ARTICLE IN PRESS
Urban Forestry & Urban Greening 3 (2005) 103–111
www.elsevier.de/ufug
Comparison of establishment methods for extensive green roofs
in southern Sweden
Tobias Emilsson, Kaj Rolf
Department of Landscape Management and Horticultural Technology, Swedish University of Agricultural Sciences,
Box 66, SE 23053 Alnarp, Sweden
Abstract
The most common technique for establishment of thin extensive green roofs in Sweden has been using prefabricated
vegetation mats. Our study investigated (1) how the establishment of green roofs in Sweden was influenced by the
establishment method (prefabricated vegetation mat, plug-plant, shoot), substrate composition and species mixture,
and (2) whether on-site construction was a possible alternative. The establishment of the vegetation, which in all cases
consisted of succulent species, was recorded using the quadrate point intercept method in fixed plots and the success
measured as frequency cover.
Prefabricated vegetation mats had higher succulent plant cover than on-site constructed roofs. There was no
difference in succulent plant cover between plots established using plug-plants compared to shoots. Shoot-established
plots had more moss than the other establishment methods. The commercial substrate ‘Roof soil’ had significantly
higher succulent plant cover than the other substrates, which might be related to a higher nutrient content. The organic
content of the non-commercial substrates was rapidly decomposed. The standard species mixture produced a higher
cover than both the mix developed for northern conditions and the mix with an increased proportion of big leaved
species. The total cover of the plots was mainly dependent on the cover of two species: Sedum album (L.) and Sedum
acre (L.). Few species managed to establish spontaneously but the establishment of woody species highlighted the need
for proper maintenance.
r 2004 Elsevier GmbH. All rights reserved.
Keywords: Crassulaceae; On-site construction; Sedum; Vegetated roofs
Introduction
Green roofs are becoming increasingly popular in
many countries. The interest for green roofs has been
related to their capacity to reduce stormwater runoff
volumes and peak flows (Bengtsson, 2002), mitigate
urban heat island effects (Akbari et al., 2001) and cool
buildings during summer months (Eumorfopoulou and
Aravantinos, 1998; Onmura et al., 2001). Green roofs
Corresponding author.
E-mail address: [email protected] (T. Emilsson).
1618-8667/$ - see front matter r 2004 Elsevier GmbH. All rights reserved.
doi:10.1016/j.ufug.2004.07.001
can also be designed to improve urban biodiversity
(Mann, 1998; Brenneisen, 2003).
Installation of green roofs requires larger investments
than conventional roofs (Wong et al., 2003). Systems
with thick substrate layers and large plants are especially
expensive since they generally require reinforcement and
reconstruction of the building unless it was designed for
the extra load from the start. Thin extensive systems can
generally be built without making any adjustments to
buildings and this reduces the cost of the system
and increases the number of possible roofs that can
be vegetated. Even though the initial cost is high,
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calculations have shown that the life cycle cost of
extensive green roofs can be lower than the cost of
conventional roofs (Wong et al., 2003). This is due to the
extended life expectancy of the roofing membrane and
to the reduced energy consumption related to summer
cooling, which might be an important factor even in
countries like Sweden where cooling during summer is
becoming increasingly important in office buildings
(Nilson et al., 1997; Wong et al., 2003). Still, the high
investment can be seen as a barrier to a widespread use
of green roofs and much would be gained if extensive
green roof systems could be installed at a lower cost.
A fundamental part of the success of an extensive
green roof installation is connected to the establishment
and development of the plant material. Failure of the
vegetation during the initial phase means that new plant
material has to be brought to the site at an additional
cost, and there is also a risk for erosion of the substrate
if it has a lower cover during an extended establishment
period (Wolfgang, 2002). The goal of establishment is a
high cover of the desired vegetation but also survival of
the established plant species. The guidelines developed
by the German organisation Forschungsgesellschaft
Landschaftsentwicklung Landschaftsbau E.V (FLL),
focus on high cover and state that a green roof should
have a projective cover of at least 60% one year after
establishment (FLL, 2002).
