Green Roof Plants:
Establishment, Viability and Maintenance
Project Report To:
Investment Agriculture Foundation of British Columbia
Michelle Nakano, Nicolas Rousseau, Deborah Henderson
April 2013
Funding for this project has been provided by Agriculture and Agri-Food Canada through the
Canadian Agricultural Adaptation Program (CAAP). In British Columbia, this program is
delivered by the Investment Agriculture Foundation of BC.
Agriculture and Agri-Food Canada (AAFC) is committed to working with industry partners.
Opinions expressed in this document are those of the authors and not necessarily those of
AAFC.
Further funding provided by the BC Landscape & Nursery Association, Kwantlen Polytechnic
University, and BCIT School of Construction and the Environment.
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SUMMARY
Extensive green roofs present an emerging market for the horticulture industry due to their
increased use in sustainable building construction for the proven environmental and cost savings
benefits of this technology. However, successful green roof plant establishment and viability in
the unique coastal maritime environment of the Pacific Northwest is of critical value for
advancing green roof infrastructure in this region.
British Columbia’s nursery and landscape sectors have been a vital component of our region’s
success with traditional rooftop gardens for over 40 years. Also known as intensive green roofs,
these plantings have substrate depths greater than 350 mm (>14”), a high diversity of plants and
maintenance requirements, and high installation costs. In contrast, extensive green roofs with
substrate depths of 50 to 150 mm (2”-6”), reduced plant selection and minimal maintenance
requirements typically have lower installation costs. However, the extreme conditions associated
with extensive green roofs present challenges for plant establishment and viability.
To determine a cost effective establishment protocol for extensive green roofs, this study
examined the combined effect of planting methods and weeding frequencies on planted and
weed species coverage for an extensive green roof with 125 mm depth of substrate located at the
British Columbia Institute of Technology (BCIT) Main Campus in Burnaby, B. C. The effects of
the three planting methods (bulbs and cuttings, plugs, and pots) and two weeding frequencies
(once and three times per year) were studied over an 18 month establishment period. Results of
the study showed that establishment from bulbs and cuttings had the highest vegetation coverage,
the lowest cost per square meter and the lowest incidence of weed species followed by plugs and
then pots. However, due to seasonal limitations associated with the planting of cuttings, plugs
provide a feasible compromise between cost, limitations, and rapid plant coverage during the
establishment period.
Results from this study should be used to enhance evidence-based practices for the green roof
industry in the Pacific Northwest. Further investigation to complement the results of this study
include larger scale testing of plugs and cuttings as the recommended propagation materials for
extensive green roof establishment, and examination of the interaction between fertilization and
weeding frequency for long term plant viability.
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INTRODUCTION
Green roof technology has been around for centuries, but the modern green roof developed in
Germany in the 1960’s has spread steadily across North America in the past few decades as a
component of sustainable building construction. The demonstrated environmental and cost
benefits of integrating vegetation and built space (i.e. green roofs) address multiple energy and
urban land use issues (MacIvor and Lundholm 2011; Perini and Magliocco 2012). Vegetation
and substrate shade ultra violet radiation and insulate against temperature extremes to reduce
costs associated with building heating and cooling and roof maintenance and replacement. Storm
water collection and delayed run-off from building roofs reduce demands on municipal
infrastructures. Evapotranspiration and the capacity of vegetation to absorb carbon and filter
particulate contribute to improved air quality and mitigate the urban heat island effect. Increased
green space, biodiversity, and sound insulation contribute to the livability of urban settings (Bell
and Spolek 2009; Getter and Rowe 2006). As a consequence of the multiple benefits the North
American green roof industry grew by 115% between 2010 and 2011 (Green Roofs for Healthy
Cities, Annual Green Roof Industry Survey 2012). As a result of market expansion, the multiple
environmental and energy efficiencies of green roof technology are being extended on building,
neighborhood, and city scales (Kohler 2008; Obendorfer et al 2007; Philippi 2006).
There are two types of green roofs - intensive and extensive. In British Columbia, accessible
rooftop gardens, also known as intensive green roofs, with depths of substrate often greater than
300 mm and high plant diversity and maintenance requirements have been a stable market for
nursery and landscape sectors for over forty years. However, installations of green roof systems
with shallower depth of substrate (50–150 mm) - known as extensive green roofs - are
increasingly being driven by jurisdictional mandates, incentives, and policies for achieving the
proven environmental and cost saving benefits of this technology (Connelly et al 2009; Getter
and Rowe 2009). Extensive green roof technology represents an emerging market for plant
production, installation and maintenance. However, plant selection, establishment, and
maintenance for these engineered, fabricated systems differ from practices for intensive plantings
that are basically similar to ground-level gardens (Snodgrass and Snodgrass 2006).
