A new study in the US has completed a five-year trial to evaluate the best plants for a promising green roof.
The study, the largest of its kind in the US, was conducted by The Chicago Botanic Garden with results featured in Plant Evaluation Notes.
The benefits of green roofs are widely documented, from cooling the buildings they sit atop (and the planet as whole) to helping with insulation, improving acoustics, increasing biodiversity and supporting rainwater runoff.
The study recognises that from municipal to commercial and industrial to residential, green roofs are becoming increasingly popular as a community asset. This is particularly happening in dense cities where ground space is scarce.
“Green roofs are being developed for greater interaction with people, whether as pleasure spaces, urban sanctuaries, or for growing food crops,” the report reads. “With this usage comes a desire to have greater plant diversity so that the green roof functions on an environmental level as well as provides a pleasing landscape.”
However, plant selection is often determined by aesthetics rather than climate and substrate considerations. Also, there is a demand for self-sustaining plants to reduce maintenance.
“When choosing plants for green roofs it is important to understand that a green roof is not the equivalent of a typical landscape elevated to the top of a building; therefore, appropriate consideration must be given to a plant’s growth habit, native ecosystem, and cultural needs, to name a few factors,” the report states.
For the study, a diverse group of 216 herbaceous and woody plants were evaluated in and array of extensive to semi-intensive green roof gardens. The roof depth ranged from four to eight inches of growing medium.
The plants were continually monitored to the evolving environmental conditions with heat and drought serving as main performance criteria.
“Heat and drought were challenges that not all plants were able to overcome; however the vast majority of plants survived with minimal supplemental water provided during the worst droughty periods,” states the report.
The Chicago Botanic Garden opened its Daniel F. and Ada L. Rice Plant Conservation Science Center, a 38,000 square foot LEED Gold-rated research and laboratory facility designed by Chicago-based architectural firm Booth Hansen. The roof spans 16,000 square feet and is divided into two 8,000 square foot green roofs located on the north and south of the building.
Of the structure and substrate, the report notes “Curvilinear aluminium bands — both decorative and structural — run from north to south across the roofs, delineating the spaces between the three growing depths. The growing medium is a mixture of expanded clay and shale, vermiculite, perlite, sand, and organic matter.”
By the end of the trial in 2014, there were a couple of key findings:
- 78 species received top ratings
- 14 per cent of the 216 plants planted between 2009 and 2011 had died
- Of the 41,561 plants planted on both green roofs, 30,568 plants were alive in 2014
Plants were also given a star rating and beyond survival were documented for bloom period, size, production, reseeding and their survival in heat/drought conditions.
The nine plants that received five star performance ratings were:
- Antennaria dioica (Common Name: Catsfoot, Stoloniferous pussytoes)
- Calamintha nepeta ssp. (Common Name: Lesser Calamint)
- Juniperus chinensis var. sargentil ‘Viridis’ (Common Name: Chinese Juniper, Sargent juniper)
- Phlox subulata ‘Apple Blossom’
- Phlox subulata ‘Emerald Blue’
- Phlox subulata ‘Snowflake’
- Rhus aromatic ‘Gro-Low’
- Sporobolus heterolepis (Common Name: Prairie Dropseed)
- Sporobolus heterolepis ‘Tara’ (Common Name: Tara Prairie Dropseed)
You can see the full list of plants in the table at the end of the report.