In the line of fire: Garden design to reduce the threat of bushfire

Topography and wind are the primary mechanisms that drive a fire’s rate of spread. They can also act together to funnel fires along gullies, around hills, through gaps in trees and so on. Wind also plays a vital role in fire spread by transporting firebrands and embers. 

Ember attack is the primary cause of house loss to bushfire in Australia, accounting for over 80 per cent of recorded losses. Ember density is highest near the bushland edge. Embers land on and can ignite houses, gardens, mulches, garden furniture, cars, outbuildings, mats, lawns, pergolas and so on. They can be trapped and accumulate against vertical surfaces, particularly in corners of buildings. Other bushfire attack mechanisms on houses include direct flame contact and radiant heat flux. 

So, to reduce possible fire intensity and risk to our homes, we can focus on reducing: 

•  the amount of available fuel in our gardens 
•  the heat yield of the vegetation 
•  the fire’s rate of spread by reducing wind speed and possibly modifying the slope 
•  the possibility of flame contact with our house 
•  the level of radiant heat flux impacting our house 
• the number of burning embers that can reach our home. 

These are all factors which we can influence through the design and construction of our gardens. Now let’s move on to an overview of design strategies to implement around your home. 

 Understanding your site and its position in the landscape 

Some places are more prone to fire than others. Whether you live in a rural or bushland area, a small town or a peri-urban area, it’s a good idea to assess your site to gain an understanding of its level of bushfire risk. Influencing factors include climate, slope and aspect, density and type of surrounding vegetation, distance to bushland, and likely direction for bushfire attack. The site’s position within the landscape such as on ridgelines or at the bottom of slopes as well as the property’s distance to roads and avenues of escape all play a role. Arranging a visit from your local fire authority or an accredited bushfire consultant is an excellent place to start. 

One factor to understand is the likely ‘fire wind’ direction. Although wind can move fires in any direction and topography also has an influence, in any location there are typical wind directions that are associated with bad fire weather. These usually involve dry winds emanating from the hot and arid interior of the continent. On the eastern seaboard of Australia, fire winds typically come from the west. In Western Australia, the reverse tends to be the case with hot, dry winds blowing from the east. By contrast, winds with a maritime influence are more cooling and tend to carry higher levels of moisture. Consideration must also be given to the direction of a potential wind change. In southern Australia, bad fire weather is associated with winds blowing from the north-west. A wind direction change from the south-west will often follow in the evening. The long northern flank of the fire can then become the head of the fire, creating dangerous fire behaviour.  

Forest fires will behave differently to heathland, woodland or grassland fires. There is also variation in fire behaviour between small and large patches of vegetation. 

Creating defendable space 

Now armed with a better understanding of where fire obtains its energy for combustion and something of the way it attacks houses, we have a few principles with which to design our gardens. Good design will help create ‘defendable space’, reducing fire intensity and the risk to the house. These include layout decisions, material choices and plant selection. Providing sufficient water supplies to maintain high moisture content in your plants is also important. 

Islands of fuel 

When it comes to planning your garden layout and plantings, physical separation of patches of vegetation reduces the overall fuel load in your garden and also a potential fire’s rate of spread. Consider arranging plantings in your garden as ‘islands’ in a ‘sea’ of non-combustible or very low fuel areas. The ‘sea’ can be areas of gravel, mown lawns, driveways, ponds or even sections of garden dominated by plants with high moisture content such as succulents. 

And it’s not just the plants. Identifying other potential fuels – organic mulches, unprotected sheds with flammable contents, outdoor furniture, bins, wooden fences, stockpiles of timber, timber retaining walls and so on – and keeping them away from the house reduces their potential attack on the house as secondary fires and is critically important. Some of these sources of fuel may burn for long periods once ignited. 

In particular, keep in mind that burning wooden fences may lose structural integrity and fall against the house if they are close enough, creating an enduring source of embers, heat and flame. 

Radiant heat barriers 

Barriers such as non-combustible garden walls, fences and even less-flammable tree species can provide a filter and obstacle to the passage of fires and embers. They can be useful in slowing the spread of fire. They also serve to shield objects behind them from radiant heat. 

However, if these barriers ignite, they can become part of the attack on a house. Radiant heat barriers should, therefore, be placed with care if they are themselves combustible. As a rule of thumb, keep them two times their height away from the house. Non-combustible materials like rock and masonry are excellent for retaining walls and other garden features designed to double as radiant heat shields.