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Often functional in purpose with limited accessibility, shallow-depth extensive green roof
systems with reduced plant selection, and minimal material and maintenance requirements
typically have much lower cost requirements than intensive green roofs (MacIvor and Lundholm
2011). While reductions in the depth of substrate presents a viable means of reducing the cost of
implementation, it also presents the potential to compromise plant establishment and viability
(Durhman et al 2007). In order to advance the capacity of the horticulture industry to access the
developing extensive green roof market, information is needed to obtain the desired effect from
the planting while reducing the rate of failure and costs for unnecessary work associated with
improper establishment (B.C. Standard for Extensive Green Roofs, BCLNA 2007).
Regardless of the depth of substrate (i.e. intensive or extensive) plant establishment on green
roofs is influenced by the initial planting method, the season of planting, provision of an
appropriate substrate and adequate irrigation, and effective maintenance during the establishment
period (Monterruso et al 2005; Dunnett and Kingsbury 2010). Additionally, the influence of the
climatic context on optimal plant coverage in the desired time frame requires regional
adaptations in green roof practices (Connelly et al 2005; Schroll et al 2011). However, limited
research is available on regional green roof plant establishment and maintenance strategies for
the unique coastal maritime environment of the Lower Mainland and Fraser Valley (BuildSmart,
Metro Vancouver 2009; Martin and Hinckley 2007). Specifically, precipitation patterns for the
region include heavy rainfall driven by eastward outflow winds during winter months with low
air temperature and high relative humidity while rainfall during the summer can be minimal,
reaching drought levels at times during July and August.
In this study the impact of planting method and maintenance frequency on the establishment of a
community of sedums and bulbous species was examined on a built-in-place green roof with a
125 mm substrate depth located in the lower mainland region of British Columbia. The first
objective of the study was to compare the effects of three plant establishment methods (pots vs.
plugs vs. cuttings), on planted and weeds species cover. The second objective was to compare
the effects of high and low weeding frequencies on plant establishment and cover. The third
objective was to evaluate the combined cost effectiveness of establishment methods and
maintenance frequencies on installation costs.
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MATERIALS AND METHODS
Study Site Description and Conditions. The research infrastructure is located on a 1400 m2
Elevated Research Platform (ERP) located on the British Columbia Institute of Technology
(BCIT) Main Campus in Burnaby, B. C., Canada (49o15'N, 123o00'W, elevation 26m). The ERP
was designed as a 10-meter high structural canopy over an outdoor learning environment, and as
an unheated infrastructure for green roof research with a weather station, a data acquisition
system, and an irrigation source. Situated in an urban setting, the ERP site is exposed to full sun
and wind and is not influenced by shading from buildings or tree cover. While the surrounding
plant community is minimal, bird flocks are common to the ERP (Fig. 1).
Figure 1. The Elevated Roof Platform (ERP) Site, BCIT Campus, Burnaby, B.C. Canada.
Source: 2012 Google.
During the period of the study (September 2010 – September 2012), the monthly minimum
average temperature ranged from 1.3o to 17.4o C and the monthly maximum average temperature
ranged from 6.1o to 26.49 o C. The average monthly precipitation values ranged from 0.36 mm to
8.6 mm (Fig. 2). The ERP weather conditions during the study were consistent with the 30 year
average for the lower mainland region of B.C.
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Figure 2. Monthly average maximum air temperatures (o C), monthly average minimum
temperatures (o C), and monthly average precipitation (mm) throughout the study (10 Sept. 2010
to 30 Sept. 2012). Data are from the BCIT Elevated Roof Platform automated weather station
(49o15ʹN, 123o00ʹW). Data for the 30 year average are from the YVR Vancouver Airport (49.19
o
N, 123.17 oW).
Plot Construction. Two research enclosures with length X width X depth dimensions of 5.0 m x
5.0 m x 125 mm were installed approximately 6 m apart on the northeast and southwest
quadrants of the ERP. Each enclosure replicated standard built-in-place extensive green roof
construction with regard to protective waterproof roof membrane, drainage panel, and substrate
and vegetation layers. Sopradrain Eco-2 drainage panel was installed above the waterproof
membrane of the ERP built up roof system and the research enclosures were installed above the
drainage panel (SopremaInc. Drummondville, QC, Canada). The roof slope of 2% provided
positive drainage for the enclosures. The wood and recycled plastic-framed enclosures included
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sides that extended 125 mm above the waterproof panel. The enclosures were subdivided into
eighteen self-contained 0.8 m wide x 1.6 m long plots using wood and recycled plastic dividers
(Fig. 3). Each plot was individually lined with Microfab root barrier fabric (SopremaInc.
Drummondville, QC, Canada). Bird deterrent frames were constructed of wood and netting for
each research enclosure.
Figure 3. ERP research enclosure design and construction used in the trial.
