1

Green Roofs as Local Habitats in Singapore

HWANG Yun Hye Assistant Professor, Department of Architecture, School of Design and Environment, National University

of Singapore, 4 Architecture Drive, Singapore 117566, akiyhh@nus.edu.sg

ABSTRACT

Much of the original tropical rainforests in Singapore have been lost due to deforestation,

and later urbanization, whereas manmade greenery on the other hand has flourished under a

national ‘garden city’ agenda. There is a question of whether manmade greenery can play a

greater role in biodiversity, bridging the gap between human development and the natural

system.

Green roofs are a possible toehold for greater diversity of flora and fauna, given the

relatively less human disturbance compared to the conventional ground level park or garden in

high-density urban areas (Brenneisen 2005, Grime 2002). However, green roofs as an

ecological measure seem to be overlooked in Singapore, which have currently only focused on

physical or aesthetical parameters (Wong, 2005).

This research therefore seeks to optimize green roofs as an ideal niche for inviting wildlife,

through the creation of minimally-disturbed habitats in a land-scarce urban context. Pilot sites

set up within the National University of Singapore were observed for their ecological

performance over the course of a year. Three inferences can be drawn from the experiment:

First, spontaneous vegetation, being self-generating and self-sustaining, can act as an

alternative material to attract more biodiversity to the roof. Second, the context of the plot, such

as its location and size, is one of the most crucial factors in determining the ecological potential

of green roofs. Third, planting palettes and landscaping elements directly impact habitats, and

these relationships can be further studied for habitat enhancement. It is expected that the

findings will be a precursor to the implementation of a network of larger scale extensive green

roofs that will shape a more ecologically sustainable future for the urban greenery in Singapore.

Key Words: Urban biodiversity, Green Roof Habitats, Spontaneous Vegetation, Singapore

2

1. INTRODUCTION

In 2007, a satellite image of Singapore revealed a 47% green coverage over the entire

city-state (CRISP, 2007). This is a promising percentage, approximating to an equivalent 333

km2 coverage of land, given the extensive urban developments that have taken place over the

entire land. As an extension of urban greenery, 55 ha of green roofs have been installed, and it

is envisioned that an additional 30 ha will be installed by 2030, with a government-funded

subsidy program launched to support 50% of installation costs (Lok, 2013). There has been

increased installation of green roofs in various locations such as multi-story carparks of public

housing, commercial buildings, and institutions. The majority of green roofs use a standardized

low-profile system brought in from the West, planted with a limited palette of hardy plants, many

of which are non-native (Tan & Sia, 2008).

The anthropocentric functions of green roofs have been emphasized through many

academic research studies as well as projects in practice: roof gardens create better urban

environmental settings, allow chances for activities, aid in energy efficiency, filter dust particles,

reduce carbon monoxide, provide acoustic insulation, foster community interaction, provide

therapeutic effects, increase building property values, improve the aesthetic of buildings, and

hide rooftop services (Wong, 2002, Tan 000). In Singapore, however, the public - and most

importantly - building professionals lack the belief that green roofs can aid in preserving and

protecting the habitat for plants and animals (Wong 2002). Highly manicured greenery is

perceived as the norm, so therefore an intensive management regime is applied to all forms of

green roofs, extensive or intensive, with regular pruning and fertilizing (Tan & Sia, 2008)

Research in the European and American contexts have proven the ecological value of

green roofs as shelter and food sources for fauna by employing landscapes of wild flowers,

grasses that naturally establish themselves (Dunnett, 2008). However, green roofs in the tropics

are hardly documented or researched on in terms of its biodiversity potential as urban

biodiversity ‘refuges’, especially with consideration of the ongoing loss of local native plants and

animals (Davison et al, 2008). In tropical landscapes, with a year-round growing period fed by

regular rainfall, and an enormously diverse palette of tropical plants, forms and structures, it is

expected that an equivalent self-establishing natural landscape of high biodiversity is a

possibility. The research engages the assumption that green roofs can become a basic urban

infrastructure that can function as a habitat.

While the rate of human usage in urban landscapes is relatively low in Singapore due to

the extremely warm tropical temperatures(Yuen, 2002), spontaneous vegetation, naturally

occurring in harsh urban conditions, could be a novel method for green roofs (Hwang, 2011).

By virtue of the process of plant succession, not all plants will necessarily be native in terms of

3

urban-adaptive growth responses, and it is this dynamic nature of plant successions that can be

a sustainable long-term method for urban landscapes (Dunnett, 2003).

