Cooling your house down can be a large expense on your budget and on the planet. Horticulturalist Claire Bickle shares how you can use plants as nature's air conditioner.

Plants and trees not only improve air quality and lower greenhouse gas emissions but whilst doing all this, reduce the temperatures of their surrounding environment day and night.

When strategically planted and looked after, trees and plants have the capacity to reduce your energy demands (i.e. the usage of air-conditioners and other related electrical appliances). This in turn reduces air pollution and greenhouse gas emissions from occurring in the first place. Many pollutants are also removed and the trees and plants store and sequester CO2.

We know they make a difference but what is the science behind trees and vegetation having such a cooling effect?

  1. Evapo-transpiration
    Trees and plants have a kind of inbuilt water air cooling system called evapo-transpiration: Trees lose water from their leaves through an evaporation process called transpiration. They lose more when experiencing direct sunlight and high temperatures. This moisture being released into the surrounding air has a cooling effect on the tree (like ourselves sweating), meaning they release less heat into the atmosphere and the moisture itself can also have a cooling effect on the air.
    There are a few environmental factors that affect the effectiveness of this process.
  2. Interrupt solar radiation
    This is where the tree canopy acts like an umbrella providing shade and ultimately blocking the sun rays from reaching the ground, various surfaces and us. It’s not just canopies that are useful in blocking the suns heat. The foliage of shrubs and small plants casting shadows will help with the heat reduction process in urban spaces.
  3. Provide shading to buildings via casting shadows from prevailing sun rays, overhead with canopy and use as green wall and green rood design options.
  4. Cool the soil directly by minimising moisture loss from the surface
    Plants and trees cool the soil directly by minimising moisture loss from the surface: Growing ground cover plants will protect the soil surface from the suns heat and reduce moisture loss as well.

Placement is key for the most effective cooling

It’s not just about planting plants and trees willy-nilly in the backyard or streetscape. You need to find the heat source and block it, so to speak.

You need to look at the aspect of your property. Where is east and west, and what part of your property is affected most and at what time of day by the sun rays? Sun hitting the surfaces on certain sides of the house and property, driveways, roofs and the like are all part of the heat trap effect.

Underpruning small trees or large shrubs will allow prevailing breezes through if they are located in a position where afternoon or morning breezes pass through. Underplanting with smaller shrubs and ground covers increases the shading layers and keeps the soil too.

When planning to plant trees, consider where the prevailing winds come from, structures, septic systems, water pipes, house footings, fences, neighbouring properties and views.

The size of tree, if it’s deciduous and root systems need to also be taken into account.

The iconic Boungainvillea about at Southbank Brisbane. Colour, hardy and functional, providing a cool shaded walkway through one of Brisbane’s busiest locations.

50 Shades of Shade

Creating layers of vegetation to trap air and act like insulation. Plant Stacking is kind of like mimicking a natural forest system.

Starting with large trees, then small trees, followed by shrubs, ground covers and climbing plants. Deep shade is created by having many layers of tree canopies.

Overhanging large shrubs and trees shade a hot driveway from the midday and afternoon western sun. This stops the concrete absorbing so much heat during the day and then releasing it into the atmosphere at night.

I call it ’50 Shades of Shade’ because there are varying depths of shade that can be obtained with the use of a variety of sized trees and other plants. Dappled shade, shade and then deep shade. Each layer traps a barrier of air – the more layers of trapped air above you the cooler it is below. It’s like the greenest insulation you could possibly install.

Air temperatures can be 7 to 15 degrees cooler than the surrounding environment.

Vertical spaces such as walls and fences can do their bit to cool our backyards and cities by acting a trellis’ for climbing plants. Plants growing in these locations stop the surfaces absorbing and reflecting solar heat. And increase the surrounding air moisture, which has a cooling effect.

Air flow

Be sure not to plant to cause a blocking effect of airflow. With trees, prune off lower branches to create an umbrella shape and allow prevailing cooling winds through.

In the humid subtropics look at narrow trunked trees such as palms – that grow up before growing out. And you need to think about heights, placement and shade tolerance.

Melia azedarach “White Cedar” is a beautiful semi-deciduous rainforest tree. This is one is planted in Ashgrove Brisbane in a playground next to the library. Here it provides shade for the playground and acts a solar block from the western sun for the library.

 

Windbreaks are one thing but the cooling effect of airflow from across a pool or nearby forest or valley are important too. Vines on trellis are another option.

Most are faster growing than a tree, provide colour or a harvest as well as privacy and screening.

Paved or concrete areas can be replaced with living ground covers. Plants can reduce this area by 10 degrees and stop reflective heat and soil moisture loss.

You can almost feel cooling green effect just by looking at this arbour, which is covered by a climbing plant. with the added shade of a large tree overhead. Below there are shrubs and ground covers adding to the cooling effect of the surrounding air.