Green roofs can either be established on-site or by
bringing prefabricated vegetation to the roof. In
Germany, where most of the development of technology
related to production and establishment of green roofs
has taken place, green roofs are most often constructed
on-site. On-site construction is generally achieved by
pumping or lifting the substrate onto the roof and then
distributing shoots, seeds or plug plants. In Sweden,
green roofs are mainly applied as prefabricated vegetation mats, which is generally one of the most expensive
ways to vegetate buildings but also a method that has a
low risk of failure and that ensures instant high plant
cover (Krupka, 1992; Schade, 2002; Dunnett and
Kingsbury, 2004). Vegetation mats are composed of
plants grown in a substrate that is fixed onto a carrier
material, e.g. geotextile, plastic net or coconut net. The
vegetation mats are lifted to the roof as fully established
vegetation during construction of the system. Vegetation
mats are currently used in southern Sweden and
Denmark but there are no comparative studies reported
where the less expensive technique of on-site establishment has been tested in this climate. The climate is less
extreme in southern Sweden compared to Germany but
the winters are slightly colder. This might influence the
survival-rate of the newly established succulent species,
since decreased substrate depths increases freezing
injuries of succulent plants (Boivin et al., 2001). Little
is known about how establishment of the on-site
constructed systems compares to the cover of a
prefabricated vegetation and what type of cover and
plant composition the consumer can expect when
deciding to use one or the other system.
Substrates are generally the same regardless of
establishment method. The main component in substrates is inorganic material with a high water-holding
capacity and low density such as pumice, lava, or
expanded clay (Roth-Kleyer, 2001). Recycled material
such as crushed roof tiles has also been used as a
component in roof substrate, even though the density is
higher than in pumice or expanded clay (Roth-Kleyer,
2001). The use of recycled material can be a way to
reduce the need for transport and to find use for a
locally available material that is otherwise worthless.
The composition of commercial substrates is surrounded with secrecy and patents, while at the same
time the basic idea of substrate composition is readily
available in guidelines developed in Germany during the
past 15–20 years (FLL, 2002). Our study is comparing
two substrates containing crushed roof tiles with a
commercial substrate. One of the substrates was
designed strictly according to the German guidelines
and the other was designed with a slightly increased
organic content since this would increase the water and
nutrient holding capacity of the substrate and possibly
improve establishment. Higher water and nutrient
holding capacity of the substrate might on the other
hand increase the possibility for establishment of weeds
that would influence the aesthetic characters of the roof
negatively.
The plants used on thin green roofs are succulents
belonging to the Crassulaceae family. The plants are
able to withstand sustained periods without water
through both biochemical and morphological adaptations. The thin substrate dries out rapidly but the
succulent morphology of the plants enables them to
store large amounts of water and thereby cope with
drought situations. Two of the plants commonly used,
Sedum album and Sedum acre, are known to express
crassulacean acid metabolism (CAM) during drought
periods (Sayed, 2001). Again, little information is
available on the survival and establishment rates of
different succulents in the Swedish climate. Most species
combinations have been developed in Germany and few
systematic studies on plant performance have been
performed in Sweden. The species mixes that were tested
in our study have been designed for both rapid cover
and aesthetics, and all have a high proportion of ground
covering Sedum species, S. album and S. acre, in the mix.
Our aim was to test how a standard mix commonly used
in Sweden and Germany compared to a mix composed
for more northern conditions and a more aesthetically
pleasing mix with a higher proportion of big-leaved and
more flowering species.
The overall aims of our study were: (1) to describe the
effects of establishment method, substrate composition
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T. Emilsson, K. Rolf / Urban Forestry & Urban Greening 3 (2005) 103–111
and species mixture on vegetation performance and
spontaneous establishment by moss and vascular
species; and (2) to test whether on-site construction
could be used for establishment of green roofs in
southern Sweden.
The success of the establishment was measured as
plant frequency cover and species composition. Other
interesting results such as aesthetic value of the systems
and the exact cost of installation are briefly discussed
but not analysed in depth.
Methods
The study was started in 2000 at the Augustenborg
botanical roof garden situated in the city of Malmö,
southern Sweden (551340 N, 13110 E). The green roofs
used in the study were thin ‘extensive green roofs’
divided into 105 vegetated roof sections measuring
1.15 m 6.5 m. The roofs were constructed on old
buildings currently used by Malmö city council and
they were located in an area with a combination of
industrial and residential buildings. The roofs were
orientated in a north-westerly direction and had an
inclination of 41. The green roofs had a maximum
water-saturated weight of around 55 kg/m2, making any
extra adjustments of the roof construction unnecessary.
