The fineness, size and shape of leaves affect their flammability.
Wide, flat and thicker leaves (such as those on maples, camellias and oaks) and those that are soft and fleshy have more plant tissue in their leaves. This usually means a higher moisture content relative to their surface area.
Leaves with a high moisture content take longer to dry out and therefore longer to catch fire.
Generally, the higher the surface-area-to-volume ratio, the more flammable a leaf will be. Some plants with high surface-area-to-volume have leaves with high levels of oils (such as paperbark, tea trees, eucalypts) or resins (conifers such as pine trees). These combined properties increase flammability.
The shape of leaves influences how easily they are caught in vegetation when they fall off the plant. If leaves are caught within plants it will increase that plant’s flammability as leaf litter dries out and ignites readily. Dead pine needles are a good example of leaves that readily catch in other plants.
Some bark types ignite more readily than others.
Bark that is loose, stringy or fibrous will ignite easily and can break off to create burning embers that are carried ahead of a bushfire.
These types of bark can also act as ladder fuels that carry fire into the canopy of a tree, increasing the intensity of the fire. Examples of such plants include stringybark eucalypts and some paperbarks.
Bark that is attached tightly to the trunk or is smooth is usually less flammable because it is more difficult to ignite and will not be easily carried as an ember, for example Box Barks.
However, some smooth-barked trees shed their bark annually and trap large ribbons of bark in their branches or on the ground below. These ribbons of bark are highly flammable, can be carried as embers and can also act as a ladder fuel, for example Manna Gums.
OILS, WAXES AND RESINS
Some chemicals that are found naturally in plants will increase their flammability.
The leaves of plants containing significant amounts of oils, waxes and resins will often have a strong scent when crushed. For example rosemary, lavender and eucalyptus have oil in their foliage and pines can have high resin content.
Waxes and resins have a similar effect of increasing flammability of plants although there are a number of characteristics that contribute to the overall flammability of a plant.
Plants with high amounts of resins or oils should be limited and placed carefully within a garden.
Dead leaves, twigs, bark and branches that are retained on the plant or accumulate on the ground or in shrubs can increase the flammability of an otherwise firewise plant.
Regular pruning and maintenance of all trees and shrubs to remove these fine fuels is necessary.
MAINTAINING THE GARDEN
Remember that establishing a garden takes time. Buildings may not change but the plants in a garden will. To ensure a garden is effective over many years it will require ongoing maintenance of the defendable space around the house.
Replacement planting will need to be considered, as well as the periodic assessment of the suitability of the plants within the garden. Use the Plant Selection Key in Section 7 to assess plant flammability.
Diseased, stressed or dead plants are more flammable and moisture content will be lower in summer when bushfires are most prevalent.
Regular maintenance of the garden must be carried out and should be included as part of overall preparation for bushfire.
Incorporate maintenance into a Bushfire Survival Plan to ensure the garden is ready for the upcoming bushfire season.
Regular maintenance actions:
Clear ground fuel from underneath plants, on and around the house.
advice about whether the plant is appropriate for a garden.
An interactive version of this key is available online at cfa.vic.gov.au/plants
This Plant Selection Key is based on Behm AL, Long AJ, Monroe MC, Randall CK, Zipperer WC, Hermansen-Baez LA (2004), Fire in the Wildland-Urban Interface: Preparing a Firewise Plant List for WUI Residents. Circular 1453, School of Forest Resources and Conservation, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Address: Southern Center for Wildland-Urban Interface Research and Information, 408 W. University Ave., Suite 306, USDA Forest Service, Gainesville, FL 32601. Email (firstname.lastname@example.org) or fax (1-352-376-4536).
The Plant Selection Key has been customised to better suit Australian conditions and is intended to provide an indication of plant flammability. The flammability of plants is highly variable and in periods of drought or in the path of an oncoming bushfire, plants will dry out and become highly flammable. If there is uncertainty about the results this key produces, seek professional advice from a plant specialist.