Green Roofs - Living Architecture Monitor - Summer 2020 - 8

PLANT PROFILE

SHOULD THERE BE POLICY SPECIFYING
ONLY NATIVE PLANTS ON GREEN ROOFS?
DR. BRADLEY ROWE, MICHIGAN STATE UNIVERSITY, (MSU) EAST LANSING

Do native plants on a green roof provide greater environmental
services than non-natives? In some situations, yes, while in others
it is difficult to make such a claim. The assumption that they are
better adapted to local climatic conditions stems from the comparison
at ground level. However, green roofs do not represent native
environments as engineered shallow growing media and more extreme
environmental conditions are the norm.

irst, if native plants are going to be mandated for green
roofs, then who decides the definition of a native plant?
The meaning of the word varies widely depending on who
you talk to. For example, the Sustainable Sites Initiative and
the EPA Green Landscaping Program have different definitions. There are definitions based on geographic regions, state
boundaries, time, ecosystems and other factors. It is quite possible to have multiple ecosystems within the same geographic
region. Throughout Michigan for example, glacial deposits have
resulted in multiple soil types in close proximity to each other.
On my own property, one corner is sandy while the rest is clay.
Plants that would thrive on these two types of soil and thus be
'native' are very different.
Much has been written about the drought tolerance mechanism of sedum. Even so, not all sedums are really suitable
for green roofs. Sedum ternatum is native to the sand dunes
along Lake Michigan, but has performed poorly in our plant
evaluations. Because of a climate that normally includes ample
rainfall most native plants in Michigan are not going to exhibit
enough drought tolerance to survive on a shallow green roof.
In contrast, many plants native to parts of California or the arid
western states are very drought tolerant or they wouldn't be able
to survive at ground level in those locations. In regions where
plants have evolved under drought condition then the use of
natives makes sense.
Whether native or not, within a given location, depth of the
growing substrate determines what types of plants can be grown

LIVING ARCHITECTURE MONITOR / SUMMER 2020 / 8

unless irrigation is provided.
Generally, as substrate depth
increases, moisture retention,
plant survivability, and the
number of potential suitable
plants also increases. For
example, in 2011, a portion of
the roof on the Molecular Plant
Sciences Building at MSU
was planted with 17 species of
native perennials and grasses.
After eight years, although the
20 cm deep roof still maintains
100 per cent coverage, the
roof consists of mainly Allium
cernuum (a Michigan native)
and several species of sedum
that moved into the bare spots.
Most of the original native herbaceous perennials and grasses
have slowly disappeared, but
may have survived in a deeper
substrate. Although these were
all native species, the same scenario would have occurred with
similar non-native species.
I think that the strongest

argument for use of native
plants is that they are usually
superior from an ecological
perspective. Plant community
influences a roof's ability to
provide wildlife habitat for
microorganisms, insects, birds,
and other animals. A good example is the monarch butterfly
(Danaus plexippus). Monarchs
have co-evolved with milkweed
(Asclepias ssp.) and their
caterpillars rely on milkweed
as their sole source of food.
The eastern US population
of monarchs has declined 80
per cent since 1990, due in
part to the removal of native
habitat. Planting milkweed on
green roofs is at least a step
in the right direction. Even
so, non-natives may provide a
food source for other species
at times when natives are not
flowering. The major factor
that determines biodiversity
and wildlife habitat is the
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Green Roofs - Living Architecture Monitor - Summer 2020

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