Substrate. Each plot received 125 mm of lava rock-based growing substrate. Each plot was
filled with 5 - 25 kg bags of substrate to a depth of 125 mm. Initial settling of the substrate was
achieved with thorough irrigation and light compaction was completed using an empty drum
roller. One additional bag of substrate was then added to each plot to achieve the final 125 mm
depth. Physical analysis confirmed that the lava rock-based substrate met industry standards for
extensive green roof media (SGS Agri-Foods Laboratories, Guelph, Ont.).The substrate was
composed of 70% mineral aggregates by volume and 5% organic matter, dry base. At
installation the saturated bulk density, dry bulk density, pore space, volumetric water retention,
and pH were 1.39 g.cm-3, 0.85g.cm-3, 25%, 35%, and 7.1 respectively (Pacific Soil Analysis,
Richmond, B.C.).
Plant Material. The plant community was composed of four Sedum species commonly used on
green roofs: Sedum album L. 'Coral Carpet' (Coral Carpet stonecrop), Sedum divergens S.
Watson (spreading stonecrop, a PNW native), Sedum kamtschaticum var. floriferum Fisch.
'Weihenstephaner Gold' (Weihenstephaner Gold stonecrop), Sedum rupestre L. 'Angelina'
(Angelina stonecrop), and two geophytes or bulbous species indigenous to the Pacific Northwest:
Allium cernuum Roth (nodding onion) and Triteleia hyacinthina (Lindl.) Greene (fool’s onion).
Plant selection criteria included survival rate, plant propagation characteristics, provenance
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relative to the green roof growing environment, adaptability to climatic conditions, and seasonal
visual appeal (Snodgrass and Snodgrass 2006).
Treatment Description and Plot Layout. The study examined the combined effect of three
types of planting methods (plants vs. plugs vs. cuttings) and two maintenance/weeding
frequencies (once (June) vs. three times (June, October, and February) per year) on plant
establishment; total of six treatments. For the purpose of the study, the two enclosures were
treated as one experimental area, thus the total number of plots was 36 and each of the six
treatments was replicated six times with treatments randomly assigned to plots across the
treatment area (Fig. 4).
North East Enclosure
BC.L
Pg.H
Pg.L (3)
(1)
(2)
P.L
P.H
P.L
(4)
(5)
(6)
BC.H
BC.H
Pg.L (7)
(8)
(9)
BC.L
Pg.H
P.L (12)
(10)
(11)
P.H
Pg.L
P.H
(13)
(14)
(15)
Pg.L
BC.L
BC.H
(16)
(17)
(18)
South West Enclosure
BC.H
P.H
Pg.H
(19)
(20)
(21)
Pg.H
BC.H
BC.L
(22)
(23)
(24)
BC.L
Pg.L
P.L (27)
(25)
(26)
P.H
BC.H
Pg.L
(28)
(29)
(30)
Pg.H
P.H
P.L
(31)
(32)
(33)
BC.L
P.L
Pg.H
(34)
(35)
(36)
Figure 4. Treatment layout for the Green Roof Plants: Establishment, Viability and Maintenance
study of six combinations of three plant establishment methods (P=pots, Pg=plugs and BC= bare
bulbs/cuttings) and two maintenance frequencies (H=high weeding, three times per year in June,
October, February, L=low weeding, once per year in June). Plot numbering is indicated in
parentheses.
Plot Establishment. Planting of the plots as per Figure 4 occurred between 13 and 23
September, 2010. Plant material in pots (57 cm3), plugs (72/flat), and bare bulbs and un-rooted
cuttings (75–100 mm shoots) was obtained by custom order and from stock material from a
regional nursery. Cuttings were stored overnight at 5oC prior to planting. All plant specimens
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were inspected and any visible weed species were removed prior to planting. Pots and plugs were
planted in moist substrate using triangular plant spacing. Sedums and bulbs in pots were spaced
and planted at 200 mm on center (8'') with 25 plants per m², and 32 pots per 1.3 m2 plot with an
initial plot cover of approximately18-28%. Spacing and planting of sedums and bulbs as plugs at
150 mm on center (6''), established 49 plants per m², 60 plants per 1.3 m2 plot, and initial plot
cover of approximately 7-17%. Bare bulbous species were planted at a depth of 30 mm (1.25'') at
a spacing of 100 mm (4'') on centre in the twelve bare bulbs/ cuttings plots with 100 bulbs per
m², and 16 plants (8 Allium cernuum Roth and 8 Triteleia hyacinthina (Lindl.) Greene) per plot.
Cuttings were scattered at a nursery rate of 1.4 kg/m², and 1.8 kg/m² for each 1.3 m 2 plot, with
average initial plot cover of 90%. After scattering, the cuttings were evenly pressed to promote
contact with the substrate. Once survival was confirmed plots established from cuttings were
thinned to approximately 50% cover prior to January 2011.
Fertilizer. Nutricote type 100, 14N-14P-14K (SunGro Horticulture, Abbotsford, B.C. Canada)
slow release fertilizer was hand-applied at a rate of 150 ml/ 1.3 m2 plot at the time of planting.
Additional fertilizer was not applied during the study.
Irrigation. All plots received irrigation to run through following planting. An automated
overhead irrigation system was constructed onsite to promote plant establishment. The system
was programmed to run for 2-minute cycles twice daily from late September until the onset of
winter rain in the Pacific Northwest. With the end of establishment irrigation in late October, the
automatic system remained available to provide optimum irrigation operating daily for 10
minutes during subsequent periods of regional drought between June and September.
