In his latest column Dr Tom Young (below), GRO Board Member and Blue-Green Infrastructure Associate at The Environmental Partnership (TEP), talks food production on green roofs and specifically ‘rooftop agrivoltaics’ with Armando Villa-Ignacio and Eliza Gross, Graduate Research Assistants at Colorado State University…
Above: Dr Tom Young, article author.
During my Nuffield travels I was lucky enough to contact Professor Jennifer Bousselot who is based in the Department of Horticulture and Landscape Architecture at Colorado State University. Jennifer has many lines of research, but the one I was most excited by was growing crops on green roofs in combination with solar panels. I sat down with Armando Villa-Ignacio and Eliza Gross, two of her very talented Masters students, who are currently studying rooftop agrivoltaics (RAV) to find out about their latest research…
Tom Young (TY): Can you tell me a bit about your career and how you got into green roofs?
Armando Villa-Ignacio (AVI): I first got interested in green roofs through a nature centre I worked at. It had a demonstration green roof and so I decided to do some research on it. That’s how I found Prof. Bousselot’s research, and I reached out with some questions. That exchange got me more interested and invested into green roofs which led onto my Masters topic.
Eliza Gross (EG): I came to working on green roofs in a rather roundabout way. My background initially was working in the mental health field after graduating with a bachelor’s degree in psychology. While the work was very impactful and important to me, ultimately, I decided that my passion for both plants and people was the one I wanted to pursue. I went back to school originally to complete a certificate in Horticulture at Front Range Community College. In my time there, I was fortunate enough to connect with Prof. Bousselot and learn about green roofs and rooftop agrivoltaics. I was so excited to find a way to connect my love for plants, sustainability and people.
Above: Image credit to Kevin Samuelson.
TY: How has research on vegetation production on green roofs developed?
AVI: Green roof culture has been around for hundreds, if not thousands, of years, dating as far back as the hanging gardens of Babylon. In modern times, vegetated green roofs are utilized in cities for various reasons like parks or for the multiple benefits green roofs provide like insulation and stormwater management.
EG: A lot of research has focused on creating green roofs that are utilized as parks or stormwater management. These green roofs are commonly planted with drought tolerant perennials and sedums. Over time, research has expanded to focus on creating more biodiverse environments on green roofs and looking into the impact it can have on pollinators. More recently the research has shifted to exploring vegetable crop production on green roofs, and even more recently to introduce solar panels to the system. There is still a lot to explore around fruiting crops and in various environments. One challenge in green roof research is how different each green roof can be. The microclimates on green roofs vary greatly, so more research in different climates needs to continue to be done in order to learn more about what crops are best suited in different green roof environments.
TY: What are your study topics?
EG: My research is focused on chile pepper production in an RAV system [rooftop agrivoltaic system]. It is the first time – that we are aware of – that fruiting crops are being studied on a green roof under solar panels. I am focusing on three cultivars of chile peppers and examining their production, water use, chlorophyll content, and plant size.
AVI: I’m currently finishing up my masters studying leafy green production in an RAV system.
TY: What are the benefits of having PV panels on green roofs?
EG: The main benefit of having PV on a green roof is the shade they provide. Green roofs are inherently a water-limited environment that is notoriously hot. Most food crops, even though they like “full-sun” really do appreciate a bit of shade. It helps reduce their heat stress and water use. The panels can also help protect plants from hail or an early snow as well. The moderated temperatures under panels provide an ideal environment for many plants.
AVI: Energy production; PV panels can provide renewable energy for the building they’re placed on. There’s a mutualistic benefit as well, where the plants are protected from extreme weather and intense solar radiation. In turn, evapotranspiration from the plants helps cool the panel, possibly increasing the efficiency of the PV panel. The shading from the panels can reduce water use in plants too, helping with irrigation requirements.
