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Chicago Botanic Garden's Green Roof Plant Research Findings

In September 2009, the Chicago Botanic Garden opened the Daniel and Ada Rice Plant Conservation Science Center, a 38,000 sq. ft. LEED Gold-rated research and laboratory facility designed by Booth Hansen, a Chicago-based architectural firm. The center's 16,000-square-foot green roof, designed by Oehme, van Sweden Landscape Architecture (Washington, D.C.), is divided into two 8,000 sq. ft. gardens that display a variety of North American native plants and exotic plants from around the world.

Five years of research on the green roof of the Daniel and Ada Rice Plant Conservation Science Center has led the Chicago Botanic Garden to release "Plant Evaluation Notes", a 22-page report on which plants were found best suited for growing on a botanical roof in Zone 5. The study purports to be the largest of its kind in the U.S. In the five-year trial, a diverse group of 216 herbaceous and woody taxa (41,561 plants) were evaluated on the "extensive" and "semi-intensive" green roof gardens. It's the depth of growing media that classifies green roofs as extensive, semi-intensive and intensive.

Extensive green roofs
-growing depth of 3 to 6 inches
-saturated weight of 15 to 30 pounds per sq. ft.
-plant palette often limited to drought-tolerant hardy succulents.

Semi-intensive green roofs
- growing medium of 6 to 12 inches
- saturated weight of 30 to 50 pounds per sq. ft.
- supports a wide variety of perennials, grasses and shrubs
- depending on the climate, irrigation may be necessary and require more maintenance to keep the green roof looking good.

Intensive green roofs
- growing medium greater than 12 inches
- saturated weight greater than 70 pounds per sq. ft.
- commonly designed for access by people; supports shrubs and trees; requires higher maintenance needs.

Seventy-eight plants received top ratings for their strong performances. Many of these plants grew equally well in two or more of the growing depths, an "impressive feat," according to the report.
A few of the strongest growers "across the board" were:

Lanceleaf tickseed (Coreopsis lanceolata)
Prairie Junegrass (Koeleria macrantha)
Purple prairie clover (Dalea purpurea)
Prairie dropseed (Sporobolus heterolepis)
Arrowleaf violet (Viola sagittata)

The most vigorously reseeding taxa were:
Ashy sunflower (Helianthus mollis)
Beach wormwood (Artemisia caudate)
Black-eyed Susan (Rudbeckia hirta)
Blue hair grass (Koeleria glauca)
Fame flower (Talinum calycinum)
Korean feather reed grass (Calamagrostis brachytricha)
Partridge pea (Chamaecrista fasciculate) - the most prolific reseeder
Prairie Goldenrod (Oligoneuron album)
Prairie Junegrass (Koeleria macrantha )
Shrubby cinquefoil (Potentilla fruticosa)
Spotted Hawkweed (Hieraceum spilophaeum 'Leopard')

The goal of the trial was not to provide supplemental irrigation. Adaptability to the growing media and environmental conditions, particular 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 drought periods. By the end of the trial in 2014, 14 percent of the 216 taxa planted between 2009 and 2011 had died. Of the 41,561 plants planted on both green roofs, 30,568 plants were alive in 2014.

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August 17, 2019, 11:02 am PDT

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