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:
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.
PLANT MOISTURE CONTENT
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.