Green Roofs - Living Architecture Monitor - Summer 2020 - 9

Plant species diversity at MSU Molecular
Plant Sciences Bldg, in June 2013
Photo courtesy Mert Eski

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tell us that deeper green roofs
planted with herbaceous perennials and grasses would be better
insulators than shallow sedum
roofs. Likewise, the same could
be argued for mitigation of the
urban heat island, air pollution,
and membrane lifespans. These
services are also a function of
substrate depth which in turn
favors larger plants. However,
there isn't any difference in
these services between native
and non-native perennial plants
of similar size and structure.
Furthermore, what about
green roof farms that are producing locally grown nutritious
food? Crops often grown on
rooftops such as beans, carrots,
cucumbers, lettuce, peppers,
spinach, and tomatoes? You
guessed it. All non-native. For
that matter, none of our major
food crops are native to North
America. Corn originated in
Central America, wheat in the

Middle East, the potato is from South America, and rice and
soybeans are native to Asia. Likewise, apples, peaches, pears, and
cherries were all introduced to America. The only food crops
eaten with any frequency that are native to North America are
blueberries, cranberries, grapes (excluding vinifera wine grapes),
pumpkins, strawberries, and sunflowers. Guess we could all
enjoy our meals of wild game, berries, and sunflower seeds. Do
we really want to outlaw rooftop farms?
So should there be policy specifying only native plants on
green roofs? No. Native plants should definitely be considered
for green roofs if the structural integrity of the roof allows for
the substrate depth needed. Are we going to outlaw sedum roofs
and thus limit the number of green roof installations and in the
process eliminate all the stormwater retention, energy savings,
and other benefits that the roof would have provided? I would
much rather see a sedum roof than no green roof at all. Sedum
are probably growing on more green roofs and have had more
to do with driving the economic engine of green roofs than any
other species. Decisions should be made on a project basis, not
by mandated public policy.

Photo courtesy B Rowe

structure of the vegetation.
Varying substrate depths will
provide favorable microenvironments for a greater
number of plant species with
variations in season of flowering, plant height, and spread.
This structural diversity in
plant species will then provide
the microhabitats for various
flora and fauna to live which
increases the overall diversity.
Both native and non-native
plants can increase diversity.
For other green roof services
the benefits of native plants is
not so clear. Numerous studies
have shown that vegetation
type influences stormwater
retention. Plant photosynthetic
metabolism has a major effect
on transpiration and plants with
larger shoot and root biomass
will intercept rainwater which
may evaporate before reaching
the substrate surface. Regarding
energy, intuition would also

Brad Rowe has been conducting green roof research at MSU since
2000. Research topics include plant selection, growing substrates, carbon
sequestration, stormwater runoff, energy conservation, and roof vegetable
production. He was the founding co-chair of the GRHC Research Committee and received the GRHC Research Award of Excellence in 2008.
Brad also teaches a course on green roofs and walls at MSU.
LIVING ARCHITECTURE MONITOR / SUMMER 2020 / 9

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Green Roofs - Living Architecture Monitor - Summer 2020

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