Pest Control. During the first 21 days of the study, greenhouse shade cloth was used to protect
the plots from avian damage until permanent protection frames were installed. The BCIT campus
is located in the Still Creek habitat with the largest crow population in B.C.’s lower mainland.
Bird damage was mitigated with the installation of netting that was not harmful to the birds.
Data Collection. Data on plant cover of the plots included measurement of total, and planted
species (Sedum album L. ‘Coral Carpet’, Sedum divergens S. Watson, Sedum kamtschaticum
var. floriferum Fisch. 'Weihenstephaner Gold', Sedum rupestre L. 'Angelina', Allium cernuum
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Roth, and Triteleia hyacinthina (Lindl.) Greene, and spontaneous weed species. Measurements
were recorded using digital images on three-month cycles January 2011 to September 2012, for a
total of eight assessments. A portable camera stand was constructed to raise a camera 180 cm
above the research plots. The digital camera, a Nikon D90 Zoom AF-S NIKKOR 18-105 mm
was suspended on the camera stand. To obtain plant cover data a grid structure (Fig. 5) was
overlaid on each digital image to facilitate observation. To remove the edge effect, the edge
squares of the grid structure were excluded from assessment. The excluded area represented
approximately 10% of the area of each plot surface. A visual assessment method commonly used
in Ecology for its simplicity and efficiency was used to study the plant cover or relative area
covered by plant species in the plot images (Damgaard, 2009). The B.C. Institute of Technology
Green Roof Research Facility Scale (GRRF) was used to assess plant cover (Table 1). Data were
recorded as a number value that represented the assessed percent plant cover attributed to each
grid square on a plot (Rousseau et al 2010).
7.5cm
140cm
7.5cm
3.75cm
75cm
3.75cm
Figure 5. Grid structure overlaid on each digital image to record plot cover.
Table 1. BCIT Green Roof Research Facility Plant Cover Scale used to assess percent plant
coverage per grid square.
Scale Score (S)
% Cover
.5
less than 5%
1
>5 to10%
2
>10% to 20%
3
>20% to 30%
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4
>30% to 40%
5
>40% to 50%
6
>50% to 60%
7
>60% to 70%
8
>70% to 80%
9
>80% to 90%
10
>90% to 100%
Data Analysis. Effect of planting method and maintenance frequency was examined on total
cover, planted species cover and weed species cover using two-way repeated measures
MANOVA. Follow-up profile analyses were done with one-way ANOVA and post-hoc means
comparison was done with Tukey-Kramer HSD test. In addition to coverage at each assessment
period we also examined the overall change in coverage from the beginning of the trial to the
end, i.e. ([cover Sept 2012-cover January 2011]/cover January 2011)*100, for planted species
using two-way ANOVA. Data were analyzed using JMP-In Version 5.1 (SAS Institute, Chicago,
IL).
RESULTS
Plant Establishment, Maintenance Frequency and Total Cover. We first examined the effect
of propagation material used for planting and maintenance frequency on total cover in plots and
found a complicated interaction of both effects with time (Table 2). However, total cover in plots
was recorded separately for planted species and weeds. Thus, we were able to get a clearer
understanding of the impact of propagation materials and maintenance regimes on plant
establishment on green roofs by examining planted species and weed cover separately.
Table 2. Summary of repeated measures MANOVA analysis of the effect of propagation
material used for planting and weeding frequency on the total plant coverage in plots over 18
months.
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F
P-value
Propagation Material
Weeding Frequency
Propagation Material X Weeding
Frequency
Time
Time X Propagation Material
128.43
1.97
0.18
Degree of
Freedom
2,30
1,30
2,30
1700.25
42.80
7,24
14,48
<0.0001
<0.0001
Time X Weeding Frequency
Time X Weeding Frequency X
Propagation Material
5.51
2.36
7,24
14,48
0.0007
0.01
<0.0001
0.18
0.84
Plant Establishment Methods and Planted Species Coverage. There was a significant
interaction between time and the propagation material on the coverage of planted species over
the course of the study (Table 3). Although planted species cover increased over time for all
three types of propagation materials, the coverage in plots established from bare bulbs and
cuttings remained the highest throughout the study (Fig. 6), except in June 2012 when the cover
in the plug plots (84.19±3.72) was similar to that of bare bulbs and cuttings (93.29±0.89) (Table
4). Initially (i.e. January 2011 and March 2011), coverage of planted species was significantly
greater in plots established with plants in pots than in plots from plugs. However by the first
growing season (June 2011) coverage of planted species was similar between the two types of
propagation materials (Fig. 6 and Table 4). From October 2011 to June 2012 planted species
coverage was significantly greater in plots from plugs. By the end of the trial (September 2012)
coverage was similar again, but due to an increase in the coverage of planted species in the plots
from pots. Overall, the coverage in the plots from pots was the most variable from one
assessment date to the next (Fig. 6).