This paper is an accumulation of activities that observe spontaneously-generated

extensive green roof habitats. There are three research activities: First, the nature of

spontaneous vegetation is studied, identified, and collected. Second, two sites with different

size, context and location are chosen and installed with simple, low-cost green roofs. Third,

vegetative and faunal changes are observed regularly and recorded in detailed documentation.

These activities aim:

• To demonstrate a unique and spontaneous tropical plant palette arising from a natural,

long-termed low-cost, low-maintenance method

• To develop a novel prototype/model for achieving high biodiversity in green roofs

• To develop a tool to assess the ecological value of green roofs

2. PROCESS

2.1. Plants and planting

Plant types for green roofs in Singapore are typically limited; no greater than 80 species

have been recommended in an official information booklet (Tan & Sia, 2008). This is in

contradiction with the diversity that naturally exists in tropical environments. Succulent-type

plants like Sedum species are typically planted, but most of these succulent plants, while being

successful at tolerating drought, are susceptible to moisture in the air or in the green roof

substrates. On the other hand, there are some other groups of plants like grasses that are

particularly hardy – they are able to thrive, though in extreme drought they may die off. In

general, green roofs require intensive irrigation and maintenance to upkeep a manicured

appearance on a regular basis. Therefore, spontaneous vegetation comes to mind to reduce

maintenance as they are able to exist perennially without human intervention and are irrigated

according to natural rainfall patterns.

Spontaneous vegetation is introduced in this research in three ways– through

spontaneous processes such as wind or avifaunal distribution, through seeding, and through

direct transplanting. The first method looks at a purely spontaneous approach, which leaves a

quarter of the green roof site bare to be colonized naturally. The bare patch is a possible resting

site for birds passing through, but the initial germination likely resulted from wind-dispersed

seeds like some grasses or sedges, which largely populate this quarter after 10 months.

For the seeding approach, a decision was made to collect the seeds and pods of

spontaneously-occurring plants from sites which have environmental stress similar to roof areas.

4

With an emphasis on flowering or fruiting plants that can attract various fauna, a total of 93

species of plants, belonging to 22 botanical families, were collected. The entire collection of

seeds was prepared by fully extracting all seeds from pods and fruits, leaving them to

thoroughly dry, and storing them in the freezer. All seeds were mixed together and sifted out

onto the green roof. Two rounds of seeding were done for SDE (School of Design and

Environment), with 1 round of seeding for AS3 (Arts and Social Sciences, Block 3) (Figure 1.).

In the mean time, about 50 species of plant seeds collected were tested for their germination

potential. To test this, the seeds were placed in trays containing good soil mix, watered and

tended to nursery conditions for 2 weeks. 20 plant species successfully germinated into

seedlings and looked healthy. This germination test proved that propagation through planting

collected seeds, which is a method rarely done for ornamental landscaping in Singapore, is a

viable approach (Figure 2).

Fig 1: Preparation of seed collected from spontaneously grown sites like Lorong Halus. Seeds were

extracted, dried and packed, according to species.

Fig 2: Germination trials for seed collected from spontaneously grown sites (left, middle), and

actual seeding process on the green roofs (right).

In order to examine the possibility of ‘instant’ spontaneous vegetation on green roofs, a

separate method of planting was applied, which involved transplanting mature plants that had

5

been proven to grow in a harsh and dry condition without any prior human intervention. Three

sites were chosen for the collection of these plants, namely: the forest edges of NUS (National

University of Singapore), a former dumping ground in Lorong Halus, and Old Holland Road at

the edge of the former KTM (Keretapi Tanah Melayu, the Malaysian rail operator) railway. A

significant plant type found was the Axonopus compressus, a grass commonly planted in

gardens around Singapore, and it was also transplanted to be monitored for changes in the

absence of the maintenance it usually receives. Although similar to the typical planting process,

the challenge was to achieve an immediate coverage at the start, thereafter allowing these

plants to become a base in which other spontaneously generated species would gradually mix

with. Instant plant coverage also prevents wind and rain erosion of the soil, the latter of which

would otherwise be bare for some time (see Figure 3).

Fig 3: Collection of plants for transplanting, here showing grasses adapted to dry sandy conditions

(left).Collection of plants like sun-loving ferns from secondary forest edges for transplanting (middle). Transplanting of collected ferns from secondary forest edges direct to site on the same day (right). Cow

grass or Axonopus compressus was also transplanted.