The green roofs were composed of three layers, the
uppermost being a vegetation layer consisting of a 4 cm
thick growing substrate and the vegetation. The second
layer was a filter layer in the form of a geotextile, which
prevented small particles from being washed from the
substrate layer into the drainage layer or out from the
system. The third layer was a drainage layer in the form
of recycled foam material, sold under the product name
AquaTop.
All plots were given a fertiliser addition of 15 g/m2 at
the time of establishment and in spring the following
year. The fertiliser was a 50:50 mixture of a long-term
fertiliser (Multicote 8 M extra N:18-P:6-K:12) and a
conventional fertiliser (ProMagna N:11-P:5-K:18).
The cost of the vegetation layer in these experimental
plots was approximately 32 h/m2 for prefabricated
vegetation, 23 h/m2 for plug-plant establishment and
14 h/m2 for shoot establishment.
105
as for the shoot-established surfaces but after establishment they were kept in a sandpit and watered regularly.
The vegetation mats were established during spring 2000
and this meant that in reality they were 4 months older
than the on-site established surfaces at the start of the
experiment and at the start of the experiment they had
almost full cover. The plots established with plug plants,
each having a soil-root volume of 65 cm3, were
established with a density of 25 plants/m2 of the
individual species mixture. Shoots were established by
distributing 150–200 g shoots/m2 of the different species
mixtures. The same type of thin plastic net as in the
vegetation mats was used to reinforce the substrate in
shoot establishment.
Substrates
The first substrate was bought from the Swedish green
roof company VegTech and is referred to in this study as
‘Roof soil’. The composition of the substrate is not
known exactly but it is a commercially available
substrate that is composed of a natural soil mixture
improved by the addition of lava rock, expanded clay,
organic material and clay. The two other substrates,
substrates A and B, were mixed by us according to the
list of content in Table 1. The main difference between
the two substrates A and B was in their organic matter
content and in the amount of crushed tiles (Table 1).
Plant mixes
All three types of plant mixtures used in this study
involved different combinations of the succulent species.
The first mixture was a standard mix commonly used for
green roof establishment in Sweden. The second mix was
designed for more northern conditions and had a large
proportion of S. acre. The last mix had a higher
proportion of big-leaved species and species that have
intense flowering (Table 2).
Experimental design
Establishment method, substrate composition and
species composition were varied in a factorial fashion
but due to time constraints prefabricated vegetation
Establishment methods
Table 1. Composition of substrates A and B expressed as
percentage by weight
Three establishment methods, pre-made vegetation
mats, plug plants and shoots, were used in the study.
The vegetation mats were made of a geotextile and a soil
substrate that was reinforced by a plastic net. The
substrate in the vegetation mats was the same as the
substrate used in the on-site establishment. The vegetation mats were established using the same methodology
Composition %-by weight
Substrate A
Substrate B
Clay
Broken limestone 8–12 mm
Crushed roof tiles 8–12 mm
Sand
Organic material (Peat)
5
5
50
37
3
5
5
43
37
10
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Table 2.
T. Emilsson, K. Rolf / Urban Forestry & Urban Greening 3 (2005) 103–111
Composition (%) of the three species mixtures standard, northern and big-leaved mixed used in the establishment
Species
Standard mix
Northern mix
Big-leaved mix
Sedum acre (L.)
Sedum album (L.)
Sedum rupestre (L.)
Sedum sexangulare (L.)
Hylotelephium ewersii (Ledebour)
Phedimus floriferus ‘Weihenstephaner Gold’ (Praeger)
Phedimus hybridus (L.)
Phedimus kamtschaticus (Fischer & C.A. Meyer)
Phedimus spurius (M. von Bieberstein)
40
40
70
10
5
5
30
30
5
10
5
Table 3.
