Green roofs have become increasingly popular in cities. They provide a wide range of benefits such as reducing cooling costs, providing storm water management and being beautiful. One of the most desirable benefits of green roofs is their ability to providing natural habitat for local wildlife and pollinators. Green roofs provide sanctuaries for birds and insects in traditionally hostile urban environments and birds have even been recorded to use green roofs as nesting sites.
However, the most common plant used on green roofs are sedums. Sedums are non-native, succulent plants. They are used because they are low-lying, drought resistant and can be grown in mats which make greening a roof less heavy or time intensive to install. While Sedums do provide some benefits of reducing energy use and helping to manage storm water, they do very little in terms of sustainability and providing viable habitat for wildlife. As an article in Scientific American put it, “A roof planted with sedum […] is no greener, from the standpoint of sustainability, than is ordinary tar or asphalt.”
The low lying, tightly bunched nature of sedums does not provide the habitat the birds require. Wild flowers and grasses are more suited to the needs of nesting birds. Additionally, native plants provide food for the birds in the form of seeds and the insects that the plants attract. Sedums do not. Instead, the non-native plants tend to attract non-native insect species. The Green Roof Innovation Testing Laboratory at the University of Toronto found that Sedum green roofs were attracting non-native bee species, and while native bee species did use the sedums, they were more likely to be pollinating native plant species.
Native plants have the advantage that they have co-evolved with the local species. Many native pollinators feed exclusively on particular plants, meaning Sedums can never replace the role of the native plant. Local plant species are finely tuned to provide the habitat and food that the wildlife needs, and to support the populations throughout the season. Wildflowers bloom at different points throughout the season, so that there is a continuous food supply. Flowers in spring and summer attract insects which baby birds need, and then in fall plants produce berries and seeds that sustain the bird populations throughout the winter.
Native plants are the right fit for green roofs as these species are ideally suited to the local environment. Find nurseries that have sourced seeds from your seed zone, and you’ll know that the plants will survive whatever weather is thrown at them. Additionally, the variety in shapes, sizes and colours of native species can keep things interesting by creating a dazzling display that changes throughout the season. Many native prairie grasses and wildflowers do well in the heat and the sun that green roofs are exposed to. Additionally, there are many species whose roots systems do well in 4 to 6 inches of soil, lending them well to the shallow growing conditions of green roofs.
Check out the city of Toronto’s Guidelines for Biodiverse Green Roofs for some plant suggestions. One great species the report suggests is Wild Strawberry (Fragaria virginiana). This plant requires only 4 inches of soil, is low growing, produces fruit to feed birds and has beautiful dense foliage that turns red in fall.
In 4 to 6 inches of soil, many different grasses and meadow flowers can establish themselves. Plant a mix of species to increase the diversity, which in turn will provide more services for wildlife. Take a look at our seed mixes for inspiration on plants that do well together. Our Shallow Groundcover mix is ideally suited for green roofs, because their root systems do well in shallow soil. It includes plants such as Slender Wheat Grass (Elymus trachycaulus), Brown Eyed Susan (Rudbeckia hirta), Evening Primrose (Oenothera biennis), and Dwarf Blazing Star (Liatris cylindracea).
If you’re considering a green roof, talk to your designer about using native plants!