Green roof plants
Green roofs are hostile sites. The combination of elevated temperatures, wind exposure and high light provide challenging conditions for plant growth. Plant selection requires careful consideration of site, microclimate, substrate and maintenance factors, linked to the desired aesthetic, functional and management outcomes for the project.
A useful way to categorise plant suitability for green roofs is required substrate depth, shown in below. See also sections on substrate in the following chapters.
Plant selection for stormwater management
If a roof is designed to soak up water and remove contaminants in the water during storm events, species that use water effectively and that accumulate nutrients should be explored. Herbaceous or shrubby species, which use more water than succulent species, will be more effective plant choices. While it may seem counter-intuitive to choose plants that have a higher water requirement over those requiring less, water may move more effectively from its landing on the substrate and back into the atmosphere with herbaceous plants as the interface. In addition, higher levels of water loss provide greater water movement and increase localised cooling of the surrounding environment.
Plant selection for aesthetics
If aesthetics are important, then select plant species that will provide interest throughout the year, and consider both foliage and flowers. The period after flowering provides interest from dried flower or seed-heads, for example, Leonotis leonurus, Agastache rugosa, ornamental Allium species, and native species Olearia axillaris. Planting in layers, with drought tolerant (seasonally dormant) species is another approach. Bulbine bulbosa, Senecio spathulatus, and other short-lived species can be added in with perennial species.
Plant selection for drought tolerance
Plants that come from ecosystems with shallow soils, such as rock outcrops, have been shown to both survive extended dry periods and make use of the high water available after rainfall and dry out the growing substrate. Successful species have also been shown to re-sprout after droughts, offering an ‘insurance policy’ if conditions are particularly harsh. These species include Dianella revoluta, Stypandra glauca and Arthropodium milleflorum.
Experience in Melbourne has shown that the succulent species Sedum xrubrotinctum and Sedum pachyphyllum were able to survive extreme dry conditions on the unirrigated shallow substrate green roof at The University of Melbourne’s Burnley campus through the summer of 2008-09. Other species that failed under the extreme conditions of 2008-09 but survived the milder 2009-10 and 2010-11 summers without irrigation were Sedum reflexum, Sedum mexicanum, Sedum spurium ‘Schorbuser Blut’.
Types of green roof plants
Succulents, particularly colourful sedums, dominate shallow substrate green roofs across temperate Europe and North America. Their low growing and/or spreading habits, great drought tolerance, seasonal flowers and contrasting foliage colours, textures and forms make them ideal candidates for green roofs. Many will benefit from some irrigation, particularly during drier months of the year. In projects with no, or minimal, irrigation, thicker-leaved succulents are the most suitable. Succulents should be planted at high density (up to 16 per square metre) to provide adequate coverage of the growing substrate and aid shading across the surface.
This category includes a range of non-woody plants, many with persistent roots or underground stems (such as rhizomes and stolons, etc.) that enable the plant to regrow and persist for many years. The most useful herbaceous perennials for Melbourne green roofs are those originating from dryland habitats. Flowering perennials are used mainly for display and seasonal interest, and many indigenous flowering plants used will also have significant habitat values. Ornamental grasses and grass-like plants, especially those forming upright tussocks, provide useful contrasts in texture and form and can be managed through pruning to maintain their shape and habit. Some may have high water needs over summer and large biomass forms could present a fire hazard in some locations.
Geophytes (bulbs, corms, and tubers) are another group of herbaceous perennials that can be extremely useful, particularly for seasonal interest and display. Many of the spring and autumn flowering geophytes are also summer dormant, making them particularly useful drought ‘avoiders’ over the warmer months of the year. Larger succulents with upright growth habits are also useful for green roofs, although their mass over time can be considerable. While many herbaceous perennials can be grown in substrate depths as little as 150 mm, irrigation will be needed for long-term success at these depths. Some caution is needed in the use of plants with vigorous rhizomes or stolons (such as some Bamboo species); they can become excessively dominant and damage green roof profile layers.
Annual and biennial plants
A range of annual and biennial plants can be used successfully on green roofs and tend to fall into two distinct groups. Quick growing annuals and ephemerals, particularly those originating from dry and arid climates, can be spectacular additions to display plantings, but will need irrigation to be sustained for longer periods. Vegetables are the other main group of annual plants used on green roofs. These require irrigation and a substrate depth of at least 200 mm. Careful plant selection and maintenance is needed to ensure annuals do not become weeds on a green roof.
Some green roofs are constructed specifically to support sports turf. Careful species selection is needed to ensure outcomes can be met: the surface and play requirements are much more demanding than for amenity turf.
Sports turf requires a designed soil or growing medium to ensure effective drainage and a substrate depth of at least 250 mm. It also requires regular irrigation, fertilising and mowing to maintain sward performance and health. Many facility managers seek expert advice on the use of sports turf on green roofs to ensure design outcomes and maintenance can be properly resourced and managed. On smaller scale green roofs, species with excessive vigour, such as Couch Grass (Cynodon dactylon) and Kikuyu (Pennisetum clandestinum), should be avoided: their rhizomes can be invasive and may damage waterproofing membranes.
Shrubs to one metre in height are best used in substrate depths of 250 mm or more. Small shrubs provide cover, display and habitat values, and often form the bulk of plantings used on green roofs with deep substrates. Increasing the substrate depth and irrigation will also increase the range of plants that can be used successfully. Excessively vigorous species should be avoided unless there will be sufficient maintenance to manage their growth, some low hedging plants could be in this category.
