How Growing Media, Fertilizers and Irrigation Impact Rooftop Food Production
Advertisement
In 2008, I started graduate school at Michigan State University, where I got my start in green roof research. I was lucky to work with Dr. Brad Rowe, who supported my idea of investigating the use of green roof technology to grow food on rooftops. Many of the early discussions about my dissertation later went into one of our first publications, “The role of green roof technology in urban agriculture”. This not only focused on the ways that green roof technology could solve problems in urban agriculture, like the obvious competition with development, but also looked at the horticultural and environmental issues that might arise from using green roof technology in agricultural production. Some of those main issues included:
Figure 1. Green roof mixed vegetable and herb plots on top of the Plant Science Building at Michigan State University. Photo: Leigh Whittinghill
Can the load capacity of existing roofs support agricultural green roofs?
How would the use of fertilizer and irrigation impact the stormwater management capacity of green roofs?
Would the green roof media change how much fertilizer should be used to produce crops?
The suitability of different crops or crop varieties for production in green roofs, and
Potential impacts of agricultural production on other environmental benefits of green roofs.
We developed four projects that became my dissertation. They were designed to start to answer some of these questions. The first of these was a comparison of mixed vegetable and herb gardens grown in 4 inches of media on a green roof (Figure 1), on green roof platforms, and in the ground. We found that production was possible in this shallow depth of growing media with yields comparable to traditional agriculture for several of the crops we looked at. Other studies have also found this. Second, a comparison of carbon sequestration in ground plots and green roof platforms for several ornamental plant communities and our mixed vegetable and herb garden. The third project was a study comparing the stormwater runoff quantity and quality of plots containing our mixed vegetable and herb gardens to plots containing a sedum mix and a native prairie plant mix roofs (Figure 2). We found that the mixed vegetable and herb garden retained a similar amount of stormwater to the sedum mix, but less than the prairie plant mix. The mixed vegetable garden also had more phosphorus in the runoff than the other plant mixes. Finally a study that explored the idea that mulching and fertility management could improve yields on an agricultural green roof (Figure 3). The results of this study were mixed. We did observe a dose response to fertilizer for most crops, but did not see significant improvement with mulching for most crops. In some cases, the living sedum mulch used for one of the mulch treatments seemed to compete with the crop plants.
Advertisement
Figure 2. Research platforms containing mixed sedum, native prairie, and the mixed vegetable and herb garden at Michigan State University Horticultural Teaching and Research Center. Photo: Leigh Whittinghill
After receiving my doctorate, I completed a Postdoctoral fellowship in the Urban Design Lab at the Earth Institute at Columbia University. While there I was able to monitor an operational rooftop farm for stormwater metrics including runoff water quantity and quality. The results of this work added to the evidence found during my dissertation that switching from an ornamental plant community to an agricultural plant community would affect this environmental benefit of green roofs. In comparing the operational rooftop farm to nearby extensive sedum roofs we saw significantly more nutrient runoff from the rooftop farm. We also saw differences in runoff water quantity and media moisture content relating to the use of irrigation on the rooftop farm.
Figure 3. A living sedum mulch mixed vegetable and herb garden plot with two pine bark mulch plots on either side at Michigan State University Horticultural Teaching and Research Center. Photo: Leigh Whittinghill
The work that I did during my postdoctoral fellowship inspired a few different research projects that followed. Based on the nutrient leaching that we saw on the operational rooftop farm, I started to look at nutrient leaching from ground level urban agriculture systems as well. I also developed a long-term project to examine the use of compost on agricultural green roofs. The farm that we worked with, and other rooftop farms add compost to their green roof media every year to provide some of the nutrients that crops need. In ground level systems, adding compost can create soil building, increasing organic matter content over time, which increases water holding capacity. On a roof, this could have implications for the weight of the green roof system. An alternative would be that organic matter from the compost would break down and potentially contribute to nutrient leaching. I developed a project which I performed at Kentucky State University to look at the effects that annual compost additions would have on crop yield, runoff water quality and green roof media properties over time. This project lasted four years. Using a control, with no compost additions and all nutrient coming from organic fertilizers, and three compost treatments, 0.33, 0.66 and 1 kg/m2 of compost addition supplemented with organic fertilizers, we took monthly runoff water quality measurements, green roof media samples three times a year, and crop yields based on the crops we were growing. At these low compost additions, based on compost additions from the rooftop farm I observed during my postdoc, we saw little effect of compost on crop yields and (an additional paper yet to be published) and little effect on runoff water quality or green roof media properties (paper in progress). We did, however, gather important data on how the green roof runoff water quality and green roof media properties changed over time, adding to the limited research on the subject.