Maintenance Frequency and Planted Species Coverage. Weeding frequency also had a
significant effect on planted species coverage (Table 3). Initially (January 2011 to June 2011)
coverage was similar for both high and low weeding frequencies. From October 2011 to
September 2012 planted species coverage was consistently higher in the high weeding frequency
plots, although the differences in planted species coverage were less than 5% (Fig. 7). Although
the interaction between Propagation Material and Weeding Frequency was not significant (Table
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2), it should be noted that in the plug and pot plots species coverage was higher for the high
weeding frequency plots than the low frequency plots; this difference was most dramatic in the
second year (Table 5).
Table 3. Summary of repeated measures MANOVA analysis of the effect of propagation
material and weeding frequency on the coverage of planted species over 18 months.
F
P-value
Propagation Material
Weeding Frequency
90.61
7.84
Degree of
Freedom
2,30
1,30
Propagation Material X Weeding
Frequency
Time
Time X Propagation Material
Time X Weeding Frequency
Time X Weeding Frequency X
Propagation Material
3.19
2,30
0.06
966.12
24.17
1.98
1.12
7,24
14,48
7,24
14,48
<0.0001
<0.0001
0.10
0.36
< 0.0001
0.009
Planted species - Propagation material
100
90
80
% cover/plot
70
60
Cuttings
50
Pots
40
Plugs
30
20
10
0
Jan 2011
March
2011
June
2011
Oct 2011 Jan 2012
March
2012
June
2012
Sep 2012
Figure 6. Effect of the type of propagation material on the coverage (mean ± s.e.) of planted
species in green roof plots from January 2011 - September 2012.
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Table 4. Summary of effect of propagation material on planted species % coverage (mean ± s.e.)
over eight assessment dates. Values within a row with different letters are significantly different
based on Tukey-Kramer means comparison (α = 0.006).
January 2011
March 2011
June 2011
October 2011
January 2012
March 2012
June 2012
September 2012
Bare bulbs, cuttings
55±0 a
47.68±1.62 a
88.89±4.12 a
94.02±0.25 a
94.69±0.15 a
94.69±0.15 a
93.29±0.89 a
94.68±0.22 a
Pots
34.22±1.26 b
41.53±1.07 b
69.89±0.73 b
73.71±1.43 b
63.37 ±3.47 b
69.88±3.78 b
65.06±8.57 b
81.08±2.83 b
Plugs
18.67±0.38 c
27.11±0.45 c
71.38±1.87 b
79.12±1.43 c
76.80±2.31 c
82.57±1.65 c
84.19±3.72 a
86.79±1.95 b
100
90
80
% cover/plt
70
60
High
50
Low
40
30
20
10
0
Jan 2011
March
2011
June 2011 Oct 2011
Jan 2012
March
2012
June 2012 Sep 2012
Figure 7. Effect of the High and Low weeding frequency on the coverage (mean ± s.e.) of
planted species in green roof plots from January 2011 – September 2012.
Table 5. Summary of effect of propagation material and weeding frequency on planted species %
coverage (mean ± s.e.) over eight assessment dates.
January 2011
March 2011
June 2011
October 2011
January 2012
Bare root
High
Low
55±0
55±0
49.5±2.68 45.85±1.73
86.67±8.33 91.11±2.38
93.95±0.42 94.08±0.32
95±0
94.8±0.14
Pots
High
Low
32.68±1.84 35.75±1.63
41.45±1.05 41.6±1.99
70.52±0.83 69.27±1.23
74.5±2.02 72.92±2.28
69.93±3.55 56.8±4.80
Plugs
High
Low
19.3±0.45 18.03±0.52
26.73±0.62 27.5±0.65
69.57±1.99 73.2±3.19
81.37±1.49 76.87±2.19
79.78±2.73 73.82±3.53
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March 2012
June 2012
September
2012
95±0
94.38±0.24 77.02±2.34 62.75±6.08 84.87±1.50 80.27±2.77
94.58±0.42 92±1.62
81.23±8.23 48.88±12.28 89.03±4.24 79.35±5.79
94.37±0.40 95±0
84.52±2.91 77.63±4.70 86.78±3.72 86.8±1.67
Increase in Planted Species Coverage over Time. In terms of the overall increase in planted
species coverage from the beginning of the trial to the end, the type of propagation material used
to plant green roofs had a significant effect, but weeding frequency did not (Table 6). While
planted species coverage increased in all plots regardless of the type of propagation material the
greatest increase in coverage was observed for plots planted with plugs (Fig. 8).
Table 6. Summary of two-way ANOVA analysis of the effect of propagation material and
weeding frequency on the increase of planted species coverage on green roofs plots from
January 2011 to September 2012.
