Effective defendable space, house construction, water and access in new and existing gardens can all be compromised by inappropriate landscaping.
The following design principles outline how defendable space can be used to reduce radiant heat, prevent flame contact and minimise ember attack on the home.
Design Principle 1
CREATE DEFENDABLE SPACE
Requirements for defendable space will vary, depending on the type of development and level of bushfire risk to the property. A bushfire site assessment (contact me if further details are required on acquiring one of these and I will put you in contact with the appropriate people) may be required as part of your planning permit process. This permit will impose the conditions and the size of your defendable space zones.
There are generally 2 zones, each having different considerations.
The Inner Zone, is the area immediately surrounding the house. Vegetation needs significant management, so that fuel is managed to a minimum level in this zone.
- Flammable objects such as plants, mulches and fences must not be located within 10 metres of vulnerable parts of a building such as windows, decks and eaves.
- Trees must not overhang the roofline or touch walls of a building. A canopy separation of 2 metres is recommended with total cover being no more than 15% at maturity.
- Grass should be no longer than 5cm in height, and all leaf litter and debris should be removed regularly.
- Shrubs should not be planted under trees.
- Planters taller than 10cm in height should not be placed under windows or other glass features.
The Outer Zone sits between the inner zone and unmanaged vegetation. Vegetation in this zone is managed to a more moderate level to decrease the ground fuel and restrict other fuels available to an approaching bushfire.
- Grass should be no longer than 10cm, and leaf and other debris should be mowed, slashed or mulched.
- Trees and shrubs should not form a continuous canopy. Overall canopy cover should be no more than 30% at maturity, with few shrubs in the under storey. Any branches within 2 metres above ground level should be removed.
- Shrubs should be in clumps of no greater than 10 metres square, which are then separated by a further 10 metres.
For both zones
- Non flammable features such as tennis courts, swimming pools, stone walls, dams, driveways or paths should be incorporated into the proposal, especially on the sides of the house where the prevailing winds are most likely to occur.
- Use materials such as brick, concrete, earth, stone, non combustible water tanks and galvanised iron to create radiant heat barriers.
- Locate driveways and paths to create separation between the house and vegetation.
- Use gravel and pebbles such as scoria, shells, recycled crushed brick etc to mulch garden beds.
Design Principle 2
REMOVE FLAMMABLE OBJECTS FROM AROUND THE HOUSE
The immediate area surrounding the house should be clear of flammable objects. There are a number of things that can be done to support this principle.
- Locate driveways, paved areas and paths against the house to create separation.
- Ensure trees are planted away from the house so they don't cause damage, especially if they fall. Ideally the should be located at least one and half times their mature height away from the house.
- Avoid flammable mulches. Great alternatives are gravel and pebbles such as scoria, shells, recycled crushed brick etc.
- Maintain grass regularly.
- Use non-combustible, moveable containers and pots that can be relocated in summer.
- Remove other flammable objects such as sheds, caravans, outdoor furniture, bbqs, gas bottles, and wood piles.
(source: CFAVIC Landscaping for Bushfires, 2011)
Design Principle 3
BREAK UP FUEL CONTINUITY
When a plant catches fire it can preheat and ignite the vegetation around it, so one of the most effective ways to reduce the spread of fire within the garden is to create separation between plants, garden beds and tree canopies.
Grouping plants and garden beds with areas of low fuel between them can help avoid spread, by breaking up the fuel continuity.
- Locate shrubs and other flammable objects away from trees.
- Clumping trees and shrubs so they don't form a continuous canopy. Separate with areas of low fuel.
- Use hard-scaped paths, non-flammable mulch and mown lawn to provide separation.
- Prune branches to a minimum of 2m above ground. This provides vertical separation between ground level fuel and the canopy.
- Tennis courts, pools, water features, driveways, stone walls etc also provide fuel seperation.
Design Principle 4
CAREFULLY SELECT, LOCATE AND MAINTAIN TREES
Surprisingly, trees can be useful during a bushfire, provided they are:
- carefully selected
- maintained frequently
- located at a safe distance from the house
Bushfire's are often accompanied by strong winds, which may cause branches to break or whole trees to fall over. Correctly selected and located trees can:
- Reduce wind speed
- Absorb radiant heat
- Filter embers
Fire is rarely sustained in the tree canopy, unless there is fire burning in the under storey. When designing:
- Avoid trees with loose or stringy bark
- Separate canopies by at least 2m
- Canopies should cover less than 15% of the inner zone or 30% of the outer zone
- Prune branches to a minimum of 2m above the ground level
- Locate trees at a safe distance from buildings. At least 1.5 times their mature height.
- Don't plant shrubs, that can carry fire into the trees, near or under trees
- Regularly remove dead leaves, bark and branches as well as leaf litter from underneath the trees.
Trees can be planted as a windbreak but are move effective in a fire of low to moderate intensity. However many things need to be considered:
- It takes time for trees to grow and they may not provide protection for many years.
- Wind direction can change and spot fires occur, allowing the fire to approach from any direction.
- There needs to be adequate separation between a building and the windbreak.
- A windbreak should not be planted within the defendable space.
- Trees should be carefully selected and then maintained frequently.
- Highly flammable trees will become a fire hazard.
- The windbreak should be planted at right angles to the prevailing winds.
- It should allow some wind to pass through.
- It should continue for at least 100m if possible.
- Well watered grass should be planted underneath windbreak and maintained regularly.
- The windbreak should be maintained regularly to remove any dead plant material or leaf litter.
(source: CFAVIC Landscaping for Bushfires, 2011)
In the next instalment we will look at some garden designs, and how apply these principles.