Planting the Green Roof and Wall in Warmer Climates Theme: Green roofs and walls design and implementation Key words: plant selection, species, subtropical climate, substrate, extensive Abstract




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Planting the Green Roof and Wall in Warmer Climates
Theme: Green roofs and walls - design and implementation

Key words: plant selection, species, subtropical climate, substrate, extensive
Abstract

A critical component of green roofs and walls is “the green”. If a green roof or wall is to succeed, the plants must grow and thrive. The purpose of this paper is to provide practitioners with skills to select appropriate plants for use on Extensive Green Roofs and Green Walls in subtropical and tropical climates. Whilst each situation will vary, certain traits identify specific plants which may be more suitable for these conditions.



Specific Site Conditions

Every roof or wall project presents a slightly different combination of site conditions.

A thorough site inspection is necessary. A practitioner is better equipped to make appropriate plant selections if he/she is familiar with these distinctive growing conditions. This needs to be balanced with the desires or needs and budget of the client.

Intensive Green Roofs (in the form of planters or roof gardens) have been widely installed around much of Australia and generally appear to be very successful, however there are many issues specific to Extensive Green Roofs or Green Walls.


Conditions to review include:

  • Local climate

  • Aspect and microclimate

  • Water

  • Substrate and weight limitations

  • Wind

  • Maintenance

  • Cultural


Local Climate

In warm climates, under direct sun, and with reflective heat, roofs can become very hot in summer (enough to fry an egg!). Most plants prefer cool root systems and can perform poorly in overly warm conditions.


In the humid subtropical and tropical zone, this is often coupled with moisture and high humidity. Rain can be intense and highly erosive and the substrate needs to be well protected; have binding additives; and have adequate drainage.
Aspect and Microclimate

Orientation to different aspects can result in very different growing conditions.

In the southern hemisphere, roofs or walls which are exposed to:


  • Westerly aspects - can be subjected to extreme afternoon sun in summer;

  • Northerly aspects - can be subjected to sun all day long, having an extended growing season in more southerly latitudes;

  • Easterly aspects – receive less intense morning sun, and shade from the afternoon sun, and;

  • Southerly aspects – shaded by adjacent buildings for the cooler months of the year – however in tropical areas, as the sun moves over the Tropic of Cancer, may receive intense summer sun for a brief period.

The location of adjacent buildings may create shadowing from sun and rain. Local industry or traffic may result in issues of dust and pollutants.


Water

Limited substrate depth results in more variable soil moisture levels. Moisture deficits are particularly marked in winter and spring in northern Australia when transpiration and evaporation levels far exceed precipitation levels and when humidity is low.


In tropical and subtropical areas it appears regular irrigation is needed on extensive roofs to avoid plant failure. Currently in South East Queensland, on site rainwater collection needs to be installed as potable water is no longer available for landscape use. It is likely this will be required in other regions in the future.
Substrate and Weight Limitations

Like natural soils, the substrate buffers extremes in temperature, moisture and nutrient availability. Extensive roofs with shallow soil profiles have been less widely installed in warmer climates. In Singapore, where more Green Roofs and Green Walls have been constructed than anywhere else in the tropics, they have found it necessary to increase soil depths alongside regular irrigation and the use of tough plants to ensure success


In warm, moist climates the organic component of the substrate breaks down rapidly, resulting in slumpage, anaerobic conditions and the blockage of filtration and drainage systems. To reduce weight, many of the recommended mixes have a high component of organic matter. This may need to be replaced with expanded volcanic gravels products such as perlite or recycled waste products such as polystyrene granules.
Many subtropical and tropical plants have strong associations with the soil microbiology. An organic approach avoiding the use of herbicides, pesticides and artificial fertilisers and inoculation with appropriate foliar and soil teas and innociulates can enhance growth and nutrient and water uptake.
Wind

Green Roofs and Green Walls are often exposed to high wind levels. In subtropical and tropical areas this can also include highly destructive cyclonic winds. Trees and plants with umbrella-like canopies or plants that are not strongly structured or well rooted may be torn from the substrate. Plants which are ground hugging or root adventitiously into the substrate will be better able to flourish in these conditions.


