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
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
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.
Green Roofs: A Growing Rooftop Resource