10
5
10
10
5
10
Experimental setup studying the effect of establishment method, substrate and species composition
Species mix
Roof soil
Substrate A
Standard mix
Pre-fabricated mats
Plug-plants
Shoots
__
__
Plug-plants
Shoots
Plug-plants
Shoots
Pre-fabricated mats
Plug-plants
Shoots
__
__
Plug-plants
Shoots
Plug-plants
Shoots
Pre-fabricated mats
Plug-plants
Shoots
__
__
Plug-plants
Shoots
Plug-plants
Shoots
Northern mix
Big-leaved mix
Substrate B
The first part of the experiment involves the column at the left and the second part includes all grey shaded areas.
mats were only constructed on the commercially
available substrate Roof soil (Table 3). The unbalanced
design meant that the analysis had to be divided in two
parts. In the first part of the experiment only Roof soil
was involved. This part of the experiment investigated
the effect of the establishment method (plugs, shoots or
pre-made mats) and species mix (standard, northern, or
big-leaved) on plant cover. In the second part of the
experiment, three types of substrates (Roof soil,
substrate A and substrate B), three species mixes
(standard, northern, or big-leaved), and two types of
establishment methods (plugs and shoots) were involved
and the effects of these parameters on cover were
measured. All treatments were randomly assigned to
plots.
Vegetation survey
The survey of the vegetation was made in autumn
2001, one year after the roof vegetation had been
established. The vegetation of the plots was analysed
using a quadrate point-intercept method (Greig-Smith,
1983). The points were arranged in a 45 cm regular
13 13 grid with each point spaced 37.5 mm apart. The
grid was constructed by two nets arranged at 25 mm
distance from each other. The use of two grids ensures
perpendicular recording of vegetation cover. The crosshairs had a diameter of 2 mm. The grid was suspended
10 cm above the substrate layer.
All vegetation that was covered by the projection of
the cross-hairs was recorded as present. Great care was
taken to record all vegetation layers by carefully moving
the higher layers without disturbing the lower. In most
cases, the vegetation consisted of a single layer. Vascular
plant species, moss or lack of vegetation was recorded.
The point-intercept measurements were complemented
by a survey of plants species in every plot in order to
include species with low cover.
The vegetation was measured in three fixed quadrants
per experimental plot. The first replicate was randomly
located in the upper part of the roof, the second
replicate was randomly located in the middle part and
the third replicate was randomly located in the lower
part of the roof, resulting in a blocked experimental
design.
All succulents were identified and labelled according
to Eggli (2003). All other vascular plants were identified
and labelled according to Flora Europaea (Tutin et al.,
1968–1980, 1993).
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107
Soil analysis
Soil was collected from three randomly selected plots
for each treatment. The three soil samples from each
treatment were mixed into one bulk sample on which all
analyses were made. All soil analyses were preformed in
triplicate. Soil density and total pore space were
analysed after Proctor hammer compaction (FLL,
2002). Organic matter content was estimated as loss on
ignition (5501, 15 h) and calculated as % dry weight. pH
was measured in 0.01 M CaCl2 solution using a WTW
pH electrode SenTix 21. Determination of the CaCO3
was carried out according to the method of Scheibler
(Hoffmann, 1991). Phosphorus and potassium contents
were determined by calcium-acetate-lactate extraction
and magnesium was determined after CaCl2 extraction.
Plant available mineral nitrogen was extracted with
CaCl2 and analysed after steam distillation. Detailed
descriptions of chemical soil analyses methods are given
in Hoffmann (1991).
Statistical analysis
The plant frequency cover data collected using the
point intercept method were transformed using an
arcsin(x0.5) transformation in order to get homogeneous
variance (Underwood, 1998). A factorial ANOVA was
used to test for significant differences between the
treatment combinations and where differences existed,
means were separated by a Tukey test. The establishment of the succulent plants failed on one plot located
close to the projection of a higher roof and it was
therefore excluded from the analysis. Throughout the
analysis, the threshold for significance was set at
Po 0:05: Data are presented as mean7SE. All statistical analyses were performed using the SPSS vs. 10
statistical programme.
Results
The establishment was successful on most plots, but
there were substantial differences in the frequency cover
between the different establishment methods, species
mixtures and soil substrates.
