Green roofs abound. Even little bird houses, like these at the Smithsonian Gardens, are popping up with their own garden rooftops. In our opinion, there are not enough yet and some serious research needs to be done in adaptive reuse, engineering and regulation for turning existing asphalt roofs into heightened oases.
Whether it’s a tool shed roof, the top of your client’s chicken coop or a new home going with plans for sustainable features, there may be opportunity for including green roofs in your next project.
Sedums and succulents seems to be all the rage right now too. Called the “trendiest members of the plant kingdom” in a 2018 article, they are making the news, highly viewed content on crafting and DYI blogs and are sold in every (I know that is a generalization… but have you found a garden center that does not sell them?) garden center. They are centerpieces and guest gifts for weddings and take-home activities at corporate staff retreats. They are everywhere. They are on green roofs too.
But, they do not need to be the only plant on green roofs. They do not even have to be included on a green roof. In other words, you can have a green roof without succulents!
This 2013 report and design guide by Lee Skabelund and Dea Kesh of Kansas State University Department of Landscape Architecture not only focuses on the native plants that can be used for green roofs but also highlights “small-scale retrofit” of roofs for a living roof.
Most of us know why we should be thinking more about the spaces above our heads for gardens. Heat reduction and water conservation is at the top of the list. Green roofs are infinitely better than dark asphalt roofs in mitigating the urban heat island effect. IN fact just painting a roof green or using white asphalt shingles may be better than the dark asphalt to reduce heat. But when it comes to storm water and pollution management, roots, soil and plants are necessary. Instead of water running down roofs or downspouts into storm drains and out in natural bodies of water create problems of flash flooding and pollution. Flash flooding occurs when all of that water from all of those roofs is funneled into narrow channels and then into narrow creeks which overflow and cause the flooding so many of us expect on most rainy days.
If you have an asphalt roof, do you know what those granules on the top of each shingle are made of? First developed in the 1930s each asphalt shingle is coated with granules intended to make the shingle last longer by keeping the UV sun rays off the shingle itself. They are made of ceramic coated minerals – usually talc or mica. With each storm event some granules end up in the water cascading into streams from thousands and thousands of roof tops. (New granules have been created that are supposed to lessen air pollution – by transforming nitrogen oxide into a substance that gets washed away with… you guessed it … rain water!) This is just one type of pollution in rain water that can be lessened by the addition of a green roof.
We understand the value of green roofs but we may be making assumptions about the plants that will do best on the roofs. Sedum, cacti and succulents seem like a natural choice because it it HOT and dry up on those roofs right? Well, sometimes. It depends on the media used in the green roof. Rather than a hot desert-like situation, we may think about green roofs more like a seasonal stream bank. At different times through the year they alternate between being inundated with water and dry.
In 2015 Chicago Botanic Garden published an assessment of plants for use on green roofs. On their new greenroof they displayed plants known for their ability to withstand the harsh conditions of growing on a roof without sacrificing aesthetics and they used other areas of the roof to trial plants for use on green roofs. Native and non-native plants feature prominently throughout and there is a lot more to the design than sedums.
There are many other studies out that will help guide you in plant selection in your area of growing. There is research on cold climates, hot and arid climates and any other situation you may be interested in. There is also a lot of research on the pollution and storm water mitigationof various types of green roofs. You will find research on pollinators supported by green roofs and how green roofs can play an important role in habitat connectivityand the reduction of fragmentation. Finally you will find many studies analyzing the growth of plants using various green roof growing systemsavailable. A simple Google Scholar search will lead you to numerous research on plants and green roofs. Even NASA has been studying green roofs.
Here you find a few of the many plants we carry that have been found to do well on green roofs. Of course the system being used and the location of the green roof all factor into plant selection, but these plants have been found, in multiple trials to handle the conditions of a green roof with, shall we say, flying colors. (Click on the scientific name for plant information from our website; click on the common name for links to other places in the blog to learn other ways these plants can be used.)
Campanula rotundifolia – Harebell
Coreopsis verticillata ‘Zagreb’ – Thread Leaf Tickseed
Dalea purpurea – Purple Prairie Clover
Dianthus gratianopolitanus ‘Firewitch’ – Firewitch Cheddar Pink
Koeleria macrantha – Prairie Junegrass
Liatris ligulistylis – Rocky Mountain Blazing Star
We have a suspicion that many other native non-succulent plants will do splendidly in various green roof situations. For example, we don’t think you should count out Antennaria neglecta (Field Pussytoes) and Antennaria plantaginifolia (Plantain-leaved Pussytoes). What plants have you tried on various sizes, shapes, location and systems of green roof? Which would you caution others to avoid? Which have been successful?