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It’s easy being green: New research supports green roof advancement in the U.S.

Richard Hawke, Associate Scientist and Plant Evaluation Manager, Chicago Botanic Garden

Imagine looking down on a major cityscape and seeing a sea of lush greens dotted with vibrant blues, yellows and Mother Nature’s other favorite hues. Normally, that type of scenery can only be found in the Midwest, over expansive prairies or western plains. However, in the future, metropolises could become “Emerald Cities,” distinguished by the green roofs crowning their skyscrapers and buildings.

Green roofs, also known as living roofs, are partially or completely covered with vegetation and have numerous social, economic and environmental benefits – from lessening storm water runoff and filtering pollutants, to adding aesthetic appeal, reducing energy costs, and more. However, to fully reap the benefits of green roofs, the plants must be right.

Chicago Botanic Garden's Green Roof Garden atop the Daniel F. and Ada L. Rice Plant Conservation Center. (C) Chicago Botanic Garden
Chicago Botanic Garden’s Green Roof Garden atop the Daniel F. and Ada L. Rice Plant Conservation Center. (C) Chicago Botanic Garden

As such, in 2009 I began research atop the green roof of the Chicago Botanic Garden’s Daniel F. and Ada L. Rice Plant Conservation Science Center. The goal of my research was to help fill the significant void that exists in understanding green roofs. Through the various plant trials that my team and I conducted on the roof, I hoped to increase the scientific and gardening communities’ knowledge about the best plants for green roof culture.

Over the course of five years, a diverse group of 216 herbaceous and woody taxa were evaluated in the extensive (growing depth of three to six inches) to semi-intensive (growing depth of six to eight inches) green roof garden. My team and I watched, documented and analyzed data collected from the green roof, keeping five criteria top of mind when considering each plant’s sustainability, including adaptability, pests/diseases, winter hardiness, non-weediness and ornamental beauty.

Throughout the study I was surprised by how well many of the plants were performing, which made me optimistic at what we might conclude. Ultimately, the study exceeded my expectations. Among our findings, we were able to compile a list of the top performing plants for green roofs, which I’m proud to say is the most extensive list of best plants for green roofs in Plant Hardiness Zone 5 to-date. The top performers includes:

  1. Pussytoes (Antennaria dioica)
  2. Dwarf calamint (Calamintha nepeta nepeta)
  3. Juniper ‘Viridis’ (Juniperus chinensis sargentii‘Viridis’)
  4. Creeping phlox ‘Emerald Blue’ (Phlox subulata ‘Emerald Blue’)
  5. Creeping phlox ‘Apple Blossom’ (Phlox subulata ‘Apple Blossom’)
  6. Creeping phlox ‘Snowflake’ (Phlox subulata ‘Snowflake’)
  7. Aromatic sumac ‘Gro-Low’ (Rhus aromatica ‘Gro-Low’)
  8. Prairie dropseed (Sporobolus heterolepis)
  9. Prairie dropseed ‘Tara’ (Sporobolus heterolepis ‘Tara’)

As more cities offer incentives to reach sustainability goals – for example, the Green Roof Rebate program in Washington, D.C., which provides base funding of $10 per square foot in targeted subwatersheds – I believe research like my teams’ is essential. It is a significant step towards maximizing the benefits and improving the longevity of green roofs, which is critical to making “Emerald Cities” a reality here in the U.S.

Looking forward, my team and I plan to expand our study. Our goal is to compile a broad list of proven plants so that everyone across the country – from businesses and architects to governmental groups and residential homeowners – has the information they need to grow a green roof.

Richard Hawke is an Associate Scientist and Plant Evaluation Manager at Chicago Botanic Garden

About Chicago Botanic Garden: The Chicago Botanic Garden is one of the world’s great living museums and conservation science centers. In 2014, more than one million people visited the Garden’s 26 gardens and four natural areas, uniquely situated on 385 acres on and around nine islands, with six miles of lake shoreline. Within the nine laboratories of the Garden’s Daniel F. and Ada L. Rice Plant Conservation Science Center, scientists and graduate students conduct a wide array of plant research. The Garden is one of only 17 public gardens accredited by the American Association of Museums. Its Lenhardt Library contains 110,000 volumes — including one of the nation’s best collections of rare botanical books.