The first part of the experiment, involving only the
Roof soil substrate, showed a significant difference in
cover between the establishment methods but also
between the species mixtures. The prefabricated vegetation mats that had been pre-grown for 4 months before
the experiment and were fully established at the start of
the experiment and still had a higher cover than shootand plug plant-established plots one year after establishment (Fig. 1). The mean succulent plant cover on the
prefabricated mats was more than 80% for all species
Fig. 1. Frequency cover of succulent plants one year after
establishment when different species mixtures and establishment methods were used on Roof soil substrate. Symbols show
mean values and bars represent standard error of the mean
value =Vegetation mats =Shoots =Plug plants).
mixtures. There was no significant difference between
the two other establishment techniques on the Roof soil;
they both had a mean succulent plant cover of
approximately 50–60%. The standard species mixture,
when used on the Roof soil substrate, had a significantly
higher cover than the other species mixtures. There was
no difference between the mix selected for more northern conditions or the mix with more big-leaved species.
The cover of moss showed a somewhat different
pattern. The only variable that had a significant
influence on the moss cover was the establishment
method. Plots established with shoots had significantly
higher moss cover than plots established with plug
plants or plots established with vegetation mats. There
was also significantly less moss on the vegetation mats
compared to the plug plant-established plots. The
composition of the species mixture used had no
significant impact on the moss cover on Roof soil
substrate (Fig. 2).
The total cover of succulent plants and moss was
affected by both establishment method and species
mixture (Figs. 1 and 2). The prefabricated vegetation
mats again had the highest cover, followed by shootestablished plots and finally plug plants. The standard
mix had a significantly higher cover than the big-leaved
species mix. There was also a block effect for the total
cover, which increased towards the edge of the roof.
The second analysis, excluding the prefabricated
vegetation mats, showed a significant effect for both
soil substrate and species mixture on frequency cover of
succulent plants. However, there was no difference in
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succulent plant cover between plug plants and shootestablished plots. There was also a significant interaction
between establishment method and soil substrate, which
showed a negative relationship of shoot establishment
on substrate A. In general, the Roof soil had a
significantly higher cover than the other substrates and
the standard mixture had the significantly highest
frequency cover of the species mixtures tested (Fig. 3).
The mean succulent plant cover was below 40% when
the northern mix or the big leaved mix were used for
shoot establishment on substrate A or plug planting on
substrate B (Fig. 3).
Fig. 2. Frequency cover of moss one year after establishment
when different species mixtures and establishment methods
were used on Roof soil substrate. The figure shows mean
values and standard error. ( =Vegetation mats =Shoots
=Plug plants).
The moss cover in the second analysis showed a
dependency on species mixture and establishment
method, but not on soil substrate. The use of the
standard species mixture resulted in less moss than the
other two species mixtures. The use of shoots generally
resulted in more moss. A significant interaction showed
a negative impact on moss cover when Roof soil was
used in combination with plug-plants (Fig. 4). There was
also a block effect for the moss cover, which increased
towards the edge of the roof.
The total cover of succulent plants and moss showed
significant positive effects of establishment with Roof
soil, when looking at all plots established with plug
plants or shoots (Figs. 3 and 4). There were also
significant interactions between establishment method
and soil substrate, which meant that the combination of
the shoot establishment on substrate A had a reduced
cover. There was also a significant block effect as the
total cover increased towards the bottom of the plots.
The total cover of the plots was basically made up of
S. acre and S. album. The other species had never more
than 7% of the total mean succulent cover. S. album was
especially favoured by the establishment with the
standard mix on vegetation mats where it constituted
close to 90% of the total succulent cover.
Few plants managed to establish spontaneously on
the thin extensive green roofs used in the study. Most of
the plants were common ruderals but there were also
some uncommon calcicole species such as Saxifraga
tridactylites and Saxifraga granulata. A total of 13
different species were found to have established spontaneously on the roof. The establishment was occasional
and showed no apparent pattern. The following plants
were found on the roofs: Cerastium semidecandrum (L.),
Senecio vulgaris (L.), Cerastium pumilum (Curtis),
Arabidopsis thaliana (L.), Poa alpina (L.), Epilobium
spp. (L.), Acer campestre (L.), Taraxacum sect. (Weber),
Fig. 3. Impact of establishment method, species mixture and substrate on succulent plant cover. The figure shows mean values and
standard error. ( =Roof soil, =Substrate A, =Substrate B).
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109
Fig. 4. Impact of establishment method, species mixture and substrate on moss cover. The figure shows mean values and standard
error ( =Roof soil, =Substrate A, =Substrate B).
