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Effects of green roof growth medium depths on
Sedum immergrauch establishment. Schaefer H.1, K.
Forrester2, V. Jost3, K. Luckett4, S. Morgan5, T.
Yan6 and W. Retzlaff1,2 1Department of
Biological Sciences 2Environmental Sciences
Program 3Jost Greenhouses 4Green Roof Blocks,
St. Louis MetalWorks, Inc. 5Department of Civil
Engineering 6Department of Mechanical
Engineering, Southern Illinois University
Edwardsville, Edwardsville, IL. 62026
(wretzla_at_siue.edu)
ABSTRACT The purpose of my project was to
determine the effect of various growth medium
depths on the establishment of Sedum immergrauch
in a green roof system. The shallower the growth
medium, the greater the physical demands on the
green roof plants. Shallow medium depth may lead
to root damage from heat and frost fluctuations
(Boivin et. al, 2001). Further, deeper medium
depth may provide additional water for plant use
during drought. A field study was
conducted to determine if growth media depths of
5, 10, 15, or 20 cm influence plant establishment
and survival of Sedum immergrauch in a green roof
system. Analysis of the results indicates after
ten weeks that there is no significant difference
in plant growth, plant performance or roof
coverage between 5, 10, 15, and 20 cm of growth
medium in this green roof system.
MATERIALS AND METHODS Roof Model Setup Green
roof models were set up in a completely
randomized design with four replications of 5,
10, 15, and 20 cm of growth medium depth. All 16
green roof models are 70 cm x 70 cm wood frames
with wafer board substrates and adhered EPDM
roofing membrane. The models have sheet metal
edging to retain the growth media. Testing
Models 4 with 5 cm of growth media with plants
4 with 10 cm of growth
media with plants 4
with 15 cm of growth media with plants
4 with 20 cm of growth media
with plants Plant Species Sedum
immergrauch Fertilizer Isobutylidene-diurea,
IBDU Nitrogen, a condensed
product of urea and isobutylaldehyde.
1,1-Diureidoisobutane Growing medium Fine
Arkalite expanded clay Irrigation By hand
initially for the first two weeks as needed. On
September 5, 2005, eighty Sedum immergrauch plugs
were planted into the sixteen model green roofs.
Each model contained five Sedum plugs randomly
assigned to growth media depths of 5, 10, 15, or
20 cm.
Figure 2. Plant growth index (htdiam1diam2)/3
of Sedum immergrauch grown in 5, 10, 15, and 20
cm growth medium depths for 10 weeks. (n20)
DATA COLLECTION The data collection methods in
the field included measuring plant growth, plant
performance, and roof coverage once a week for
ten weeks. Plant growth rate was quantified by
measuring each plants height and width in two
perpendicular directions. A growth index was
calculated for each plant by averaging the three
individual growth measurements (Monterusso et
al., 2005). Plant performance was quantified
by rating each plant on a scale of zero to five,
(0 - dead, 1 - stressed plant showing visible
wilting or browning, 2 - plant that showed little
change since planting, 4 - healthy plant
exhibiting a large amount of growth, and 5 -
exceptional growth and fullness) (Monterusso et
al., 2005). Roof coverage was quantified using
a 6 x 6 circle coverage grid (each circle is 3.8
cm in diameter) in 4 quadrants and counting the
circles that have plants in them. All recorded
data has been analyzed using SAS to determine if
green roof growing media depth alters Sedum
establishment (ANOVA for a completely randomized
design, a lt 0.05).
Figure 3. Plant performance of Sedum immergrauch
grown in 5, 10, 15, and 20 cm growth medium
depths for 10 weeks. (n20)
Figure 1. Green roof models with 5, 10, 15, and
20 cm growth medium depths at the SIUE
Environmental Sciences Field Site.
INTRODUCTION Today, building sustainable
communities has become a global challenge that
presents exciting new opportunities for the
widespread use of green roofs. Green roofs help
mitigate the impact of high-density commercial
and residential development by restoring
displaced vegetation (VanWoert et al., 2005).