Shrubs up to two metres high can be used where substrate depths are at least 600 mm. They provide screening, space definition, ground coverage and seasonal flowers. Like any plant group, shrubs require careful selection and consideration of their maintenance needs. Plants with dense, upright habits should only be used where there is minimal wind exposure and/or significant protection can be afforded to support the canopy and prevent wind forces. Hedges and screening shrubs will require regular maintenance, including pruning and removal of biomass off the roof.
While many small trees (to five metres) can be successfully grown on substrate depths of 600 mm, depths of 1,000 mm or greater will ensure the best outcomes are achieved. Trees are dominant elements in any landscape, and on a green roof trees will generally be stunted in height and spread, when compared to those planted at ground level. The greater the roof exposure and overall site ‘hostility’, the more important tree selection becomes. Trees with sparse canopies, flexible stems and high tolerance to heat are best in areas of high wind exposure, although some form of anchorage will always be needed to manage them successfully.
Suitable plants for green roofs in Victoria
Bold text indicated native species.
Provided as a guide only, and should not be considered as an exhaustive list or suitable for all sites.
|Low growing succulents||Small and/or thin leaves||Crassula multicava Sedum mexicanum, S. reflexum, S. sexangulare|
|Thick leaves and/or stems||Carpobrotus rossii, C. modestus Disphyma clavellatum Carpobrotus edulis
Kleinia mandraliscae, K. repens
Mesembryanthemum echinatum, M. lehmanii, M. floribundum
Sedum xrubrotinctum, xGraptosedum ‘Bert Swanwick’, xSedeveria ‘Pat’s Pink’
|Annual and biennial plants||Plants for floral display||Calandrinia eremaea, C. polyandra Calendula officinalisTagetes patula, T. erecta
|Culinary herbs and vegetables||Ocimum basilicumPetroselinum crispumSalvia officinalis, S. ‘Greek Skies’Thymus vulgaris
With suitable substrate and irrigation, most vegetables that can be grown in containers should succeed on a green roof
|Turf||Amenity turf||Zoysia macranthaStenotaphrum secundatum Zoysia speciesFestuca arundinacea
Festuca rubra ‘Commutata’
|Sports turf||Cynodon dactylonPennisetum clandestinum Digitaria didactylaLolium perenne|
|Herbaceous perennials||Upright flowering perennials||Brachyscome ciliaris, B. multifida
Calotis cuneifolia Chrysocephalum apiculatum, C. semipapposumLeptorhynchos tenuifolius
Podolepis jaceoides Rhodanthe anthemoides
Veronica gracilis, V. perfoliata
Agastache species and cultivars
Euphorbia rigida, E. myrsinites
Hylotelephium ‘Matrona’, H. ‘Autumn Joy’
Hylotelephium cauticola ‘Ruby Glow’
S. nemorosa cultivars
|Low, spreading ground covers||Dichondra repensEinadia nutansEutaxia microphylla Grevillea lanigera
Tradescantia pallida ‘Purpurea’
Thymus pseudolanuginosus, T. serpyllum
|Geophytes (bulbs, corms, tubers, etc)||Arthropodium milleflorum Bulbine bulbosa, B. crassa, B. vagans
Pelargonium rodneyanum Allium species and cultivars
|Larger succulents (upright and rosette forms)||Aeonium arboreumAeonium haworthiiAloe mitriformis
Crassula falcata, C. ovata ‘Blue Bird’, C. tetragonaEcheveria ximbricata
|Grasses||Austrodanthonia caespitosa, A. setacea
Austrostipa scabra Chloris truncata
Deyeuxia quadriseta Dichelachne crinitaOrthrosanthus multiflorus
|Flowering plants with ‘grass-like’ foliage||Anigozanthos cultivarsConostylis species & cultivarsD. caerulea, Dianella revoluta, D. tasmanica species and cultivars
Ficinia nodosaLomandra micrantha, L. multiflora and cultivars
Liriope species and cultivars
|Small shrubs (to 1 m)||Acacia amblygonaCorrea glabra, C. reflexa, C. decumbens and cultivarsOlearia axillaris
Buxus sempervirens and B. microphylla species and cultivars
Gaura lindheimeri species and cultivars
Lavandula species and cultivars
Nandina domestica ‘Nana’
Plectranthus ciliatus, P. parviflorus
Santolina magonica, S. chamaecyparissus, S. neapolitana cultivars
|Shrubs (to 2 m)||Callistemon ‘Little John’ or ‘Captain Cook’ Correa albaEremophila debilisGrevillea obtusifolia, G. rosmarinifolia
Westringia species and cultivars
Cistus species and hybrids
Juniperus horizontalis, J. sabina
Raphiolepis umbellata, R. indica species and cultivars
Rosmarinus species and cultivars
|Small trees (to 5 m)||Trees||Acacia cognata cultivars, A. pendula, A. stenophyllaBrachychiton rupestrisEucalyptus caesia ‘Silver Princess’, E. dolichorhyncha, E. macrocarpa, E. paucifloraFicus microcarpa var. hillii
Arbutus species and hybrids
Lagerstroemia indica xfauerii cultivars
Malus ioensis ‘Plena’
Quercus ilex, Q. suber, Q. coccifera
Olea europaea ‘Tolley’s Upright’ or ‘Swan Hill’
|Tree-like forms||Dracaena dracoYucca gigantea|