Green Roofs - Living Architecture Monitor - Spring 2018 - 10

shallower roof. Plant diversity decreased
every year, but plummeted in 2016.
The major die off was likely due to
drought conditions during the spring
and summer of 2016. During a 57 day
period, May 16 to July 11, total rainfall
amounted to 3.28 cm (1.29 in). About
a third of that fell on one day, July 1.
Comparatively, normal rainfall for May,
June, and July is 8.5, 8.8, and 7.2 cm
(3.35, 3.46, and 2.83 in), respectively.
The Molecular Plant Sciences study
points out the fact that supplemental
irrigation may be critical to maintaining species diversity on a green roof.
Irrigation may be necessary or the
substrate must be deep enough to
support the plants selected. Apparently, 20 cm (8 in) is not deep enough
for long-term survival of herbaceous
perennials and grasses in East Lansing,
Michigan. Overall, a balance must
be found between providing enough
water to maintain plant health and
aesthetics while allowing the substrate
to dry out enough to provide maximum
stormwater storage capacity. To increase
water use efficiency, moisture sensors
could be installed in the substrate that
would trigger an irrigation event when
water availability reaches a minimum
threshold.
If the decision is made to provide
irrigation, the next question is by
what method. When comparing the
efficiency of overhead, drip, and subirrigation on green roofs, we found
that overhead was most favorable for
water distribution and retention as
well as plant health (Rowe et al., 2014,
Ecological Engineering 64:306-313).
Because green roof substrates tend to
be coarse to allow adequate drainage,
limited capillary movement resulted
in the majority of the water draining directly to the bottom and out
of the system for both the drip and
sub-irrigation treatments. In contrast,
overhead irrigation distributed water
over 100 per cent of the surface, wetted
LIVING ARCHITECTURE MONITOR / SPRING 2018 / 10

"THE ULTIMATE GOAL IS
TO MAXIMIZE IRRIGATION
EFFICIENCY AND REGARDLESS
OF IRRIGATION METHOD,
WATER THAT DOES NOT
REACH THE ROOT ZONE AND
BECOME AVAILABLE TO THE
PLANT IS WASTED."
- DR. BRADLEY ROWE

the entire area in a much shorter period of time, and resulted in less wasted
water. This conclusion could be altered
depending on factors such as windy
conditions, frequency and duration of
irrigation events, spacing of emitters,
and whether the roof was just planted
or had time to become established.
Even so, drip irrigation may be more
practical for irrigating individual plants

such as trees, shrubs, or vegetables that
may be found on an intensive roof. The
ultimate goal is to maximize irrigation
efficiency and regardless of irrigation
method, water that does not reach the
root zone and become available to the
plant is wasted.
In addition, the use of moisture
retention fabric (MRF) and cups in
drainage layers are other methods to
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