Wildflowers: Planting Living Roofs for Native Pollinators Part II
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Plants and Pollinators
One of 22 species of Penstemon thriving on the Mordecai Children’s Garden living roof at the Denver Botanic Garden. With over 600 taxa, it is North America’s most biologically diverse living roof. Its accessible trails take visitors through a wonderland of diverse forms of native and exotic beauty (Dvorak and Bousselot 2021). The stripes inside these beardtongue flower tubes point the way to nectar, which a bee has found (center flower). Photo: Bruce Dvorak
Spring is on the rise in the Northern Hemisphere as wildflowers begin to unfold their annual ultraviolet “light show.” Birds and bees see some of the same colors we can see, plus more. Although humans can’t see the solar spectrum called ultraviolet light, the eyes of birds and bees can, and it allows them to roam across landscapes in search of nectar-filled wildflowers.
Eyes began to evolve on primitive life forms about 540 million years ago, but these primitive forms of life could not see much other than faint colorless blotches of light (Parizotto and Lamberts 2011). Pollination first began to evolve about 300 million years ago through an elegant ecological banter between flowers, birds, and insects (Darwell, Fox et al. 2014). Flowers need cross-pollination to reproduce, and insects need nectar as food. Flowers began to develop elaborate displays of color, like billboards used to entice insects and birds in for a good meal (Briscoe and Chittka 2001). However, visitors leave with a little something extra—pollen, which is spread to a neighboring flower to cross-pollinate. About 100 million years ago, there was a rapid increase in plant, insect, and bird diversity as they developed eyes to see the many evolving colors of nectar-producing flowers (Ren, Labandeira et al. 2009). In addition to bees, flies, moths, butterflies, beetles, and other insects, some birds also feed on nectar. Hummingbirds are well-known nectar feeders, but orioles, mockingbirds, grosbeaks, tanagers, and several species of warblers will also search out nectar. Bats are the only flying mammals, and they love to feed on nectar.
Why Urban Landscapes Need Pollinators
In the modern world, bees and other pollinators are attracted to diverse plantings, which also helps improve the productivity of food crop production (Bretzel, Vannucchi et al. 2017). Since fruits and vegetables are dependent upon pollination, we need pollinators for our own health and survival! Therefore, it seems that sustainable and resilient forms of urban planning should include provisions for native pollinators across developed regions so that the people living there might have access to healthy and sustainable sources of fresh and nutrient-rich food, especially in food deserts (Irvine 2012).
Since urban land is valuable, the multiple ecosystem services produced by living roofs make them one of the least costly and potentially most biologically rich interventions available to secure habitat for native pollinators (Francis and Lorimer 2011). Some countries (FLL 2018), urbanized regions, or cities take biodiversity seriously because the people living there recognize that humans live in an integral relationship with biodiversity (Kowarik 2011). Good planning and urban design practices always include financial rewards; however, some developers also invest in the biodiversity of the land they are developing (Slootwig 2005), including green roofs. For example, the Google Kirkland campus was developed by SRM Development through a form of corporate social responsibility as a way to give back to the community. The property was developed with “gifted” parkland to the city, and biodiverse living roofs on the headquarters buildings that feature native wildflowers, beehives, and some ground-level food crops grown and served in its cafeteria (Dvorak and Rottle 2021).
Even though we may not see what pollinators are looking for, science and principles of ecological design can guide decision-makers to make land-use decisions that are good for all living things. As Aldo Leopold wrote, “A land ethic… enlarges the boundaries of the community to include soils, waters, plants, and animals, or collectively: the land” (Leopold 1966). In dense urban areas where there is a paucity of pollinator habitat, biodiverse living roofs can enlarge the boundaries of habitat for pollinators (Dvorak and Bousselot 2021). Public policies and corporate mission statements (Slootwig 2005) that activate some form of a land ethic help make ecoregions more reliant and livable for all living things through biodiversity planning concepts, including living roofs (Meissner 2022).