Advertisement
I hope to build on this work in future projects that will continue to answer the questions posed in my first paper. Since that paper was published, the number of agricultural green roofs has continued to grow, but the peer review research is still quite limited. While we have learned that production on extensive green roofs is possible, the number of agricultural rooftops using intensive media depths continues to grow. We have learned a little about nutrient leaching from agricultural green roofs, but there is still a lot to learn about optimal fertility management to minimize leaching. I hope to continue work on examining the use of compost on agricultural green roofs and how this impacts runoff water quality, yields, and media properties over time. Less work has been done examining the impacts of other environmental benefits of green roofs, and such work is important to enable rooftop farmers and urban centers to reap all of the benefits possible from this type of rooftop agriculture.
Key Research Findings:
Figure 4. Green roof research platforms at Kentucky State University Harold R Benson Research and Demonstration Farm. Photo: Leigh Whittinghill
Crop production in extensive media depths (4 in) is possible.
Crop production on green roofs, in shallow or deeper media can be as productive as comparable ground level and traditional agriculture systems.
Carbon sequestration is possible on green roof systems, including mixed vegetable gardens. The extent of carbon sequestration will depend on the plant community on the green roof.
The use of mulches could improve production, but living sedum mulch may just compete with the vegetable crops for applied fertilizers.
Switching from low maintenance ornamental plants to high maintenance crop plants increases nutrient leaching from the green roof.
The use of irrigation reduces the stormwater holding capacity of green roofs.
Use of compost rather than fertilizers as a nutrient source does not prevent nutrient leaching.
Advertisement
Leigh Whittinghill is an Assistant Agricultural Scientist at the Connecticut Agricultural Experiment Station in the Department of Environmental Science and Forestry. Her current work focuses on runoff water quality from a variety of urban agricultural production systems and other urban agriculture production practices such as the use of wading pools as containers and cut-and-come-again greens. She was awarded the 2023 Jeffrey L. Bruce Awards Of Excellence in Research by Green Roofs for Healthy Cities.
References
Greenroofs.com. The Green Roof and Wall Project Database 2021. http://www.greenroofs.com/projects
Whittinghill, L.J., P. Culligan, R. Plunz, and D. Hseuh, 2016. Stormwater performance of a full scale rooftop farm: Runoff water quality. Ecological Engineering. 91: 195-206. http://dx.doi.org/10.1016/j.ecoleng.2016.01.047
Whittinghill, L.J. Jackson, C, and P. Poudel. 2024. The Effects of Compost Addition to Agricultural Green Roofs on Runoff Water Quality. HortScience. 59(3):307–322. 2024. https://doi.org/10.21273/HORTSCI17556-23
Whittinghill, L.J. and P. Poudel. 2020. Yields of relay cropped greens grown in green roof production systems. Proceedings from the Urban Food Systems Symposium. Virtual Conference. Oct, 2020. https://newprairiepress.org/ufss/2020/ proceedings/1
Whittinghill, L. J. and D.B. Rowe. 2012. The role of green roof technology in urban agriculture. Renewable Agriculture and Food Systems 27:314-322. https://doi.org/10.1017/S174217051100038X
Whittinghill, L.J., D.B. Rowe, J. Andresen, and B.M. Cregg. 2015. A comparison of storm water runoff from two traditional and one vegetable producing extensive green roof. Urban Ecosystems. 8(1): 13-29. https://doi.org/10.1007/s11252-014-0386-8
Whittinghill, L.J., D.B. Rowe and B.M. Cregg. 2013. Evaluation of Vegetable Production on Extensive Green Roofs. Agroecology and Sustainable Food Systems 37(4):465-484. http://dx.doi.org/10.1080/21683565.2012.756847
Whittinghill, L.J., D.B. Rowe, M. Ngouajio, and B.M. Cregg. 2016. Evaluation of nutrient management and mulching strategies for vegetable production on an extensive green roof. Agroecology and Sustainable Food Systems 40(4): 297-318. http://dx.doi.org/10.1080/21683565.2015.1129011
Whittinghill, L.J., D.B. Rowe, R. Schutzki, and B.M. Cregg. 2014. Carbon sequestration in landscapes at grade and on extensive green roofs. Landscape and Urban Planning 123:41-48. http://dx.doi.org/10.1016/j.landurbplan.2013.11.015