F
Propagation Material
Weeding Frequency
179.60
0.11
Degree of
Freedom
2,30
1,30
Propagation Material X Weeding
Frequency
2.95
2,30
450
P-value
0.07
<0.001
0.74
c
% Increase in coverage
400
350
300
250
b
200
150
a
100
50
0
Cuttings
Pots
Plugs
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Figure 8. Increase in planted species coverage from January 2011 to September 2012 in green
roof plots planted with three different types of propagation materials.
Costs for Planted Species Propagation Material. A comparison of material costs at planting
(2010) for the three types of propagation material indicated that cuttings had the lowest cost/m2
followed by plugs and then pots (Fig. 9). However, by the end of the study (2012), the cost/m2
for plants in plugs and pots had decreased by approximately 31% while the cost/m2 for cuttings
remained the same (Table 7).
Cost in dollars (Cdn)
Propagation material cost/m2 2010 and 2012
50
45
40
35
30
25
20
15
10
5
0
Cuttings 2010 Cuttings 2012
Plugs 2010
Plugs 2012
Pots 2010
Pots 2012
Propagation material
Figure 9. Comparison of propagation material costs/m2 2010 and 2012.
Table 7. Percent change in propagation material cost/m2 from 2010 to 2012
Propagation
Material
2010
2012
% Change
Cuttings
26.40
26.40
0
Plugs
41.65
31.85
- 30.7
Pots
44.5
33.75
- 31.8
Propagation Material, Maintenance Frequency and Weed Cover. Weeds, or species of plants
that occurred spontaneously in the plots were identified as typical of the Pacific Northwest and
green roofs (Table 8). (Rousseau, et al 2009).
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Table 8. Elevated Roof Platform: main weed species and life history
Annual Life History
Cardamine hirsuta
snapweed
Medicago lupulina
black medic
Poa annua
annual bluegrass
Perennial Life History
Alnus rubra
Marchantia sp.
red alder
liverwort
Bryophytes
Oxalis corniculata
moss
creeping oxalis
Cerastium vulgatum
Sagina procumbens
mouse-ear chickweed
bird’s eye pearlwort
Epilobium spp.
Salix species
willowherb, fireweed
willow
Festuca rubra var. rubra
Taraxacum officinale
creeping red fescue
dandelion
Hypochoeris radicata
Veroncia spp.
hairy cat’s ear
speedwell
Weed coverage in green roof plots was significantly affected by the type of propagation material,
the weeding frequency and the interaction of the two (Table 9). There were significant time
effects on weed coverage over the period of the trial (Table 9). The lowest weed cover was found
in plots planted with the bare bulbs and cuttings and the highest weed cover in plots established
from plants in pots (Fig. 10). Not surprisingly, weed cover was higher and more variable from
one assessment date to the next in the low weeding frequency plots (Fig. 11). However, the
impact of weeding frequency was complicated by the type of propagation material - for example
for plots planted with bare bulbs and cuttings the weed cover was similar between High and Low
weeding frequency treatments (Fig. 12). In contrast for plots established from plants in pots there
were significant differences between the two weeding regimes (Fig. 12). These results
demonstrated that weeding frequency for green roof plots depended on the type of propagation
material initially used and potential for weed species in the propagation material at planting.
Therefore, a high weeding frequency made a significant difference when the propagation
material was plants from pots. For plants from plug material weeding frequency also resulted in
significant differences in weed cover; however these differences in coverage were smaller than
for plants in pots (Table 10).
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Table 9. Summary of repeated measures MANOVA analysis of the effect of propagation
material and weeding frequency on weed cover in green roof plots over eighteen months.
F
P-value
Propagation Material
Weeding Frequency
49.27
22.71
Degree of
Freedom
2,30
1,30
Propagation Material X Weeding
Frequency
Time
Time X Propagation Material
Time X Weeding Frequency
Time X Weeding Frequency X
Propagation Material
9.50
2,30
0.0006
30.30
8.66
5.60
1.81
7,24
14,48
7,24
14,48
<0.0001
<0.0001
0.0006
0.07
45
40
% Weed cover - Propagation material
Cuttings
35
Pots
30
Plugs
% weed cover/plot
< 0.0001
< 0.0001
25
20
15
10
5
0
Oct 2010Jan 2011 March
2011
June
2011
Oct 2011Jan 2012 March
2012
June
2012
Sep
2012
Figure 10. Effect of the type of propagation material on weed coverage (mean ± s.e.) in green
roof plots over eighteen months.
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35
High frequency
% weed cover/plot
30
Low frequency
25
20
15
10
5
0
Jan 2011 March 2011 June 2011
Oct 2011
Jan 2012 March 2012 June 2012
Sep 2012
Figure 11. Effect of High (June, October and February) and Low (June) weeding frequency on
weed coverage (mean ± s.e.) in green roof plots over eighteen months.
Bare root/High
70
Bare root/Low
Pots/ High
60
% weed cover/plot
Pots/ Low
50
Plugs/High
Plugs/Low
40
30
20
10
0
Jan 2011
March 2011 June 2011
Oct 2011
Jan 2012
March 2012 June 2012
Sep 2012
Figure 12. Effect of propagation material and weeding frequency on weed coverage (mean ± s.e.)
in green roof plots over eighteen months.