Fig 4: Schematic plans of green roof at SDE1 (left) at 626 m2, divided into 4 sectors, and at AS3 (right upper) at 79 m2, divided in 2 sectors. The plans indicate how various planting methods were

applied to different sectors to research the effects and impacts on flora development.

6

2.2. Site and construction

The two sites, typical concrete roofs, were selected within the National University of

Singapore (NUS) campus. The campus is built on hilly terrain, with pockets of remnant

secondary forest patches in between. The larger 626 m2 sized SDE1 roof is located at the edge

of a 5 storey linear building cluster in the School of Design and Environment, which fronts a

major road and woodland park beyond; the smaller 79 m2 sized AS3 roof is located on the 5th

floor open void deck area, in the midst of a building cluster within the 7 storey Arts and Social

Science faculty. Both sites are non-accessible to public and have little overshadowing effects

from adjacent buildings. The existing NUS buildings date back to as far as 1978-1980, thereby

approximating these reinforced concrete roofs to be slightly more than 30 years old, but the

structural conditions have been certified by a professional engineer to be in good condition to

withstand the typical allowable imposed load of 4kN/m2 for green roofs.

The roof sites were firstly grouted and waterproofed, and aluminium L-sections were

used as edging for the garden, in which soil was spread over 20mm thick drainage cells and a

protection sheet. The soil mixes, a typical green roof mix comprising of loamy soil, compost and

pumice in a 2:1:1 ratio, was applied to a general depth of 50 mm. In order to test the effect of

higher soil levels in relation to the fostering of more diverse species, mounds between heights

of 100mm to 300mm were proposed in certain areas. The SDE and AS3 sites were divided into

4 and 2 sections respectively to test various methods of creating habitats (Figure 4). The

construction comes up to approximately S$104.14 per square metre. Minimum maintenance is

proposed with no pruning or watering except when needed, and a simple irrigation system of

rubber hoses was provided for initial plant establishment stage, as well as for back up purposes

should there be no rainfall for more than 5 days. 100-200mm diameter logs and a 300mm deep

pond of 1m2 were also installed as potential shelter for fauna. Table 1 shows the construction

and planting specification of the two sites.

Table 1: Overall Summary of Methodology for Sites SDE and AS3

SDE1 Roof AS3 Roof Area 626 m² 79 m²

Dimensions 27.7 x 23.9 m (approx. square form)

13.3 x 5.9 m (rectangle form)

Description 5th floor (roof) (135m AMSL) 5th floor (roof) (135m AMSL)

Orientation Approx. N-S orientation (along axis of square form)

Approx. N-S orientation (along long side of rectangle form)

Overall

Schedule Commenced Dec 2012

Completed end Dec 2012 Commenced Dec 2012

Completed end Dec 2012

7

Materials 100x100mm, 3mm L-section aluminium edging with pre-cut side drainage

Profile 50mm deep topsoil, compost, pumice in 2 : 1 : 1 mix ratio

25mm water retention/drainage cells, covered by filter sheet water proofing layer, with silicon sealant and grouting of roof tile gaps

Layout 4 sectors. See figure 4 2 sectors. See figure 4 Method 25 % first seeding (93species)

25 % second seeding (70? Species)

10% first transplanting (ornamental

ground over like cow grass)

25% second transplanting (11 species

000,000)

15% bare soil

50 % first seeding (93species)

25% first transplanting (Turf)

25% bare soil

Fig 5: Construction process: (top row, left to right) setting up of aluminium L-section edges; laying

of green roof cells; laying of protective filter sheet; installing of pond; (bottom row, left to right) use of crane equipment for material movements; waterproofing application; spreading of soil over green roof

area; arrangement of logs for fauna shelter.

2.3. Mapping and monitoring

A process was set up to map and monitor the growth of vegetation on the green roofs

over time (Figure 6). The process comprised of setting up a 1m x 1m grid onto the green roofs

to aid in a concise photographic aerial mapping of each section every two months. A high-

resolution camera mounted onto a 6m high pole for remotely-controlled photo-taking was set up

to capture grid by grid of the green roofs with the intention of achieving an overall flat and planar

view of the roofs. The stitched photographs were then translated into a graphic of circles in

8

which each circle represents a 10 cm radius on site. By using different coloured circles, the

progress and development of different species of spontaneous vegetation on the roofs were

mapped. Standard eye-level views and panorama photographs were also taken, which were

used for analyzing plant states, health, massing, as well as faunal activities (Figure 7).

