, grasshoppers, praying mantis egg sacks, goldfinch, song sparrows, killdeer, and hawks observed at the edges. It is also important to know that no ticks have been reported from the green roof!

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A purple coneflower plant (Echinacea purpurea) blooms mid-September on the University of Hartford meadow roof. Purple coneflower pollen attracts many species of bees, butterflies, hummingbirds, and moths. Coneflower seeds attract many species of birds including goldfinches, sparrows, and chickadees. Photo: Bruce Dvorak

This project demonstrates how green roofs planted with wildflowers with a variety of bloom periods and cycles, tends to attract insects and birds throughout the year. The maintenance approach allows for volunteer species that are not overly aggressive to mingle with the planted species. This approach minimizes disturbances to the green roof substrates, allows new species to add to ecosystem complexity, and keep maintenance costs affordable. However, this approach requires knowledgeable caretakers that keep watch of the green roof. Just as there are friendly volunteer species, there are some that may need immediate attention before an undesirable outbreak occurs.

3. Provide Structural Support

Natural environments are rarely uniform and are not composed of monocultures, or even plantings of only a few species. In nature, ecosystems often exhibit varied structural elements. This can be found in different forms and sizes of plants, and the inclusion of dead or decaying trees such as snags, (upright dead trees), or downed logs. Although we can interpret elements as messy, wildlife makes good use of them. Wildlife uses structural elements for places to rest, find food or nest. For example, many birds, butterflies, and moths prefer to have multiple choices in their preferred settings and are attracted to places with varied plant heights to perch, rest, hide or take shelter from the elements.

A roof garden exhibit at the Phipps Center for Sustainable Landscapes in Pittsburg, PA. This green roof makes use of native grasses, wildflowers, shrubs and small trees. These offer varied heights, materials and habitats for wildlife to use. Photo: Bruce Dvorak

Another way to provide structural needs through plants or plant parts is the sourcing of woody material. Some insects require wood to bore into for housing can be added to green roofs as loose laid logs, or by the addition of a built structure, such as an insect hotel. Insect hotels can be made to mimic downed or standing logs that insects may occupy for part of their life cycle and prove to be useful on green roofs (Latty, 2016). Solitary bees use dead wood, hollow plant stalks or even the ground to make tunnels for nests. Insect hotels are designed to attract solitary insects. Solitary bees don’t have a hive to live in or protect, so they are usually less aggressive than hive bees. Insect hotels can be made from bamboo stalks, dead wood and other fibrous parts of plants. By including loose laid logs or insect hotels, green roofs can be made to attract and house a variety of wildlife that are looking for these elements, even on rooftops.

Left: A driftwood log is host to shelf fungi, and a micro-habitat for insects. This accessible green roof demonstration on Vancouver Island, Canada utilizes native plants, logs, and wildflowers to attract and support wildlife. Photo: Bruce Dvorak

Right: An insect hotel at Schob Nature Preserve in College Station, Texas. This “insect hotel” was designed by students and faculty for solitary bees. An adjacent wildflower meadow and edge of woods supplies a wide range of sources of pollen. Small-scaled insect hotels like this one can easily be incorporated into green roofs. Photo: Bruce Dvorak

This small pool of water on a green roof is edged with smooth stones and plants that like to be wet. The stones provide a perch for birds and insects to land on and get a drink. The stones also hold the growing medium in place and help keep water clear. Photo: Bruce Dvorak

4. Attract with Water

Small pools, ponds, and creeks were once very common features of landscapes before they became urbanized. The removal of these features to make way for human settlements systematically removed these habitat elements that wildlife depends on to make way for buildings, parking lots and plazas. In our efforts to “modernize” or “improve” landscapes, we typically make water features that exclude important elements that make them useful for some wildlife, like plants along the edge or downed logs or stones emerging into the water.

Green roofs can be designed to support water in the form of small pools or ponds. While ponds may be a special kind of installation and not typical, small pools or places that gather water can be added to green roofs in simple ways. An impervious liner can be added to a roof at locations where growing media is pulled back. Stone or some other elements may be needed to hold the media in place, prevent erosion and reduce siltation.

Water Adjacent to a Green Roof

If it is not possible to support water habitats on a green roof, then providing water nearby a green roof at the ground level helps support wildlife. Planting the edges of the water with native vegetation, logs or stones helps support all forms of wildlife. A green roof in close proximity to water offers a place for wildlife to perch, rest or find food.

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When Wildlife are Pests on a Green Roof

Plants attract many kinds of insects and wildlife. Sometimes, insects can create problems on green roofs, even desirable insects. Bees, wasps, mosquitoes and other stinging or biting insects can be attracted to green roofs. For extensive or semi-intensive green roofs without public access these may pose little concern. However, with roof gardens, and accessible green roofs, maintenance crews and building owners will need periodic observation of the wildlife on a green roof to ward off any issues, or perhaps inform the public about how bees or other wildlife may be present. Insects such as aphids, beetles, ants, caterpillars can cause damage to plants and potentially attract other critters to a green roof. Some birds can disturb or dig up growth media or make nests on a green roof. These events should be addressed by the person or people assigned as stewards or champions of the green roof and assure its proper recovery and maintenance. Natural remedies should be applied if possible. If chemicals are used, they should be used sparingly, and with full knowledge of any potential contamination of runoff, overspray could cause an issue with the green roof, building or ground level (McIntyre & Snodgrass, 2010).

*A small pond at the ground level is shown here with native emergent vegetation and a stone bottom was made to clean up any phosphorus or nitrogen left over from the greywater harvesting cycle of this* *Living Building. A green roof sits above a classroom at the Phipps Center for Sustainable Landscapes and can function as a perch or safe haven away from threats at the ground level. Photo: Bruce Dvorak*

Summary

Because human developments on the land have significantly damaged or removed habitat for wildlife, it is important to make use of green roofs to reintroduce important elements of ecosystems to support and feed or house wildlife. In this article, four key strategies to feed wildlife with plants were discussed including:

Wildlife depends upon native plants for food and attracting beneficial insects and insectivorous wildlife.
Bunch grasses can be used to host, feed and support habitat for many species of insects and birds. Consult local conservation guides to identify plant species to trail on a green roof.
Wildflowers selected for different forms, heights, and blooming periods help attract and feed wildlife. It is important to select the appropriate substrate depth for grasses and wildflowers.
Structural support can be incorporated into green roofs by including varied sizes and forms of plants like grasses, wildflowers, shrubs, and small trees. Structural elements can also include loose laid logs or branches.
Where logs or other natural materials are not feasible for a green roof, built structures like insect hotels can be implemented to support solitary bees and other insects.
Small ponds and pools need to be reintroduced into urban landscapes. Small ponds or pools can be incorporated into green roofs. If it is not possible or feasible to include open water on a green roof, it is important to provide a source of water at the ground level nearby a green roof.

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Green Roof Biodiversity Courses

To learn about how green roofs can support heat-tolerant plants and biodiversity, see the Living Architecture Academy Course: Case Studies of Biodiverse Green Roofs, by Bruce Dvorak.

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 Emily Ahlstedt with Recover Green Roofs for arranging an educational visit to the University of Hartford green roof and sharing information about the green roof. Also, thank you to Katina Bentley with Recover Green Roofs for sharing her wonderful photography of wildlife on the University of Hartford Green Roof.

Four Strategies to Feed Wildlife with Green Roofs

Introduction

1. Incorporate Grasses on Green Roofs

2. Include Wildflowers

3. Provide Structural Support

4. Attract with Water

Water Adjacent to a Green Roof

When Wildlife are Pests on a Green Roof

Summary

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