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Table 10. Summary of effect of propagation material and weeding frequency on % coverage
(mean ± s.e.) of weeds over eight assessment dates.
January 2011
March 2011
June 2011
October 2011
January 2012
March 2012
June 2012
September 2012
Bare bulbs, cuttings
High
Low
0±0
0±0
1.27±0.82 0.51±0.09
1.54±0.16 2.36±0.44
2.67±0.34 2.91±0.45
0.72±0.22 1.08±0.36
0.88±0.10 1.4±0.15
3.73±1.75 4.77±1.73
1.92±1.09 0.3±0.11
Pots
High
Low
3.63±0.28 3.05±0.29
7.15±1.13 6.33±0.60
14.37±1.35 36.25±7.77
9.18±2.15 12.7±1.89
11.38±1.71 32.37±4.75
8.57±1.40 28.2±6.10
16.45±8.38 46.87±11.89
9.23±3.11 18.38±5.11
Plugs
High
Low
3.47±0.51 3.2±0.20
6.33±0.35 6.03±0.66
9.18±2.01 15.22±2.71
3.02±0.58 4.09±1.30
6.98±1.47 16.72±2.69
4.43±0.76 11.58±2.03
7.72±4.49 16.62±6.57
1.68±0.27 5.48±0.94
DISCUSSION
The objective of this study was to determine a cost effective establishment protocol for extensive
green roofs, in the unique climactic conditions of Metro Vancouver. The rate of establishment
(measured by coverage of planted species) is influenced by a number of factors including the
growth characteristics of the planted species; the condition of the propagation material; and the
depth and properties of the green roof substrate (Rowe 2006, 2007, 2009). In this study the
influence of two factors - propagation material and maintenance frequency - on the establishment
of plants in a 125 mm depth of substrate was examined. At the end of 18 months, all six
combinations of propagation material and maintenance frequency had a total cover (of planted
species) that exceeded the industry standard of 60% minimum ground cover within the 12-18
month establishment period (Fig. 6). (FLL 2002; B.C. Standard for Extensive Green Roofs,
BCLNA 2007). The hardy succulents and bulbs used in this study were selected for their
adaptability and reliable performance in extreme climate conditions in shallow substrates. In
response to high relative humidity at planting adventitious roots emerged from multiple growth
tips on the sedum cuttings after two weeks and this direct rooting into the green roof substrate
provided rapid and uniform plot coverage.
In terms of propagation material, our results showed that plots established from bulbs and
cuttings had the fastest and highest rate of coverage, the lowest cost/m2 and the lowest incidence
of spontaneous weed species. However, there are risks associated with green roof establishment
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from cuttings in the Pacific Northwest. Plant selection for green roof design is limited to sedum
species, and due to the regional climate the seasonal availability of cuttings and planting is
restricted to April-May and June-September. Time required for transport and planting are
important to cutting survival due to their short shelf-life. Cuttings are also subject to damage and
loss due to poor handling and desiccation and loss due to wind, water and erosion, and to
disturbance by birds as reported by Snodgrass and Snodgrass, (2006) and Dunnett and Kingsbury
(2010). Our plots were relatively small and did not reflect the implications of these risks for large
scale installations of extensive green roofs.
In comparison, plants with established root systems in plugs and pots expand the green roof plant
selection, as well as extend the planting season in the Pacific Northwest from just after frost in
spring through the fall. In addition, these plants have a longer shelf life and greater potential to
survive as reported by Hoover (2010). When planted in moist substrate followed by irrigation to
run through, both plugs and pots were less susceptible than cuttings to the risk of desiccation and
wind and water erosion, but not damage by birds. In contrast to the rapid and direct rooting by
cuttings, the slower establishment rate for plants grown in organic media in plugs and pots was
attributed to the acclimation of the root zone to the inorganic green roof substrate as confirmed
by Getter and Rowe (2007). However, it was noted that although cuttings had 89% coverage in
the first growing season, the average coverage of 70% for plots from plugs and pots also
exceeded the FLL guideline for establishment cover (Table 4).
It would be expected that where the depth of substrate accommodates the larger root mass of a
plant grown in a pot, establishment and spread would be faster than from plugs as reported by
Snodgrass and Snodgrass (2006). However, in studies by Getter and Rowe (2007), Edwards and
Proffitt (2009), and Hoover (2010) plant establishment was also influenced by the depth of
substrate relative to the size of the plant root mass, the condition of the root mass, and age of the
plant tissue at planting. In the 125 mm depth of substrate used in our study, plant plugs with an
8.9 cm deep root mass were able to spread across the entire root surface; whereas plant roots
from 95 cm deep pots grew mostly in a horizontal direction. In our study much of the plant
material was custom grown while some plants were from stock that had longer periods of growth
in containers. The stock material had more potential to be root bound which influenced the rate
of plant establishment and coverage. For example, from January to June 2011 coverage had
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increased by 74% in plots from plugs compared to 51% in plots from pots (Table 4). In the
second growing season coverage in the plots from pots was within 6.5% of that of plugs and at
the end of the study pot coverage was within 14% of cuttings (Figure 6). As confirmed by
Snodgrass and McIntyre (2010), it was noted that custom grown plant material in the juvenile
tissue state tended to adapt more easily to the new environment. A combination of these factors
contributed to the notable increase of coverage by plugs to within 8% of cuttings by the end of
the study (Figure 6). As previously established by Emilsson (2008), the results of this study
confirmed that the planting method chosen to establish an extensive green roof should depend
more on the rate of coverage required and the potential cost, as the final vegetation cover was
comparable across cuttings and plugs at the end of the study.
