Green Roofs and Plant Selection
Green roofs are hostile sites for plants.
The mixture of wind exposure, high temperatures, and increased light provide challenging conditions for plant growth. The selection of plants should involve a careful consideration of the area, substrate, microclimate, and maintenance factors, which are linked to the desired functional, aesthetic, and management outcomes of the project.
Choosing Plants for Stormwater Management
For roofs designed to remove contaminants and soak up water during storms, you should ideally explore the species that use water effectively and accumulate nutrients. Shrubby or herbaceous species, which naturally use up more water than the succulent species are often the most effective choices.
Although it might seem rather counter-intuitive to select plants that have a high water requirement, it’s good to note that water moves a bit more effectively from its landing on the substrate and back to the atmosphere with the herbaceous plants serving as the interface. Additionally, the higher levels of water loss offer much greater water movement and in turn have higher localized cooling for the surrounding environment.
Choosing Plants for Aesthetics
If aesthetics are especially important to you, then you need to find plant species that will have some interest throughout the year, taking into account both foliage and flowers. For example, the period after flowering can provide interest from the dried seed heads or flowers in species like ornamental Allium species, Agastache rugosa, Leonotis leonurus, and native species Olearia axillaris.
Another approach is to plant in layers with seasonally dormant or drought tolerant plant species such as Senecio spathulatus, Bulbine bulbosa, and similar species with short lives. All of these can be combined with the perennial species.
Choosing Plants for Drought Tolerance
Plants that originate from ecosystems with shallow soils like rock outcrops have been proven to survive for extended dry periods and make use of the water available after rainfall and dry the growing substrate out. Some species have successfully been shown to re-sprout after droughts, essentially providing an insurance policy in case the conditions get particularly harsh. These species include Stypandra glauca, Arthropodium milleflorum and Dianella revolute.
Experience has shown that the succulent species, including plat varieties such as; ‘hens and chics‘, or Sempervivum Ruby-Heart, Sedum pachyphyllum and even Sedum xrubrotinctum are able to survive the extremely dry, un-irrigated conditions of a shallow substrate green roof. Under extreme conditions, other species failed, but some survived the milder summers without irrigation, which were Sedum spurium ‘Schorbuser Blut’, Sedum mexicanum, and Sedum reflexum.
Types of Green Roof Plants
The succulent species, especially the colorful sedums, generally dominate the shallow substrate green roof across the temperate North America and Europe. Their incredible drought tolerance, low spreading and/or growing habits, seasonal flowers, along with their contrasting foliage colors, forms and textures make them great candidates for green roofs.
Most will benefit from some irrigation, especially during the drier months of the year. In green roof projects with minimal or no irrigation, succulents with thick leaves are more suitable. Succulents need to be planted at higher densities of up to about 16 a square meter to aid in shading across the surface and for adequate coverage of the growing substrate.
In this category, there’s a wide range of non-woody plants, many with underground stems or persistent roots such as stolons, rhizomes, etc. that allow the plant to persist and regrow for many years. The most useful herbaceous perennials ideal for Melbourne green roofs are those that originate from the dryland habitats.
The flowering perennials are mostly used for seasonal and display interest, and many indigenous flowering plants also have considerable habitat values. Grass-like plants and ornamental grasses, especially those that form upright tussocks offer useful contrasts in both their texture and form, and can be pruned to maintain their habit and shape. Some usually have high water needs over summer, and the large biomass forms might present a fire hazard in some locations.
Another group of herbaceous perennial plants that can be extremely useful include Geophytes (corms, bulbs, and tubers), especially for seasonal display and interest. Many of the autumn and spring flowering geophytes are also summer dormant, which makes them useful drought avoiders during the warmer months of the year.
The larger, upright growing succulents can also be used on green roofs, though they can significantly increase their weight over time. Although a lot of the herbaceous perennials can be grown in substrate depths of as little as 150 mm, irrigation will be necessary to allow for long-term success at these depths. Some caution will be required in the use of plants with vigorous stolons or rhizomes, such as some Bamboo species, and they can be excessively dormant and even damage the layers of the roof profile.
Biennial and Annual Plants
A wide range of biennial and annual plants can be successfully used on green roofs, and can be broadly categorized into two groups. Quick growing ephemerals and annuals that mainly originate from the arid and dry climates can be great additions to display plantings, though they will need regular irrigation for them to be sustained for prolonged periods.
The other main group of annual plants that can be used on green roofs is vegetables. Of course, these will require irrigation and a substrate of at least 200 mm in depth. A careful selection of plants and prudent maintenance is required to ensure that the plants don’t become or appear as weeds on the roof.
Some roofs are specifically constructed to be able to support sports turf. A careful selection of species is required to make sure that desired outcome is achieved. The play and surface requirements are often more demanding compared to amenity turf.
Sports turf also require a designed growing medium or soil that allows for maximum drainage, along with a substrate depth of at least 250 mm. it also needs regular irrigation, mowing, and fertilizing in order to keep a sward health and performance.
Many facility owners or managers usually go for professional advice on how to use of sports turf on their green roofs to make sure that the maintenance and design outcomes can be managed and resourced properly. For the green roofs on a smaller-scale, species that have excessive vigor, such as Pennisetum clandestinum (Kikuyu) and Cynodon dactylon (couch grass) should be avoided, since their rhizomes can be invasive and could damage the waterproofing membranes.
Small shrubs grow to about one meter in height, they are best used in substrates with depths of 250 mm or more. The small shrubs typically offer display, cover, and other habitat perks, and are actually the most used plantings on green roofs that use the deep substrates.
If the depth of the substrate is increased and irrigation done as needed, it can increase the range of plants that can be used effectively. Nonetheless, the excessively aggressive plants should be avoided unless there’s adequate maintenance routine in place to manage their growth. Some of the low hedging plants can be placed under this category.
These grow to up to two meters in heights and can be used where the depth of the substrate is at least 600 mm. they offer space definition, screening, seasonal flowers, and ground coverage. Just like any other plant group, shrubs will need careful selection along with meticulous consideration for their maintenance needs.
Species with upright, dense habits should only be used where there’s minimal wind exposure and significant protection, which can help to support the canopy and keep away the strong wind currents. Screening and hedge shrubs will need regular maintenance, including pruning and periodic removal of biomass from the roof.
Small trees of up to 5 meters in length can be grown on substrates of 600 mm in depths, though depths of 1000 mm or more will ensure better outcomes. Trees are quite dominant in any landscape, and on a roof, they tend to be spread out and stunted in height compared to those at the ground level.
In general, the greater the roof exposure and the overall hostility of the surrounding, the more critical the selection of trees become. Trees with flexible stems, sparse canopies, and high heat tolerance are usually best in areas of high wind exposure, though they’ll need some kind of anchorage to manage them effectively.