Maintenance

Plants which grow quickly, create a lot of litter, are subject to dieback or that are short lived require regular maintenance. This may not be possible on an extensive Green Wall or Green Roof. Where this is not possible, select slow growing, long lived plants. Herbaceous clumping plants that root adventitiously into the substrate are preferable to shrubs or woody groundcovers. To avoid infiltration by weeds (eg Fig or Umbrella Trees), a dense planting cover is required.


Cultural

In warm climates there appears to be an expectation that landscapes will look good all year round. This is in contrast to cooler climates where there is an acceptance that during winter plantings may die down or disappear and that replanting may be required in spring.



The Plants

As would be expected, the most successful plants are those that naturally grow in similar conditions - exposed rocky cliffs, high in the canopy of trees and in areas of variable rainfall:



  • Lithophytes;

  • Epiphytes, and ;

  • Succulent plants.

With plant biodiversity increasing as one approaches the equator, there is a large palette of species to choose from.
Other traits of plants likely to perform well on Green Roofs and Green Walls include:

  • Slow growing, long lived plants requiring minimal maintenance;

  • Plants tolerant of variable soil moisture conditions if irrigation is not available or continuous;

  • Plants tolerant of harsh sun and hot temperatures along with humidity and moisture – conversely, tolerant of shade if these are the existing conditions;

  • Plants which clump or root into the substrate – these are less likely to fail during windy conditions, and:

  • Plants that do not contain flammable oils or produce quantities of dry foliage which may catch alight.

High in the branches of trees or on cliff faces, plants are subjected to many of the climatic extremes that most resemble those of green roofs or green walls – in particular variable water availability, wind exposure, and the need to be well rooted into the support or substrate.


Some genera including suitable species include: Aechmea, Portea, Quesnelia, Neoregelia, Platycerium, Drynaria, Tradescantia, Pteris, Plectranthus, Bulbine, Epidendrum, Dendrobium.
The other plants that perform well on green roofs are succulents. Once again they are adapted to variable moisture regimes and those from warmer climates, are adapted to warmer summer temperatures and summer rain.
Some genera including suitable species include: Agave, Aloe, Senecio, Sedum, Graptoveria, Kalanchoe and Euphorbia.

Be Cautious

Certain plants are less likely to perform or prove suitable for Green Roofs or Green Walls in subtropical and tropical areas. These include:



  • Climatically inappropriate plants – those from temperate and Mediterranean climates – either non-native or native (eg Thymus spp, Lavendula spp, Anigozanthus cvs);

  • Schlerophyl plants – these plants don’t store water or go into dormancy – if they run out of water, they can often die;

  • Forest dwellers – these plants are not suited to exposed conditions;

  • Plants that require specific soil conditions or fungal/bacterial associations;

  • Fast growing plants – producing high quantities of litter, or requiring regular pruning;

  • Plants with large canopy to root ratio, brittle branches/roots or that may be damaged in high winds;

  • Plants lacking foliage density, that may be subject to weed infiltration;

  • Short lived plants (eg Brachyscome cvs, Scaevola cvs, Westringia spp), and;

  • Flammable plants with a high oil content or that die back leaving a mass of dried leaves (eg: Melaleauca spp, Pennesetum spp Themeda australis etc).



Conclusion

Look around our tropical and subtropical towns and cities and note how the warmth, moisture and humidity fosters aerial plant growth. Left undisturbed, walls and structures naturally become covered in plants. These include tree species such as the native Fig Trees (Ficus benjamina, F. rubiginosa and F. virens) and umbrella trees (Schefflera actinophylla). In shaded locations, ferns such as elk and staghorn (Platycerium bifurcatum, P. hillii and P. superbum) basket fern (Drynaria rigidula), Fishbone fern (Nephrolepis cordifolia and N. hirsutula), Cliff Brake Fern (Pteris vittata) colonise walls.


We can learn from nature by understanding our “site” and by selecting appropriate plants – in particular slow growing, low maintenance epiphytic, lithophytic and succulent plants.
Coastal northern Australia has great climates for establishing green walls and green roofs and the potential for the successful use of many excellent plant species. Let’s make Brisbane the Green Roof and Green Wall capital of Australia.


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