It is widely accepted that weeding, watering, fertilizing and monitoring are critical to effective
plant establishment and coverage during the first twelve to eighteen months after green roof
installation (FLL 2002; BC Standard for Extensive Green Roofs, BCLNA 2007; Snodgrass and
Snodgrass 2006; Dunnett and Kingsbury 2010; Hoover 2010). The occurrence of weeds on green
roofs may be due to several factors including the geographic location, wind, people, birds, and
the substrate and plant material used. In this study the primary source of weed species was
ascribed to some stock plant material in pots and plugs and wind and birds as secondary sources.
Weed pressure was most apparent in plots established with pots, but less so in plugs and almost
negligible in plots from bulbs and cuttings (Fig. 10). In the second growing season weed cover
was close to 50% in the plots from pots with low weeding frequency. However, after weeding in
June 2012 planted species coverage in these plots increased to almost 80% which enabled the
planted species to compete and suppress weed coverage at approximately 18% until the end of
the study (Table 10). Based on these results and in combination with monitoring of the planted
species coverage during the establishment period, a minimum weeding frequency of three times
per year is recommended to mitigate the potential impact of weed pressure on planted species
coverage for plants from containers (Fig. 12). In contrast, weeding once per year combined with
monitoring of planted species coverage is a minimum for plugs and cuttings.
In 2009 extensive green roof installation costs in North America ranged from $80 –
200/m2 (BuildSmart, Metro Vancouver, 2009). In 2012 the average installation cost in B.C. was
$166/m2 (R. Schwenger, Architek Inc., personal communication, October 24, 2012) (P. O’Brien,
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Holland Landscaping, personal communication, January 22, 2013). While it is difficult to
accurately separate costs associated with individual green roof situations, plant material and
specialized substrate have been identified as major installation costs (BuildSmart, Metro
Vancouver, 2009), (R. Schwenger, Architek Inc., personal communication, October 24, 2012).
Further, unit costs for hand weeding during establishment are also difficult to quantify, but Peck
and Kuhn (n.d.) have reported that higher maintenance costs of $13 - $21/m2 have been used
during the establishment phase to budget for the size of a project, the timing of installation,
irrigation, and the size and types of plants used. According to Philippi (2006) potential cost
reductions in these areas to improve the cost benefit ratio for green roof installation will be
subject to market development. It was noted that the 31% reduction in the cost of plugs and pots
between the beginning of our study in 2010 to the end in 2012 was attributed to increased market
competition and economies of scale in the production of green roof plant material (Table 7), (R.
Nataros, N.A.T.S. Nursery, personal communication, October 5, 2012). (H. Argen, N.A.T.S.
Nursery, personal communication, November 29, 2012). As instructed by the B.C. Standard for
Extensive Green Roofs (2007), the choice of establishment method would depend on the most
cost-effective means of achieving the aims of the green roof planting given site specific risks and
restrictions. Although higher in cost when compared to cuttings, based on our results and the
development of the green roof plant market, plugs provide a feasible compromise between cost,
risks, and rapid coverage during establishment. Where the seasonal timing of installation and the
availability of water allow, a combination of plugs and cuttings would further mitigate costs and
enhance the rate of green roof establishment and coverage.
Recommendations for Further Study.
The B.C. Standard for Extensive Green Roofs (2007) specifies the Canadian Nursery and
Landscape Association: Canadian Standards for Nursery Stock (2006) as the pertinent standard
for vegetative cover. However, this standard refers to nursery stock for ground level landscapes
and does not seek to address the unique growing conditions associated with extensive green
roofs. Shallow depths of inorganic substrates, limited or no supplemental irrigation, and
extremes in climatic and site conditions are limiting factors for green roof plant establishment
and viability. Therefore, a recommendation is that the results of this study should be used to
enhance the B.C. Standard for Extensive Green Roofs.
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While this study examined the impact of high and low weeding frequency on plant establishment
and coverage, further investigation would complement the results of this study and enhance
evidence-based practices for the green roof industry in the Pacific Northwest. Two areas for
further investigation are larger scale testing of plugs and cuttings as the propagation materials for
extensive green roof establishment, and the interaction between fertilization and weeding
frequency for long term plant viability.
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