Selecting Bushfire Resistant Plants and Farm Fire Safety

Ken Walker

How bushfire resistant plants can help to save lives.

Imagine a bushfire is racing towards your house. Furthermore, You wonder if the preparation made during the spring and previous years will be enough to protect you and your family. The television adverts told you to expect ember attack and radiant heat. Maybe you wish you had put more emphasis on planting bushfire resistant plants.

Over many years researchers have been studying the effects of bushfires on plants. Furthermore, there has been an analysis of plant performance and there is now quite a comprehensive list of plants to help prevent the impact of bushfire. These bushfire resistant plants can be designed into any garden and when strategically placed, provide excellent barriers.

In the sections that follow we will look at the activities, one may undertake to ultimately include bushfire resistant plants in their garden. Thereby providing, not only protection but a pleasing overall garden landscape.

How bushfires destroy houses.

Before we jump into the deep end with designs and what bushfire resistant plants we need for our property. It’s important to understand how bushfire impacts and destroys houses.
Generally, firefighters have three guiding principles in relation to bushfire behaviour. These are weather, fuel and topography.

hot-dayWeather:

Most Australians understand that when the bush drys out in the summer and we are faced with high winds. The fire services will likely call a total fire ban. There are many other minor factors which go into determining the potential fire danger rating (FDR). Even so, It could be said that weather and the associated wind is the driving force or engine behind a bushfire.

Topography (the slope of the ground):

Similarly, the slope of the ground has a significant impact on the spread of fire. Travelling up the slope at a much faster rate than on the level ground or downhill slopes. For example, if you live on the top of a mountain, an approaching fire will move very fast and generally be even more intense.

Also, northerly slopes (aspects) get more sunlight and dry out a lot quicker than southerly slopes. Subsequently providing a longer period where if given the right conditions (namely an ignition source) it will burn. Living in a valley rather than on top of a hill is certainly prefered. Firefighters never like to be positioned on top of a hill during a bushfire because it’s very dangerous.

Vegetation and fuel loads:

You may ask “isn’t vegetation what we are speaking about in this article”. Yes, you are right and in reality, it doesn’t matter how resistant a plant is, the effects of bushfire if, given the right conditions, even fire-resistant plants will burn. However, we are going to look at vegetation (bushfire resistant plants) that are less likely to burn quickly and contribute to the intensity of a bushfire.

As a rule, the fuel that concerns us most, in terms of house survival, is dry fine fuels. These are fuel being 6mm diameter or less. Even though other fuels, above 6mm in diameter, will burn. They contribute less to the intensity of the fire front. The dangers associated with fine fuels is one reason the rural fire services ask you to clear all leaves and sticks from around your home, including the gutters.

Heatwaves

Your house may be in the firing line

There are three main ways a house will catch fire during a bushfire. These are ember attack, radiant heat and direct flame contact. Firstly, embers can be devastating and will accumulate in corners and swirl around in the wind. Potentially burning your skin, eyes and anything else that is exposed. You may determine that you should be able to deal with this situation.

It is very unlikely and there are literally millions of embers and smaller sparks impacting every part of your house. Often contributing to ignition and the likely burning down your favourite asset. Don’t worry too much, bushfire resistant plants can be used to lessen the impact and we will analysis this later in the article.

Secondly, radiant heat or electromagnetic waves often ignite houses where there is vegetation within close proximity. Again, bushfire resistant plants can have a positive impact on this. Thirdly, direct flame contact with your house is going to rapidly heat the timbers or even break the glass and start burning.

Why plan a garden using bushfire resistant plants

In short, I think we have established that the purpose of planting bushfire resistant plants is to create a barrier. Stopping embers, reducing radiant heat and minimising or stopping direct flame contact. Therefore, it’s now appropriate to start to talk about the design of the garden and putting together a plan.

A disclaimer: in many states, there is legislation governing the design and building of structures/houses in bushfire prone areas. Check with your local building surveyor for information on local legislation. We can provide commentary or a comprehensive report on a holistic approach to farm and property safety. If you require to advise on house protection please call STG Fire Safety Training on (03) 9005 1767.

Designing for already established houses

What is more, and assuming your house is already established and lived in. It’s time to determine where the most likely direction a fire would approach from. Generally, in Victoria and during the summer months it would be from the north. Conditions across Australia are generally risk assessed and subsequently treated in a similar way. Even so, on a wind change, the fire may approach from the south/west direction (Victoria).

So most of the bushfire resistant plants will be heavily established in these directions. The south and eastern direction may not be a priority but will ideally still have some bushfire resistant plants included just in case! Accordingly, planting towards the north and around to the south-west will have a significant impact on the approaching fire if done correctly.

How wide will our defendable space need to be?

Inner zone: Particularly important here is radiant heat and direct flame contact. Starting at the house and moving outwards. We need to ensure that there is no potential for any flame contact and furthermore a reduction in the impact of radiant heat. As a rule, if the un-managed bush or shrub is located within 50 metres, then there is a real need to ensure that fuel within 50 metres is well managed.

Similarly, vegetation within 10 to 30 meters should be managed down to a very low height including, where possible, maintaining a high moisture content by watering. As guide vegetation, other than lawn and grass, should not be above waist level. A should be managed well and maintained with high moisture content. There are specific shrubs and small deciduous trees that are suited to this and we will cover them later.

