Determining the flammability of plants is not straightforward. Although it can be tested under controlled circumstances in a laboratory, the flammability of a plant may vary in a bushfire, where the conditions are often unpredictable. Some plants are more flammable than others but all plants in a garden – living and dead – can provide fuel for a bushfire.
Plant flammability is described as a combination of:
Flammability will vary depending on:
a plant’s age, health, physical structure and chemical content
the daily and seasonal climatic variations
location of the plant in relation to other vegetation and flammable objects
the specific part of a plant – some parts of plants are also more flammable than others.
Foliage moisture content is the most critical factor that determines plant flammability. It influences how readily a plant will ignite.
Plants with high foliage moisture content will not burn until sufficient moisture in its foliage has been removed.
Plants with low moisture content will ignite more rapidly and continue to burn when the ignition source is removed.
Plants in the path of an oncoming bushfire will dry out as a result of the radiant heat and wind generated by the fire. Even fully hydrated plants will eventually dry out and burn if they are exposed to bushfire heat for long enough.
The arrangement of vegetation within a garden, rather than the flammability of individual plants, has a greater impact on how a bushfire will spread.
In most high bushfire risk areas, houses are located in close proximity to unmanaged vegetation. Some popular garden plants have become environmental weeds by escaping to the bush and displacing native species. Environmental weeds often contribute to high fuel loads, which increases bushfire risk. Priority should be given to removing environmental weeds within the property.
Avoid planting environmental weeds. Contact local council to find out which weed species are a problem in the area. The Department of Primary Industries also has information about weed species at dpi.vic.gov.au
When selecting plants, consider using local native species with low flammability. These are well suited to local conditions and will add to the habitat value of the bushland.
Moisture content depends on a number of interacting factors:
The time of day:
Before sunrise, plants will typically have their maximum moisture content (influenced by the moisture content of the soil and humidity).
As they transpire during the day their foliage moisture content decreases until the plant stops transpiring after sunset.
Generally plants are most flammable in the mid- to late-afternoon when their foliage moisture content is at its lowest.
During summer as the soil dries out, the moisture content of the plant will decrease and the flammability of the plant will increase.
The part of the plant:
The leaves and new growth on a plant will generally have a higher moisture content than the stems or branches.
Dead leaves and twigs have a very low moisture content that is driven by the relative humidity. On hot, dry days they become highly flammable as fine fuels.
Where it is planted:
The amount of sun or shade, the availability of water, drainage and soil type will affect plant moisture content.
High temperatures, low humidity and periods of drought will increase the flammability of plants.
The age of the plant and its growth stage:
Many plants start as moisture-rich shoots but become woody as they age. As plants approach the end of their life they tend to dry out.
New growth on a plant will usually be soft and fleshy and become woody after the growing season.
The following plant characteristics are used throughout the Plant Selection Key. They all contribute to plant flammability to varying degrees and should not be considered in isolation.
This influences the distribution and density of foliage within the plant.
Choose plants with open and loose branching as well as leaves that are thinly spread.
Plants with closely packed leaves and branches have more fuel available within the plant and are usually more flammable.
Plants with branches at least 2 metres above the ground are better than those with continuous foliage from the ground to the canopy. Under-pruning increases separation.
Separation between ground fuel and foliage on the rest of the plant prevents lower branches acting as ladder fuels.
This describes the overall appearance of the plant.
In coarse textured plants, it is easy to distinguish each branch or leaf from a distance of 3 metres.
Plants with a coarse texture have a lower surface-area-to-volume ratio making them less flammable than plants with a fine texture.
This describes the amount and arrangement of fuel within the plant.
A dense plant is difficult to place a hand into and is not easy to see through.
Plants that are very dense are often more flammable as there is a higher fuel load readily available to burn.