Fig 6: Mapping process by taking sequential photographs using a high-resolution camera mounted

on a 6m high pole.

Fig 7: Panorama of green roof after seven months (July 2013), flourishing with a variety of plants, grown mostly from seed as well as some self-sown

3. INFERENCE

Over the course of nine months, flora and fauna have flourished on the roofs, and this

demonstrates the great potential of typical extensive green roofs in supporting naturally-

occurring landscapes and wildlife. At the local scale of a green roof, the findings obtained from

the research, shown below, can be further reviewed for ecological guidelines for habitats in

Singapore.

3.1. Macro-context: Location and Plot size

The immediate surrounding environment of the green roof is one of most important factors

in achieving spontaneous vegetation, as it affects the availability of birdlife visiting the site. As

the SDE roof is near a park with small secondary forest patches, compared to the AS3 roof

being a more built-up part of the campus, it naturally witnesses more instances of birdlife, as

9

well as larger numbers and variety of species. The presence of a large mature Ficus benjamina

tree within SDE, a proven source of fruits for birds, is proof of the symbiotic relationship

between the green roofs and ground landscape that greatly encourages wildlife.

The different green roof sizes also play a key role in determining the amount of species

observed at each roof habitat. Between the two roofs, the green roof at SDE1 is almost 8 times

larger than AS3. It is observed that the larger site had a more diverse habitat and attracted

more species for both flora and fauna. Although at this point more research is required to

pinpoint the “minimum” size for a green roof habitat that can sustain a substantial level of flora

and fauna, initial observations have already proven that 79 m2 too small to create a diverse

habitat, especially for birds for which sufficient planting is crucial to create a sense of security

(Figure 8).

Fig8: Timeline of observed flora(left) and fauna(right) species diversity at the two green roof sites SDE1 and AS3 over a period of 10 months.

3.2. Plant-based Habitat Characteristics

Most spontaneously-occuring plants are self-generating because of the ease of seed

dispersal, and tolerance towards harsh conditions. In the process of seed collection, the

conventional way of storing seeds through refrigeration after drying was carried out, but it was

observed that some seeds may have been stored beyond its viable period. Therefore, seeds

should be sown as soon as possible without any refrigerated storage period. This may have

affected the germination results on site and therefore more research needs to be carried out to

maximize the germination rate of each species.

Certain botanical families of plants thrive particularly well even without maintanence – for

example the Fabaceae, Asteraceae, Gramineae, and Cyperaceae families – and these grow

more vigorously to become the different layers that form the clusters of habitats. In terms of their

coverage patterns, spontaneous vegetation has demonstrated its ability to set the stage for

various shelter types required by fauna by covering the initial bare ground, in either clumping or

10

spreading methods. Crotalaria spp plants, for example, grow tall above 1m height, and in due

time thin out at the bottom as they abort leaves lower down, thereafter creating shelter spaces

below for birds such as the Spotted Dove, Asian Glossy Starling, Oriental Magpie Robin, and

Yellow-vented Bulbul. The flowering upper parts of Crotalaria spp plants provide nectar for a

variety of bees, while the pods are hosts for the Pea Blue butterfly caterpillar. Based on field

observations, Table 2 shows the key flora species that ‘activate’ the habitat, as well as the

associated fauna. Table2: Plant-based Habitat Characteristics (key species and main drivers of the green roof habitats)

Flora Species Species Function Associated Fauna

Crotalaria pallida Host plant (larval, pupae) Pea Blue butterfly

Nectar source Butterflies, bees, wasps and other insects Shelter Spotted Dove, other birds

Leucaena leucocephala Shelter (perches) Yellow-vented Bulbul, other birds

Melinis repens Seed source Scaly-breasted Munia Shelter Small birds, insects

Pennisetum purpureum

Seed source Scaly-breasted Munia Shelter Small birds, insects

Sporobolus indicus Seed source Scaly-breasted Munia Shelter Small birds, insects

Bidens alba Nectar source Butterflies, bees, wasps and other insects

3.3. Habitat enhancements

Earth mounds of 100mm to 300mm were shown to provide better conditions for plant

growth as compared to the rest of the green roof. Naturalized plants appeared first on the

mounds and flourished faster- a likely result of the mounds being more moisture-retentive than

the rest of the shallow soil base. The finding gives rise to the idea of variety in surfaces and

mounds as a good strategy to create fast-establishing niches of plants. The use of landscape

features like fallen logs and water features also appear to enhance the function of the green

roof as a habitat, by providing additional shelter, rest areas, and water source for wildlife. The

fallen logs have been observed to be the preferred bird perching areas as opposed to bare

ground, while the water feature has been a drinking source for birds like the Collared Kingfisher

and the Oriental Magpie Robin. The composition of soil and soil quality may also have some

effect on spontaneous plant growth. Additional research is necessary to optimize a green roof

habitat that is able to maximize flora and fauna within limited size and conditions.