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Wildflowers for Pollinators
In this issue, Pollinators Part II, I focus on a few of the top North American wildflowers that thrive on living roofs and support pollinators of all kinds: bees, butterflies, birds, pollinator flies, bats, ants, and more. While there is a diversity of forms of life that pollinate, bees, with over 4,000 species native to North America, are at the top of the productivity list (Moissett and Buchanan 2010). Bees easily find flowers on rooftops, even prairie roofs located 12 stories or 30 meters (100 feet) above a dense urban streetscape (Tonietto, Fant et al. 2011). To find pollinator plants for your region, begin here. To make living roofs useful to pollinators, learn about plant/pollinator relationships as part of the living roof design process, and specify an appropriate irrigation system and substrate depth to keep plants thriving. The plants featured here, are also part of the living roof case studies in the co-authored chapters of Ecoregional Green Roofs (Dvorak 2021).
Penstemon
A drift of Smooth Beardtongue (Penstemon glaber) thrives on this biodiverse demonstration living roof at the Berry Biodiversity Conservation Center LEED Gold certified building (Dvorak and Bousselot 2021). Several bumblebees were found scouting out blooms for nectar on these, one of 9 species of Penstemon planted on this roof. Photo: Bruce Dvorak
There are over 150 species of Penstemon (Beardtongue) native to North America and they attract a wide variety of wildlife (Kimball and Wilson 2009). Few species of Penstemon have been trialed on living roofs; however, Penstemon digitalis (Beardtongue) is a superstar performer in many regions where it grows across the lower 48 in the U.S. and southern Canada. As a pollinator, it attracts honey bees, bumblebees, native bees, and several kinds of flies, butterflies, moths, and beetles. It also attracts sweat bees, leafcutter bees, and is frequented by newly hatched bumblebees. Hummingbirds favor red-flowered penstemons with tubular petals that are difficult for insects to enter. Beardtongue blooms at the end of spring when nectar resources can be running thin and can bloom for many weeks into the summer. Thus, plant beardtongue to keep pollinators busy well into the summer.
Monarda
Lavender blooms of Wild bergamot (Monarda fistulosa) thrive on the roof garden of the Kansas City Public Library as part of a biodiverse roof with 15 cm-deep substrates (6 inches). At bloom in early June, this patch sits 15 meters (50 ft) above the downtown streetscape. The accessible roof garden displays native prairie vegetation that once blanketed the upland bluffs of the Missouri River. Photo: Bruce Dvorak
Monarda is a small genus of about 12 species, each important for pollinating insects including bees and Monarch butterflies. Wild bergamot or bee balm (Monarda fistulosa), is one of the most common taxa available in trade; however, other species of monarda serve local populations of pollinators and could be explored for use on green roofs. Monarda serves specialist bees, bumblebees, predatory wasps, hummingbirds, hawk moths, and small black sweat bees (Dufourea monardae), which are partial to the Midwest and Northeast portions of North America. Bee balm, its common name is a reference to the many medical uses of the plant. Bee balm can spread by rhizomes, can tolerate dry conditions but needs some supplemental watering.
Goldenrod
Goldenrod (Solidago) is a late-summer and fall-blooming perennial wildflower with over 130 species that are native to North America. It is an easy plant to establish from seed, has a good track record on green roofs, and likes soils that are not nutrient-rich. Some of the popular goldenrods that have been trialed on green roofs include Stiff goldenrod (Solidago rigida), Rocky Mountain goldenrod (Solidago multiradiata), Canada goldenrod (Solidago canadensis var. lepida), California goldenrod (Solidago velutina ssp. Californica), Missouri goldenrod (Solidago missouriensis) Canada goldenrod (Solidago altissima), Solidago golden baby (Solidago x ‘golden baby’), and Mt. Albert goldenrod (Solidago simplex) (Dvorak 2021).
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LDS Conference Center living roof. This 1 hectare-rooftop meadow (2 acres) has a 15 cm (6 in) deep substrate and an irrigation system supplied from rainwater harvested from the site (Dvorak and Woodfin 2021). Since 2000, this urban rooftop meadow has kept pollinators active with its diverse planting which includes this yellow blooming drift of goldenrod (Solidago spp.) seen here in mid-July. Since the valley is largely built-out, there is little chance of protecting pollinator habitat from being developed. Living roofs, such as this one, can make significant ecological investments to secure habitat for pollinators for future generations. (Photo: Bruce Dvorak, July 2018)
Goldenrods are a magnet for bees, beetles, butterflies, moths, flies, and wasps (Long 2020). Solidago is known to be a host plant for several butterflies including Baltimore (Euphydryas phaeton), Creamy Checkerspot (Chlosyne palla), Eastern Checkerspot (Chlosyne harrisii), Pearly Checkerspot, (Chlosyne gabbii), and Streamside Checkerspot (Chlosyne nycteis). Moths also make use of goldenrods and common species include: Dart Moths (various species), Goldenrod Gall Moth (Epiblema scudderiana), Oblique-striped Emerald Moth (Synchlora bistriaria). Many birds also enjoy the small seed head including finches such as White-throated Sparrows, Chipping Sparrows, House Sparrows, Buntings, Juncos, Siskins, and Redpolls. Goldenrods can also be visited by bats, which look for large light-colored flowers that remain open during the night (Long 2020).