Table 4.
Chemical and physical characteristics of three extensive green roofs substrates
Density (dry)
Total pore space
Organic content
pH
CaCO3
P2O5
K2O
Mg
N mineral
N total
g/cm3
%
%
(g/100g dry soil)
(mg/100g dry soil)
(mg/100g dry soil)
(mg/100g dry soil)
(mg/100g dry soil)
(mg/100g dry soil)
Substrate A
Substrate B
Roof soil
1.4770.02
44.0170.79
1.0270.01a
7.4970.04
1.5470.17
3.3870.82
5.0070.00
1.0370.12a
1.6270.2
68.57710.86
1.4870.01
43.3470.42
1.6070.03b
7.4970.02
1.6470.15
2.8670.53
6.3370.33
2.0070.06ab
1.5170.16
67.4374.72
1.3770.01a
46.6770.35a
5.2570.04c
7.3570.00a
10.8770.79a
3.9871.40
15.0071.00a
2.7770.38b
1.7870.21
219.27739.29a
Values are mean7SE. Significant differences between values not sharing a common index letter.
Poa annua (L.), Hieracium pilosella (L.), S. tridactylites
(L.), Erophila verna (L.) and S. granulata (L.).
The Roof soil differed from substrates A and B in all
of the variables investigated except available phosphorous, magnesium and mineral nitrogen (Table 4).
Differences between substrates A and B were only
found in respect to the organic content of the substrate.
The organic content of substrate A decreased from 3%
to 1.02% and that of substrate B from 10% to 1.60%
(Table 4).
Discussion
Results from the first year showed that establishment
with prefabricated vegetation achieved higher cover
than the other methods. The high cover of the
prefabricated vegetation mats can be explained by the
fact that they were pre-grown under favourable conditions for several months prior to establishment and that
they had an almost complete cover at the start of the
experiment. The plants on the on-site established plots
did not grow to a comparable cover during the first year
and this means that vegetation mats can have an
advantage if high cover is needed during the first year.
However, the vegetation mats that were used on our
research plots were more than twice as expensive as the
vegetation layer of shoot establishment and close to
30% more expensive than the vegetation layer of the
plug plant establishment.
It was interesting to note that the study did not
indicate any difference in the cover of succulent plants
between the other two methods. Plug plants are
generally believed to be less sensitive to environmental
conditions than shoots, since the pre-grown plants have
a developed root system and canopy, but there was
nothing in this study that suggested an advantage for
the use of plug plants in a Swedish climate. In this study,
the establishment of shoots functioned just as well and
due to its lower price would be the preferable method.
There were some problems with birds removing some of
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the plug plants in their search for food and it is possible
that this had a negative effect on the possibility of plug
plants to create a high cover.
At the same time, the plots established with shoots
generally had more moss than the other establishment
methods. Moss is usually no problem on green roofs,
since it adds to the total cover and absorbs water. It can
also have an aesthetic advantage since it remains green
and growing even in the late autumn and winter.
However, it can become a problem if too large a
proportion of the roof is totally covered with moss.
Moss has a tendency to attract birds that are looking for
food and nesting material. Birds dig in the soil substrate
in their search for food and spread moss and plants
parts, which can eventually end up in the rain gutter and
cause clogging. Some people might also view roofs with
food-seeking birds and a high moss cover as a less
attractive outcome of an investment in a green roof.
Future studies will show whether the moss increases and
comes to dominate the shoot-established system.
The commercial substrate used in this study led to a
better performance of the succulent vegetation, something that was most probably due to the higher total
nitrogen content (Fig. 4). The positive effect of nitrogen
fertilisation on Sedum vegetation has been shown in
other studies (Fischer and Jauch, 2002). At the same
time, it is important to remember that the nitrogen
content of roof substrates is meant to be low in order to
minimise the negative effects on stormwater quality and
that too high nitrogen concentrations might make the
plants more susceptible to freezing injury (FLL, 2002).
