The Chicago Botanic Garden has released the results of a green roof study to determine which plants are best suited for growing on a botanical roof. Five years of research on the green roof of the Garden’s Daniel F. and Ada L. Rice Plant Conservation Science Center has led to the publication of Plant Evaluation Notes, which highlight the best plants for green roofs in Zone 5. The study is the largest of its kind ever in the U.S.
A green roof consists of growing plants that help insulate the building below from temperature extremes in hot or cold weather. The plants also help control the amount of rainwater run-off that occurs from the roof of a building. The plants used are intended to be self-sustaining varieties that require little if any care.
A diverse group of 216 herbaceous and woody plants were evaluated in the extensive (growing depth of 3 to 6 inches) to semi-intensive (growing depth of 6 to 8 inches) green roof garden. Nine plants received five star “excellent” ratings for their overall performance and survivability, including Antennaria dioica, Calamintha nepeta ssp. nepeta, Juniperus chinensis var. sargentii ‘Viridis’, Phlox sublata ‘Apple Blossom’, Phlox subulata ‘Emerald Blue’, Phlox subulata ‘Snowflake’, Rhus aromatica ‘Gro-Low’, Sporobolus heterolepis, and Sporobolus heterolepis ‘Tara’.
Top-rated plants consistently displayed good vigor and robust habits, superior ornamental qualities, disease resistance, heat and drought tolerance, and winter hardiness/ survivability throughout the evaluation period. Additionally, 69 plants received four-star “good” ratings for their strong performances.
“Ultimately, the success of a green roof is due to the success of the plants growing on it,” said Richard Hawke, plant evaluation manager at the Chicago Botanic Garden. “Plant trials like the one undertaken here are crucial to increasing the knowledge about the best plants for green roof culture.”
The Plant Conservation Science Center is a 38,000-square-foot, LEED gold-rated research laboratory with two 8,000-square-foot gardens, one on the north and and one on the south side of the central clerestory.
The Ellis Goodman Family Foundation Green Roof Garden South features regional and national native plants, many of which are not currently used as rooftop plants, while the Josephine P. & John J. Louis Foundation Green Roof Garden North features a mix of plants known as good green roof plants, plus native and exotic plants that have potential for green roof use.
“The sky’s the limit for plants we can grow on green roofs,” Hawke said. “We will continue to incorporate more plants into our trials as the Garden further develops its recommended list of best plants for roof gardens.”