11

3.4. Minimal maintenance

Inaccessibility, commonly perceived as an inconvenience in an urban setting, has

become an advantage for spontaneous planting as the lack of human disturbance means that

habitats will be able to thrive without damage. Generally, minimal maintenance of these green

roofs, involving only low-intensity weeding while eliminating fertilizer and regular irrigation, is

recommended to be carried out as the plants can flourish over time and achieve a balanced

species diversity in the habitats without human intervention. As a baseline requirement, the

green roofs should not damage any part of the buildings, thus effecting the need for periodic

monitoring and controlling of any growth of trees and roots to prevent potential structural issues.

Also, it would be beneficial to demonstrate the economic impact of these experimental plots

through further investigation and analysis of their life cycle and costs.

3.5. Perceptual changes

An online and on-site preference survey of AS3 green roof was carried out to

understand public perception at the time when most bare land had been covered by

spontaneous vegetation. Being directly adjacent to the offices of the Arts and Science Faculty,

some offices have windows with views out towards the roof area. Findings from the survey were

generally positive – the online survey of 25 respondents to date showed that the green roof

garnered mainly positive votes, with only 4 dislikes. Although the most of the surveyed had

welcomed the green, there was a notable lean towards a more manicured look amongst the

locals, as compared to the foreigners who could be more familiar with natural forms of

landscape. It can be inferred that for green roofs within close proximity to commonly-used areas,

such as in the case of AS3, it is important to consider different maintenance needs in order

avoid visual and aesthetic downsides in the form of “a wild appearance” or obstruction of views.

4. DISCUSSION

The objective of creating green roofs in a tropical context may change from a human-

centred one to a more ecologically sustainable urban refuge for fauna, as it becomes more

convincing to implement green roof habitats in this region. In light of Singapore’s recent policy to

increase the country’s population, land scarcity will become more inevitable, therefore

landscape architects face the increasingly greater challenge of generating alternative ways to

maintain and incorporate greenery into a high density city. Roof areas may be the safest and

most suitable places for the integration of biodiversity into the context of urban developments.

12

The findings of this research show that there is great potential for a city-wide network of

green roof habitats and this may be the springboard to a biodiversity boost in the city. This

potential needs to be further studied by analysing regional faunal sources, fragmentation and

connection variables, culminating in the designation of large patches of roofs as biodiversity

hubs that are linked by smaller patches. The involvement of respective governmental bodies as

well as city planning authorities will be crucial to review and strategise the idea of a city-wide

master plan of green roof habitats.

Landscape is not static but changing. Considering the occurrence of ongoing

redevelopments all the time, long term implications of green roof habitats need to be understood

as well. For example, demolition or construction activities in the vicinity of established individual

green roof habitats may disturb the latter’s flora and fauna dynamics as well. For a clearer

understanding of the impact of such activities on green roof habitats, the research is projected

to be monitored for another two years.

REFERENCES

Angelia Sia, 2010, Growing Gardens in the Skies, Vol2: 28-31p. CITYGREEN, Singapore Alvey, A.A. (2006). Promoting and preserving biodiversity in the urban forest. Urban Forestry and Urban

Greening, 5, 195-201. Baumann, N. (2006). Ground-nesting birds on green roofs in Switzerland: preliminary observations.

Urban Habitats, 4, 37–50. Bowler, D.E., Buyung-Ali, L., Knight, T.M., Pullin, A.S. (2010). Urban greening to cool towns and cities: a

systematic review of the empirical evidence. Landscape Urban Plan, 97, 147-155. Brenneisen, S. (2004). Green roofs: how nature returns to the city. Acta Horticulturae, 643, 289-293. Briffett, C. 1986. A Guide to The Common Birds of Singapore. Singapore Science Centre, Singapore. Brook, B.W., Sodhi, N. S., Ng, P. K. L., (2003). Catastrophic extinctions follow deforestation in

Singapore. Nature, 424, 420–423. doi:10.1038/nature01795 Burke, K. (2003). Green roofs and regenerative design strategies: The Gap’s 901 Cherry Project,

Greening rooftops for sustainable communities. Corlett, R. T. (1992). The ecological transformation of Singapore, 1819– 1990. Journal of Biogeography,

19(4), 411–420. CUGE and Nparks, Singapore, (2010) A Concise Guide to Rooftop Greening CUGE, 2010. Guidelines on design loads for rooftop greenery. National Parks Board, Singapore. CUGE, 2010. Guidelines on design for safety on rooftop greenery. National Parks Board, Singapore. CUGE, 2010. Guidelines on filter, drainage and root penetration barrier layers for rooftop greenery.