In short, there should be no inappropriate plants or similar within 10 to 20 metres of the house. As well as, anything that could contribute to direct flame contact or dangerous levels of radiation. This is the last resort defendable space that may help to save your lives and property.

Defendable outer zone

Outer zone: This zone is where it all happens in relation to bushfire resistant plants. To begin with, the objective is going to be to create a barrier to stop or at least lessen ember attack. In reality, it is almost impossible to totally stop the ember attack but we can make a significant positive impact. A continuous barrier here would be ideal, but this could impact on your view or even look a little untidy.

Bushfire resistant plants & trees can be clumped strategically closer and further away from the house to still provide that continuous barrier. However, they should be no closer to the outer edge of the zone than 10 to 20 metres. Consequently, when you put your plan on paper it will seem more logical and well designed for fire safety.

Before looking at suitable bushfire resistant plants we need to understand the results we are looking for.

fire resistant plans

Critical Scenarios and risk assessments

Scenario: Let’s imagine the ember attack is starting to occur and hitting into our vegetation barriers. If the trees are too high the embers will get under them. Especially when the trees start to mature. On the other hand, shrubs that are too low will allow embers to go up an over them. Especially on days where high winds are present (20 km/h and above).

What we need to do is try and create a continuous barrier using trees that grow 10 to 15 metres and fill in the under area with shrubs (see important note), which have green vegetation from top to bottom during the summer.

Important:

These fill-in shrubs should never be planted directly under the trees. To do so, may create a ladder for the fire to travel up into the canopy of the tree. To overcome this plant the fire-resistant plants (shrubs) four to six metres away from the outer foliage of the tree. The preferred side is the side closest to the house.

Later on, in this article, we will look at some types of tree and shrubs that may suit your area. As mentioned, these barriers can be clumped in small lots and together will form an almost continuous barrier. It may be a medium-term strategy to carry out this activity but it is certainly worth the effort.

Once this work has been done it will only be effective if vegetation is again well managed during the summer.

Gardens using bushfire resistant plants

Whether using barriers created by fruit trees or well-maintained vegetable gardens. There are a variety of options that will suit the most critical Gardner. Furthermore, the selection of appropriate bushfire resistant plants is very long, which is included at the end of this article. Finally, we will look at the characteristics of good bushfire resistant plants.

Texture:

Plants with a coarse texture generally have a lower surface area to volume ratio. Decreasing flammability as compared to plants with a finer texture. Remember earlier in the article we indicated that fuels less than 6mm in diameter contribute to fire intensity far greater than thicker fuels.

Plant Density:

The density of foliage is an important factor. The greater the density the more available fuel to burn. I.e. consider a hedge – fuel within a hedge will be very dry as compared to the outer layer. Some hedges do make quite good barriers but the dry fuel must be managed well.
Leaves:
Again, the fineness, size and shape of leaves contribute to its potential flammability. Especially where there is significant heat stress associated with hot weather.

bark on gum tree

Bark:

It is appropriate to spend a little time on bark because of its potential to spread a bushfire. Every year in most states of Australia our forestry organisation perform controlled burns. This often reduces the impact of bushfire on flora and fauna. One of the main reason this is done is to reduce ladder fuels.

Ladder fuels provide a pathway for the fire to travel up the tree and often to the forest canopy. Where this occurs we call the type of fire a crown fire, which is very intense and generally contributes to spot fire ahead of the main fire front.

Some barks (candle barks) have been known to cause spot fire some 40 kilometres ahead of the main fire front. Carried in the wind and often resulting in catastrophic devastation.

Reducing the bark or ladder fuels reduces the risk and potential of the fire crowning. So our selection of plants should be tightly attached to the tree trunk. The fibrous and loose bark will increase substantially the ember attack. It must also be noted that even smooth tightly attached barks will shed at different times during the year and must be managed as part of your emergency fire plan.

Oils, Waxes and Resins:

There are a number of plants which contain quite high levels of oils, waxes and resins. Our selection of bushfire resistant plants will have lower levels. A general test you can do is to break up the foliage in your hand. If it has a strong scent then there is probably quite a lot of oils and resins present. They will contribute to a more intense fire.

Retention of dead material:

All fine fuels, even on bushfire resistant plants, must be managed. Dense foliage like hedges stores a lot of very fine dead fuel. When this material burns it is often so intense that it will cause the green foliage on the outer part of the plant to also burn.

In conclusion

Throughout Australia bushfire spreads rapidly and fine fuels can increase intensity substantially. In terms of providing an effective fire barrier, there are activities property owners can undertake that will lessen the impact on one’s family and property. Selecting bushfire resistant plants is just one part of an effective emergency preparedness plan for the bushfire.

The section, positioning and characteristics of such barriers are critical in reducing ember attack, radiation and potential flame contact. Incorporating bushfire resistant plants into a holistic emergency preparedness strategy the effectiveness may be the difference between life and death.

Bibliography:
  • Landscaping for Bushfire – CFA Victoria.
  • Fire Resistant and Retardant Plants – Australian Plants Society (Victoria)

List of Bushfire Resistant Plans

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