In the city, bee habitat and foraging opportunities become smaller and more distant from each other. These segments of green space have become known as “habitat patches,” disconnected pieces of habitat that animals can move between to achieve the effect of a larger ecosystem. These patches occur in cities and can take the form of ravines, parks, gardens and so on.
Despite the fact that pollinators such as birds, bees and butterflies are better at moving between patches than less mobile species, a continuous habitat is always preferable. Green roofs are seen as a way to make up for ecological habitat fragmentation. However, studies and guidelines about where and how to best construct green roofs for pollinators are just emerging.
Research on the topic of green roofs as pollinator habitats has been fairly limited, but with cities like Toronto adopting bylaws that mandate green roof implementation, there’s an opportunity to study what design decisions are most critical to their success. Also in New York City, they have a tax abatement program in place, for those who build green roofs. So hopefully green roofs can be encouraged through tax policies or other ones.
Green roof planting choices have been shown to play a part in attracting specific bee species. Sedum species, which are drought-tolerant succulent plants, have always been the most popular choice for green roofs due to their hardiness under extreme conditions, long flowering period and low maintenance requirements. In fact in Toronto for example, a great majority of green roofs are planted with sedum.
Despite many green roofs being opportune places for bees to inhabit, research has shown that the location of the green roof matters. The higher the roof, the fewer bees were found there. Green roofs implemented above the eighth story would not benefit from any additional nesting resources or attract bees.
This doesn’t mean that green roofs atop skyscrapers are useless, but that they should focus on other benefits such as rainwater retention, air quality improvement and thermal cooling.
In large cities many new high-rise buildings are being built with a “tower and podium” configuration, whereby the first few floors of the building have a wide floor area, often covering most of the podium, and the tower is set back from the edge of the building.
The roof of the podium is often used as communal space for the building’s occupants and presents a good spot for a biodiverse green roof that could serve bees’ needs. The study further shows that a decline in green space area within a 600-metre radius around each rooftop results in decreasing species diversity and abundance.
Though the appeal of planting green roofs with sedum is evident, limiting the plant palette solely to sedum species could be a lost opportunity to promote native plant and pollinator species in urban environments. At its worst, this practice could cause non-native bee species to have a leg up on natives as both groups compete for pollen.
It’s important to not only consider plant communities on green roofs, but also the building height and its proximity to other habitat patches to provide as much foraging habitat as possible for bees.
We still need new research into nesting opportunities for ground-nesting bees in the green roof growing space, as well as the connectivity between ground level landscapes and green roofs, to better understand the ecological value of green roofs in sprawling urban regions.
Installing a green roof is like putting together a complicated puzzle, and because of this, it's not something you should DIY. During the initial design discussions—for this site, there were a lot—be sure to bring on a structural engineer. The first thing you want to do is to find out how much weight the roof can support, whether it’s a lot or a little. That will change the kinds of designs you can entertain.
The average green roof needs a minimum of 2.5 to 3 inches of growing media. If you’re going to grow vegetables, you need media to be 7 inches deep. But the more media you have, the more it will weigh.
Sedum is the standard green roof plant. It’s extremely drought tolerant, low maintenance, and never needs to be reseeded or watered. It’s a plant adapted to growing on mountaintops in low-nutrient soil. Any flowers that fall off decompose and form the fertilizer the plant needs, so it does its own little composting in a way.
Green roofs are made of layers designed to protect the building structure from water while holding and sustaining the plants. The bottom layer is a root barrier, typically made of very thick plastic sheeting that will prevent any strong roots from digging into the building.
Roots are drawn to carbon. Some of the more affordable roofing materials are made primarily of carbon. It’s one of those unlucky coincidences. You have to work hard to keep them separate.
Several inches of edging material—typically stones or gravel—will then be used to form a root-free border separating the growing area from any vertical elements like walls or, in this case, the sides of the skylights. Metal slats placed between the rocks and the growth media help ensure separation while retaining drainage.
Next is a moisture-retention layer that looks something like an egg-crate mattress. Enough water to sustain the plants will collect in the divots of the egg crate, while excess water can drain out through holes atop the raised bumps.
What happens to the water when it rains? Where does the water go? The answer is that it goes the same place it would if you didn’t have a green roof. The roof is designed so the water percolates through the green roof and can flow to the gutters and downspouts in the same pattern as a normal roof.
Over the moisture layer goes a filter fabric specially engineered to hold in the dirt while draining out any water. There are a range of filter fabrics and growing media types, and it’s important for them to be compatible. Matching them incorrectly could result in clogged fabric or media that washes away. Then it’s time for the growing media—a.k.a. dirt—and putting in the plants.
The average cost for a bare-bones green roof—including the design, permitting, and installation—will typically run between $18 and $22 per square foot. A deeper or more specialized roof, like this one, can cost more, between $30-50 per square foot.
Bees in the city: U of T experts on designing green roofs for pollinators. (n.d.). Retrieved May 15, 2018, from https://www.utoronto.ca/news/bees-city-u-t-experts-designing-green-roofs-pollinators
Eldredge, B. (2016, September 27). How to install a green roof on a private home. Retrieved from https://www.curbed.com/2016/9/27/12830392/green-roof-installation-design-private-home