The type of roof planting that is of the greatest
general interest and application in my project is
called an extensive green roof. An extensive
green roof system consists of shallow soil depth,
low weight, low maintenance, and wide plant
diversity (Dunnett and Nolan, 2004).
Although several plant species have been studied
for potential extensive green roof utilization,
the most preferred plants usually come from the
genus Sedum. Sedum plugs with established root
systems may quickly spread out their roots and
form dense vegetation in a green roof system.
Sedums are most commonly known for being tolerant
to extreme conditions of temperature, wind, sun
exposure, and drought that exist on exposed
rooftops. Plant species, growth medium
depth, and water availability are all factors in
determining the appropriate planting density of
this species to achieve optimal green roof
coverage. Having the drought tolerance
characteristic is essential for the long
life-span of an extensive green roof because of
limited irrigation and shallow substrate depth.
Deeper substrate depths may provide a greater
water holding capacity that optimizes plant
growth resulting in faster plant coverage
(VanWoert et al., 2005). Therefore, growth media
depths may alter Sedum immergrauch establishment
on green roofs.
Figure 4. Percent roof coverage by Sedum
immergrauch grown in 5, 10, 15, and 20 cm growth
medium depths for 10 weeks. (n16)
RESULTS AND DISCUSSION Growth Index There was
no statistical difference in plant growth between
5, 10, 15, or 20 cm of growth medium for 10 weeks
(Figure 2). Performance There was no
statistical difference in plant performance
between 5, 10, 15, or 20 cm of growth medium for
10 weeks (Figure 3). Roof Coverage There was
no statistical difference in percent roof
coverage between 5, 10, 15, or 20 cm of growth
medium for 9 weeks (Figure 4). At the 10th week
measurement, percent roof coverage in the 5 cm
depth was less than the roof coverage in the 15
cm depth. Fast establishment, fast substrate
coverage, and low mortality are all desirable
characteristics for Sedum immergrauch. Fast
initial growth is important because the faster
the plants cover the substrate surface, the fewer
the number of plants required (Monterusso et al.,
2005). In this study, under this set of growing
conditions, growth medium depths of 5, 10, 15 and
20 cm did not have any effect on the
establishment of Sedum immergrauch. It remains
to be seen whether, under different temperature
and moisture conditions, medium depth will
influence Sedum establishment in a green roof
system.
CONCLUSION The primary goal during green roof
establishment is to achieve 100 plant survival
and 100 roof coverage as soon as possible.
Different growth medium depths of 5, 10, 15, and
20 cm had no effect on the survival and growth of
Sedum immergrauch in my investigation. No Sedum
plants were lost in this study and, at the end of
the 10 week study period, more that 30 of the
green roof system was covered with plant foliage.
These green roof systems will continue to be
monitored for the next 2 years to assess plant
growth, performance, and roof coverage. It
remains to be seen whether growth medium depth
will alter any of these characteristics following
successful establishment.
LITERATURE CITED Boivin, M.A., Lamy, A. Gosselin,
and B. Dansereau. 2001. Effect of artificial
substrate depth on freezing injury of six
herbaceous perennials grown in a green roof
system. Hort Technology 11(3) 409-412. Dunnett,
N. and A. Nolan. 2004. The effect of substrate
depth and supplementary watering on the growth of
nine herbaceous perennials in a semi-extensive
green roof. Acta Hort. 643 305-309. Monterusso,
M.A., D.B. Rowe, and C.L. Rugh. 2005.
Establishment and persistence of Sedum spp. and
native taxa for green roof applications.
HortScience 40(2) 391-396. VanWoert, N. D., D.
B. Rowe, J. A. Andresen, C. L. Rugh, R. T.
Fernandez, and L. Xiao. 2005. Green Roof
Stormwater Retention Effects of Roof Surface,
Slope, and Media Depth. J. Environ. Qual.
341036-1044. VanWoert, N.D., D.B. Rowe, J.A.
Andresen, C.L. Rugh, and L. Xiao. 2005. Watering
regime and green roof substrate design affect
Sedum plant growth. HortScience 40(3) 659-664.