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What Grows on a Green Roof?

What Grows on a Green Roof?

The Chicago Botanic Garden has released the results of the largest green roof plant study ever conducted in the United States. 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 highlighting the most extensive list of best plants for green roofs in Zone 5.

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 6 to 8 inches) green roof garden. Nine taxa received five-star excellent ratings for their overall performance and survivability, including Antennaria dioica, Calamintha nepeta ssp. nepeta, Juniperus chinensis var. sargentii ‘Viridis’, Phlox subulata ‘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, sixty-nine taxa 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,” says 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 Science Center, which opened in September 2009, is a 38,000-square-foot, LEED gold-rated research laboratory with two 8,000-square-foot gardens on the north and south sides of the building’s 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 taxa into our trials as the Garden further develops its recommended list of best plants for roof gardens.”

Five Things to Think About When Starting a Rooftop Garden
by Richard Hawke

1. Think about what you want the green roof to accomplish. Is it for economical and environmental reasons only, or are you also looking to have a pleasing garden? An extensive green roof is more utilitarian with a less diverse plant palette than an intensive type, which has a deeper growing medium that supports more types of plants.

2. Whether you're building new or retrofitting an existing building, engage a structural engineer right away to determine the structure’s suitability and viability.

3. Keep in mind that a green roof (regardless of the type) is not simply a garden or landscape elevated to the top of a building. Choosing the right plants is essential to a successful green roof.

4. Check green roof literature, websites and blogs for the best information on plants for your region and the type of green roof you're planning. Our Plant Evaluation Notes on green roof plants is the perfect place to start locally.

5. If you’re planning a semi-intensive system (greater than 6 inches of growing depth) with a broad diversity of plants, then plan for roof access to maintain the plantings.

Learn More:

Green Roofs: A Growing Rooftop Resource 


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