The higher organic content of substrate B compared to
A did not have any direct effect since the organic
material in both substrates was almost completely
decomposed during the first year. The peat material
that was used in our substrate was not suited for a green
roof substrate since it did not resist decomposition. The
rapid decay of materials, and subsequent decrease in
nutrient- and water-holding capacity, may have negative
impact on the ability of plants to grow at a high rate,
spread and maintain a desirable aesthetic character. The
rapid decay of organic material may also have
consequences for the quality of stormwater, since it
might result in leakage of nutrients to the stormwater
system. The decay and transport of organic substances
from green roofs can in itself also have effects on the
colour and quality of the water (Marx and Kolb, 2002).
It is therefore important to choose a form of organic
material that is stable and that will be able to maintain
a nutrient-holding capacity over an extended period
of time.
The alternative species mixtures developed to increase
the succulent cover during establishment did not work
as well as the standard mix. The advantage found for the
standard species mixture can mainly be attributed to
the higher S. album content. The vegetatively spreading
S. album was hardy and functioned well as a ground
cover on this thin soil. In most cases there were no
differences between the northern mixture and the
mixture with a higher content of big leaved species.
The big-leaved species had little influence on the total
cover of the plots but they had an important impact on
the aesthetic function of the roof. However, these
complementary species were found on most plots. It
was also found that S. acre might acquire a degraded
look after flowering, especially on the more nutrient-rich
Roof soil, where dense stands of grey dead flower stalks
produced an unattractive appearance. It is interesting
to note that the prefabricated vegetation mats gave
S. album a competitive advantage. Establishment by
plug plants and shoots more closely followed the initial
species composition.
The absolute cover of the surfaces established on-site
was close to the threshold value of 60% cover defined by
FLL (2002), but it was only the standard species mix on
the Roof soil that had a cover that was higher than 60%.
Some of the on-site established plots that had been
established with the northern or the big-leaved mix did
not achieve a high succulent plant cover, e.g. the plots
established with the northern and big-leaved species mix
using plug plants on substrate B and the shoot
establishment on substrate A fell below 40% cover,
which can hardly be seen as a successful establishment.
Some of the spontaneous established species were
probably brought to the research plot with the shoot
mixtures, plug plants or vegetation mats, since no such
biotopes are known to be present in the surroundings. It
is important to note that the occasional establishment of
species such as A. campestre highlights the need for
proper maintenance. Woody species can cause serious
problems with root penetration of the waterproof
membrane on the roof, even if these plants occur at
low frequency and with low cover. Annual maintenance
is needed even on 4 cm thin roofs. Furthermore, the low
capability for spontaneous establishment questions the
ability of thin extensive green roofs with engineered
substrates to function as stepping stones, exchange
biotopes and refuges for rare plant species that have
difficulties in surviving in the urban environment. The
green roofs might have a higher biological value than
conventional roofs, but it seems unlikely that the thin
roof type used in this study can compare with, for
example, an urban park or grassed land.
Conclusion
Our research shows that on-site establishment is a
functional alternative to vegetation mats when establishing green roofs in Sweden. On-site construction is still
rare in Sweden and some of the variables, in particular
ARTICLE IN PRESS
T. Emilsson, K. Rolf / Urban Forestry & Urban Greening 3 (2005) 103–111
the development of new substrates, have to be improved
in order to have rapid plant cover. The varying results
from the three different species mixtures also shows
that there is important work to be done regarding
the development of new mixtures that ensure high cover
and aesthetically pleasing vegetation during the first
year. Vegetation mats have an advantage in exposed
sites or when high initial cover is needed for other
reasons, but the differences in cost between the
prefabricated vegetation mats and establishment by
shoots or plug plants can motivate the use of on-site
establishment on more roofs. The benefits of green
roofs to the urban environment are not fully realised
until there is a substantial amount of green roofs in
the urban landscape and developing cheaper establishment methods would be one step towards building
more green roofs. The economic conditions have to
be investigated in more detail before on-site establishment can be applied in full scale but it seems economically feasible given the low cost achieved for our
plots.
The establishment is only the first part of the life of a
green roof and an important future study would be the
comparison between establishment success and longterm success and especially the long-term maintenance
requirements of the installation since these factors are
related to the life-cycle cost of the green roof.
Acknowledgements
This work was funded by FORMAS. The authors
would like to thank the two anonymous reviewers and
Jan Erik Mattson for valuable comments on manuscripts, and Jan-Eric Englund for help with the
statistical analysis.
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