Green Roofs - Living Architecture Monitor - Spring 2018 - 9

PLANT PROFILE

THE EFFECT OF IRRIGATION ON
STORMWATER MANAGEMENT
BY DR. BRADLEY ROWE, MICHIGAN STATE UNIVERSITY, EAST LANSING

The ability of a green roof to retain stormwater depends on many factors
including plant species, substrate depth and composition, and the preexisting moisture present before a given rain event. All of these factors can
be influenced by irrigation practices which in turn may have a positive or
negative influence on a green roofs ability to retain stormwater.

P

lant species can influence stormwater retention. Plant
photosynthetic metabolism has a major effect on transpiration (see my 2016 discussion comparing C3, C4, and
CAM plants, Living Architecture Monitor 18(1):24-29).
In addition, those plants with more biomass have a greater
ability to intercept rainwater with their foliage and allow it to
evaporate before reaching the substrate surface. Nagase and
Dunnett (2012, Landscape and Urban Planning 104:356-363)
showed that plant types such as forbs and grasses that possessed greater shoot and root biomass were more effective in
reducing water runoff than those with less biomass such as
sedum.
Tied to plant species is substrate depth and composition.
Deeper substrates are able to hold more water and therefore
are able to support plants with greater biomass. Changes in
composition can also influence water holding capacity and
capillary movement. As aerated pore space decreases, water
holding capacity increases and vice versa.
In addition, pre-existing substrate moisture present before
a given rain event is a major factor. To achieve maximum
LIVINGARCHITECTUREMONITOR.COM

stormwater retention we want the roof to be dried out before
each rain event. However, a minimum moisture level must be
maintained if one wishes to keep alive the herbaceous perennials and grasses that possess this greater biomass. Because of
their higher transpiration rates many of these plants may die
during periods of drought. In addition, in some cases a shallower depth supporting sedum may actually be able to hold
more rainwater since this shallow depth may be completely dry,
whereas a deeper depth may still contain pre-existing moisture.
A good example is the roof on the Molecular Plant Sciences
Building at Michigan State University. This roof was planted
in a substrate depth of 20 cm (8 in) with 17 species of native
herbaceous perennials and grasses during 2011. The roof was
watered as needed the first two years to ensure establishment,
but thereafter it had to rely on natural rainfall. After eight
years, ten of the original 17 species have disappeared. The roof
is now dominated by Allium cernuum with a limited number of
Asclepias tuberosa, Coreopsis lanceolata, Geum triflorum, Koeleria
macrantha, Penstemon hirsutus and Sporobolus heterolepis still
present. There is also sedum infiltrating from the surrounding
LIVING ARCHITECTURE MONITOR / SPRING 2018 / 9


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Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Spring 2018

Table of Contents
Green Roofs - Living Architecture Monitor - Spring 2018 - I
Green Roofs - Living Architecture Monitor - Spring 2018 - Cover1
Green Roofs - Living Architecture Monitor - Spring 2018 - Cover2
Green Roofs - Living Architecture Monitor - Spring 2018 - Table of Contents
Green Roofs - Living Architecture Monitor - Spring 2018 - A1
Green Roofs - Living Architecture Monitor - Spring 2018 - 1
Green Roofs - Living Architecture Monitor - Spring 2018 - 2
Green Roofs - Living Architecture Monitor - Spring 2018 - 3
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Green Roofs - Living Architecture Monitor - Spring 2018 - 34
Green Roofs - Living Architecture Monitor - Spring 2018 - Cover3
Green Roofs - Living Architecture Monitor - Spring 2018 - Cover4
Green Roofs - Living Architecture Monitor - Spring 2018 - M1
Green Roofs - Living Architecture Monitor - Spring 2018 - AD1
Green Roofs - Living Architecture Monitor - Spring 2018 - AD2
Green Roofs - Living Architecture Monitor - Spring 2018 - AD3
Green Roofs - Living Architecture Monitor - Spring 2018 - AD4
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https://www.nxtbook.com/dawson/greenroofs/lam_2016fall
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https://www.nxtbook.com/dawson/greenroofs/lam_2013winter
https://www.nxtbook.com/dawson/greenroofs/lam_2013fall
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https://www.nxtbook.com/dawson/greenroofs/lam_2012fall
https://www.nxtbook.com/dawson/greenroofs/lam_2012summer
https://www.nxtbook.com/dawson/greenroofs/lam_2012spring
https://www.nxtbook.com/dawson/greenroofs/lam_2012winter
https://www.nxtbook.com/dawson/greenroofs/lam_2011fall
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https://www.nxtbook.com/dawson/greenroofs/lam_2011winter
https://www.nxtbook.com/dawson/greenroofs/lam_2010fall
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https://www.nxtbook.com/dawson/greenroofs/lam_2010winter
https://www.nxtbook.com/dawson/greenroofs/lam_2009fall
https://www.nxtbook.com/dawson/greenroofs/lam_2009summer
https://www.nxtbook.com/dawson/greenroofs/lam_2009spring
https://www.nxtbook.com/dawson/greenroofs/lam_2009winter
https://www.nxtbook.com/dawson/greenroofs/lam_2008fall
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