National Parks Board, Singapore. CUGE, 2010. Guidelines on substrate layer for rooftop greenery. National Parks Board, Singapore. CUGE, 2009. Specification for soil mixture for general landscape use. National Parks Board, Singapore. CUGE, 2009. Specification for composts & mulches. National Parks Board, Singapore. Devine-Wright, P., Fuller, R.A., Gaston, K.J., Irvine, K.N., Warren, P.H., (2007). Psychological benefits of

green space increase with biodiversity. Biology Letters. 3, 390–394. doi:10.1098/rsbl.2007.0149 Dunnett, N. and Hitchmough, J. 2003. The dynamic landscape: design, ecology and management of

naturalistic urban planning. Spon press, London and New York.

13

Engleback, L., Gran, G., Nicholson, B. (2003). Green roofs: their potential for conserving biodiversity in urban areas. English Nature Research Report 498.

Grime J. P. (2002). Plant strategies, vegetation processes and ecosystem properties. Wiley. Grimm, N.B., Faeth, S.H., Golubiewski, N.E., Redman, C.L., Wu, J., Bai, X., Briggs, J.M. (2008). Global

change and the ecology of cities. Science, 319, 756–760. doi: 10.1126/science.1150195. Hwang, Y. H., 2010. Observation of Urban Spontaneous Vegetation as Landscape Material, Journal of

Landscape Architecture in Asia, Volume 5: 61-71. Kuhn, N. 2006. Intentions for the Unintentional: Spontaneous Vegetation as the Basis for Innovative

Planting Design in Urban area. Journal of Landscape Architecture (autumn 2006): 46-53. Lundholm, J.T. (2006). Green roofs and facades: A habitat template approach. Urban Habitats. 4, 87-101. Miller, J.R. (2005). Biodiversity conservation and the extinction of experience. Trends Ecol, 20, 430–434. Mitchell, R., Popham, F. (2008). Effect of exposure to natural environment on health inequalities: an

observational population study. Lancet, 372, 1655–1660. doi:10.1016/S0140-6736(08)61689-X. Nathalie Baumann, 2011, Green Roofs in Switzwerland, Vol2: 80-83p. CITYGREEN, Singapore National Parks Board, Singapore, (2012) http://www.skyrisegreenery.com/index.php National Parks Board, Singapore, (2011) New Incentives To Promote Skyrise Greenery In Singapore NParks and NUS, 2002. Handbook on Skyrise Greening in Singapore in Singapore, National Parks Board

and NUS, Singapore NUS SDE Department of Building, 2006. Green Technology Laboratory. Retrieved Dec 2010 from NUS

SDE website: http://www.greenlab.bdg.nus.edu.sg/t Ong, C.L., Wong, Sia, A., Tay, S.f., Wong, N.H., Wong, R., (2003). Life cycle analysis of rooftop

gardens in Singapore. Building and Enviorment, 38, 499-509. Shiew, F. T., 1986. A Guide To The Wildflowers of Singapore, Singapore Science Centre, Singapore. Sia, A., Tan, P.Y. (2008). A selection of plants for green roofs in Singapore (2nd ed.). Singapore: National

Parks board, Singapore. Sukopp, H. (1998). Urban ecology—Scientific and practical aspects. In Breuste J, Feldmann H, Uhlmann

O, eds . Berlin: Springer. Stephan Brenneisen, 2006. Space for Urban Wildlife : Designing Green Roofs as Habitats in Switzerland,

Urban Habitats, Vol4-1: 27-36 Tredici, D. P. 2006. Brave New Ecology. The magazine of Landscape Architecture. 46-49. United Nations Population Fund. (2007). State of the World Population 2007: Unleashing the Potential of

Urban Growth. Wong, N. H., et al., 2002. Life cycle cost analysis of rooftop gardens in Singapore, Building and

Environment 38 (2003):499-509. Yeo A., (2001). The grass is greener on the upper side. The Straits Times, H1, July 4, 2001.