EG: As Armando mentioned, plants transpire and cool the air around the panels allowing them to operate more efficiently. Even a one-degree difference can greatly impact the energy capture of solar panels. A common struggle as we grapple with addressing the various challenges of climate change is an “either-or” mindset. We can either grow crops or produce renewable energy. The reality is we can do both in the same space, and that is a really exciting innovation and step towards climate change solutions!
TY: What are the main findings of your research so far?
AVI: My research looked at various [types of] PV panels’ influence on leafy greens. This included polycrystalline opaque panels, silicon bifacial panels, frameless cadmium telluride (CdTe) thin-film panels, and 40% semi-transparent CdTe thin-film panel. What we’ve discovered so far is that leafy greens – specifically arugula, kale, lettuce, spinach, and Swiss chard – grown under the 40% semi-transparent panel produced higher fresh weight compared to the other PV panel treatments and full sun control. Leafy greens also used less water under the shade of the opaque PV panels.
EG: In my first year of research, we found that wind was a bigger challenge than we expected. A wind event really impacted our plants under the panels which set back their fruiting significantly. Additionally, we are finding that the amount of shade the silicon-based panels we currently have access to on the Hydro green roof is more than the chile peppers prefer. They offer about 80-90% shade and chile peppers prefer 30-40% shade. This has resulted in the full sun plants producing significantly more peppers in our first year than those under the panels. However, we are seeing significantly reduced water use in the first third of the growing season under the panels compared to the full sun plot.
TY: What are your next steps in your research?
AVI: My next step is to study for a PhD. I will be studying a perennial crop (raspberries) under PV panels. We’ll look at flowering, fruit production, as well as how a perennial crop influences panel efficiency.
EG: As my second year of data collection starts, we have made some changes to explore some of the challenges found in the first year. There are solar panels that are semi-transparent as Armando mentioned. We aren’t able to install semi-transparent PV panels into the grid currently, so will use a shade cloth to simulate the type of environment that could be possible with those panels. This will provide 40% shade which is around the ideal shade levels for chile pepper growth. We are also adding new microclimate sensors so that we can measure and characterise how wind impacts the growing conditions.
TY: What are the wider possibilities of vegetation production on green roofs?
EG: There are so many possibilities for the future of green roofs. Our research is just the beginning because as far as we know, food crops have not been studied like this in an RAV system. When I look at our current RAV system, I sometimes think how in 10 years it may look entirely different in ways that I can’t currently imagine.
AVI: In terms of RAV, so much is still unknown. As of now, we’re characterizing what crops can grow in RAV, since the environment on a rooftop is different than on the ground. This includes more intense sunlight, higher winds, and coarser substrate. I hope to investigate nutritional differences between crops grown on a green roof and the ground, and between crops grown in full sun on a green roof and under PV panels.
EG: I hope to see an expansion of different solar panel options designed for RAV, as well as more research examining different vegetable and fruiting crops on green roofs. I also think there is potential for research to explore how biodiverse green roofs could support animal and insect migration acting as corridors through urban areas. It is really an exciting time to be in green roof research because there are so many potential uses for them in supporting our world and our cities.
TY: Do you have any good examples of good urban farms on green roofs?
AVI: Brooklyn Grange in New York City comes to mind. It’s almost 5 acres large, exclusively on rooftops. Also, IGA Duchemin Urban Rooftop Farm in Montréal, which is on a grocery store that is supplied by the green roof!
EG: It isn’t an urban farm but the green roof that comes to mind for me is Maggie Daley Park in Chicago because it really re-imagines what a green roof can look like. I grew up outside of Chicago often visiting this park, but it wasn’t until a couple of years ago that I learned it is actually a green roof built on top of a parking structure. It effectively creates more green space in the midst of a major city and is also fully accessible. I also must mention Boston Medical Centre. They are growing crops on their green roof to supply fresh produce to their patients. I love the idea of utilizing buildings like a hospital to also nourish folks.
Contact Dr Tom Young:
Tel: 0207 5899 400
Email: tomyoung@tep.uk.com
X (formerly Twitter): @DrGreenRoof