Fire Retardants

Planting Trees as Living Fire Breaks

Using fire-retardant trees to landscape around your homestead can significantly increase the safety of buildings in the case of a wild fire event. Conversely having very flammable trees near to a house makes the risk of the building catching fire much greater. To have effective living firebreaks you must choose the right tree species, and then manage your grounds well.

Fire-retardant trees can quell a fire in three ways. Firstly they act as a shield against the immense radiant heat generated by an approaching fire, so the house is protected from the intense temperatures that can shatter windows and melt materials. Then a large canopied tree, or a strategically placed hedge, acts to trap the burning material carried in the intense fire winds. This reduces the amount of flaming matter that can reach and ignite the building itself. Finally the stored moisture in the leaves acts as all water does, it suppresses the fire’s energy. Many times even a single tree has saved a house or shed. That’s why fire-retardant trees are living firebreaks.

Which trees are fire-retardant?

The best trees for fire-retardant properties are those which have soft leaves with a high moisture content, smooth and non-peeling barks, and low volatility oils in their foliage. Generally speaking this includes the majority of deciduous trees and some evergreens from the sub-tropics and rain forests. The worst trees for fire are the eucalypts.

“Eucalypts seem designed to cultivate fire, as much as they are cultivated by it. The trees use fire to husband their own harvest and their features encourage its spread: fine leaves which ignite easily and burn fast & intensely; high oil content of leaves and twigs ready to vaporise; heavy litter fall (non-decomposing leaves, bark, twigs and branches) in dry weather, laying kindling beneath them; open-work canopies of hanging foliage which encourages updraughts during fire; and peeling bark readily whipped off by wind to start new fires wherever it lands.” (1)

Amongst Australian native trees, gums, and particularly those with peeling bark are the most flammable trees. Fine-leaved acacias and casuarinas with smooth barks are less dangerous, although they are not as fire-retardant as deciduous and sub-tropical trees listed above.

A rough and ready test for flammability is to take a small bunch of leaves to a gas stove. Make sure you have a bucket of water to drop them into. Then slowly approach the flame with the bunch of leaves. The foliage that catches fire farthest from the flame and flares the most is the type to avoid near your building. Some leaves will be very hard to burn even if they touching the flame. They are the best to use of course. Remember that the bark also plays a role, trees with loose flaky bark are more risky. This can include some deciduous ones, such as silver birch.

Designing living firebreaks

If you want to use trees to reduce your bushfire risk, the first thing to do is to assess your site. Every site is unique and behaves differently in a wild fire, and each fire will be different too. You must note all the factors that will reduce the fire threat and those that will exacerbate the risks. Walk around your home and assess your site methodically in all directions.

Important considerations include the topography, because fire accelerates uphill, so hill tops sites are much more at risk that those nestled in a valley: a fire will double its rate of spread for every 10 degrees upslope and halve the rate of spread for every 10 degrees downslope.(2) Also the prevailing wind directions are critical. In WA the most likely directions for the wind to carry a fire are those to the east/north-east and the north-west to south-west. So these two considerations of wind and slope will already determine a lot about your fire hazard. From them you can tell where your living firebreaks must be concentrated. Then there are the specifics of where you live, do you live next to the bush, or a plantation or other eucalypt trees? Dry grassy areas are also a great hazard. Recreation sites nearby can also be risky because often wild fires start as camp fires that escape.

There will be positive considerations too, aspects that will provide more fire retardant protection include roads and gravel driveways, green lawns, water bodies including swimming pools, ponds and dams and established deciduous trees.

Good Management

The suggestions made here will not guarantee your safety in a wild fire. They must be part of a complete fire planning system that includes building design and construction materials, water availability to fight a fire, meeting legal firebreak requirements and then ongoing diligence in managing your site. In relation to living vegetation this means that your garden must be well cared for to maximise the effects of fire retardant vegetation. It means that you must plant your ground covers and shrubs with a view to flammability, and never plant flammable bushes under the eaves of a building. It also means that you must prune all dead wood off fire retardant trees, so they present only healthy living foliage. You must also diligently rake up all dead leaves. Remember that in times of drought only the deeper rooted deciduous trees will maintain a high water content in their leaves. So if your living firebreaks look drought stressed put on some occasional trickle irrigation. If your water is limited then the number of trees should be reduced, but strategically placed.

References:

(1)Joan Webster “The Complete Bushfire Safety Book (Random House Australia 2000) p.3 This book contains a complete chart of fire-retardant plants.
(2)Fire & Emergency Services of WA, Volunteer Learning Manual “Wildfire Behaviour” (FESA Training Academy, Forrestfield, 2000) p.13

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