Climatic factors will vary with geographic location as well as with the site aspect and height and even from effects of surrounding buildings. It is important to understand the likely climate on-site in order to inform decisions about which plant species are suitable for the site. There are no hard and fast rules about what constitutes too much wind or shade or other factors; rather, these are environmental gradients (for example, low wind to high wind) and often the best approach is to estimate the worst case scenario for plant growth that is likely on-site, and design with that in mind.
- Wind – average wind speeds are greater at height than at ground level. Winds may be strong around the edges of buildings, or from the down draft caused by tall buildings. It is necessary to understand the likely wind load that a green roof, wall or facade will be subjected to, so that it can be built to withstand the forces. Wind at high elevation will also influence temperature, and wind has a direct dehydrating effect on vegetation, therefore influencing species selection and irrigation requirements. See the Freshwater Place and Victorian Desalination Project green roof case studies to learn more about the challenges of wind.
- Rainfall and irrigation – rainfall in Melbourne is generally not sufficient to support a green roof, wall or facade throughout the year. It is important to establish whether rainwater or another water source can be harvested from other areas on-site, and stored to supply an irrigation system. This will avoid or minimise the need to use potable water for irrigation. It is useful to carry out an irrigation water demand analysis, to estimate water needs.
- Solar radiation – light intensity tends to be greater at height than at ground level. At height there are fewer structures, no vegetation to absorb solar radiation and increased reflection from adjoining building and surfaces (such as glass and light-coloured walls). Conversely, there are some roofs and walls that may receive significantly less solar radiation, due to intense shading by nearby buildings. Shadowing and shading analysis can be used to assess areas of light and shade on a site and possible changes over the year (for example, at the equinox) and over time (for example, adjoining new building development).
- Temperature – in urban environments temperatures tend to increase with elevation, due to the increased thermal mass of built structures and the commensurate heat gain. Assessing the likely temperature range on a site is crucial in planting design, particularly in extreme temperature events. While cold temperatures are rarely a problem for vegetation in Melbourne, there can be localised green roof situations where this could be a factor in plant selection.
- Microclimate – enclosed spaces such as urban canyons can create their own microclimate where wind turbulence, pooling of pollution, humidity and temperature can be intensified. The localised climate of these areas will change the growing conditions for plants and needs to be considered when planning and designing green roofs and walls.
Climate and rooftop vegetable production
The Pop Up Patch is a subscriber-based edible gardening club, based on a car park roof behind Federation Square, in central Melbourne. The Little Veggie Patch Co. runs the garden and has to allow for the different climate that comes from being based on a roof. The concrete roof stores heat and the warmer temperatures mean that species that might not normally have been considered suitable for the area can grow. Some species that are usually annual begin acting like perennials. For instance, they have found that capsicums and chillies survive and continue to fruit through the winter months in this location. The warmer temperatures necessitate growing substrate with a high water-holding capacity. In addition, the winds at height dry the surface of the substrate, so drip irrigation underneath a layer of mulch is highly recommended.