The Green Roof Test

Results from a five-year study of soil depth and plant performance on Chicago Botanic Gardens’ green roofs are set to be released this spring. A recent article in Landscape Architecture Magazine by Lauren Mandel highlighted the research project.

One section of the roof garden just after installation in Fall of 2009.
A section of cool-season grasses on the roof garden just after installation in Fall of 2009.

The roof on the Daniel F. and Ada Rice Plant Conservation Science Center is designed as an outdoor classroom and living laboratory. It’s also the site of a five-year evaluation study directed by Plant Evaluation Manager, Richard Hawke. The study involves more than 200 plant species. Three soil levels (four, six, and eight inches deep) are used throughout the roof in criss-crossing zones that allow plant assessment under different growing conditions. Plant performance falls under four criteria: cultural adaptability, resistance to disease and pests, winter hardiness, and ornamental quality.

Pathways traverse the roof gardens, allowing for viewing and evaluation. Photo taken just after installation in Fall 2009.
Pathways traverse the roof gardens, allowing for viewing and evaluation. Photo taken just after installation in Fall 2009.

The roof gardens were designed by Oehme, Van Sweden Landscape Architecture in 2008 and installed in the Fall of 2009. Shannon Currey, Hoffman Nursery’s Marketing Director, visited the roof just after it was installed and talked with Hawke about the evaluation program. Grasses and sedges are prominent on the roofs and include a mix of cool and warm season species. John Hoffman visited the project in 2013 and spoke with Hawke about how the grasses were performing.

The garden in 2013, when John Hoffman visited.
The garden in 2013, when John Hoffman visited.

Several genera are in the mix, including Andropogon, Bouteloua, Calamagrostis, Carex, Eragrostis, Festuca, Molinia, Schizachyrium, Sesleria, and Sporobolus. In most of the plots, grasses are mixed with an herbaceous species. The team compares performance of these and other plant species against Sedum species used on the roofs.

Grasses under evaluation
Grasses and sedges are an important part of the species mix on the roof gardens. In 2013, the grasses were maturing well and providing data for the final report.

The design of the garden balances research and aesthetics, with plots near the building consisting of more reliable species that will provide an impressive display. Plots further from the building and public access contain more species that haven’t been tested before. The possibility of plant failure is real, and that possibility is part of what makes this research valuable.

In the recent magazine article, Hawke provided some preliminary recommendations for grasses and planting depth. (Click on the plant name to see our plant profile):

Roof garden and interpretive signs in 2013
In 2013, the garden continued to provide data for the report. Interpretive signs inform visitors about the project.

We look forward to more recommendations for grasses, sedges, and planting depths. Results from the five-year study are scheduled for release this spring. Hawke and his team will publish a detailed report, which will be available on the garden’s plant evaluation page when it’s released. You can also get more info on the garden’s green roofs on the CBG Green Roof Garden page.

View from the green roof garden
The view toward the trial gardens from the green roof of the Rice Conservation Science Center.

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