Coreopsis
This patch of coreopsis thrives 3 stories above downtown Kansas City (Dvorak and Skabelund 2021). It supports native grasses, herbs, and pollinator habitats. Living roofs protect waterproofing systems from hail damage, exposure to sunlight and deliver ecosystem services for 60-100 years or more. Across the street, bare rooftops contribute to urban heat islands, heat gain inside buildings, rapid runoff during heavy downpours, and need replacement every 20-30 years. Photo: Bruce Dvorak
One of the most cheerful and long-blooming wildflowers that serve a wide variety of pollinating insects includes some of the 45 species of Coreopsis native to North America. Some of the popular plants installed on green roofs include Plains coreopsis (Coreopsis tinctoria), giant sea dahlia (Coreopsis gigantea), beach coreopsis (Coreopsis maritima), lance-leaved coreopsis (Coreopsis lanceolata), golden wave (Coreopsis intermedia), largeflower tickseed (Coreopsis grandiflora ‘Leading Lady Lauren’) (Dvorak 2021).
Some of the popular visitors of coreopsis include long-horned bees (Melissodes spp.), small carpenter bees (Ceratina spp.), leafcutter bees (Megachile spp.), cuckoo bees (Coelioxys spp.), and sulfur butterflies. Some species of Coreopsis are short-lived plants, but coreopsis can reseed new plants if flower stalks are left or seeds are harvested and replanted.
Salvia
Although there are over 900-3,000 species in the genus Salvia across the globe, North America has 63 species in the lower 48 US states. Some superstar performers include perennial and annual forms of salvia. Salvia coccinea is an annual form of salvia that has a long bloom period and is actively growing on green roofs in August, even in climates that experience hot weather (when supplemental watering is provided). Salvia grows best when mixed in with other taller perennial vegetation, which helps support its growth. However, many forms of salvia are short in stature and cover the ground. Red salvia is great for attracting hummingbirds, as honey bees do not see the color red. So, diverse plantings with targeted biodiversity goals can help designers help select colors specific to pollinators.
Summary
Through plant selection, green roofs can be designed to support a wide diversity of wildlife. Pollinators are counting on us to make habitat for their persistence and our mutual well-being. With some pre-planning, a developer considering a green roof can easily make the jump from an extensive green roof to a semi-intensive green roof, for at least some portion of a roof. To make habitat for pollinators, it is important to recall that inclusion of pollinator habitat on green roofs takes into account several special considerations.
The annual Red salvia in bloom during August (left and center red spikes) grows with another pollen-laden annual Rudbeckia hirta (yellow blooms) on the kitchen house living roof at Camp Young Judea (Dvorak and Skabelund 2021). This semi-intensive living roof is based on the Blackland Prairie ecoregion. The roof is drip-irrigated with harvested rainwater. Red salvia produces seed which drops to the substrate and reproduces from seed each year. The red salvia attracts hummingbirds and black-eyed Susan’s attracts bees. Photo: Bruce Dvorak
Just because we can’t see ultraviolet light, it doesn’t mean it is not there. Birds and bees see a lot that we don’t see when looking for their habitat needs. Some investigation may be necessary to select plants for the needs of local pollinators.
Specify local sources of native plants, to encourage genetic continuity. Seek out plant nurseries that specialize in the production of native plants. Native plants work on green roofs when the green roof is appropriately designed to do so, by experienced designers. Native plants can fail on green roofs when necessary design provisions are ignored.
Pick a diversity of wildflowers to cover all seasons of the region. If the native landscape goes dormant, pollinators are often also less active or dormant.
Every urban food garden needs pollinators, including rooftop agriculture. Rooftop food gardens should make some provisions by including patches of wildflowers.
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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.
References
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