Some plants and trees are more flammable than others
Deadly wildfires in Hawaii this month were partly fueled by plants, particularly invasive grasses that have taken over land once occupied by sugar and pineapple plantations.
Some plants are more flammable than others, but “there’s no such thing as a fireproof plant,” said Michelle Steinberg, wildfire director for the National Fire Protection Association. All plants can catch fire under the right conditions.
These conditions include inappropriate pruning, inadequate irrigation, and poor sanitation practices that allow dry and dead plant parts to remain on the soil surface in vulnerable areas.
If you live in a fire risk area (or an area where climate change is increasing fire risk) and are choosing plants for your garden, knowing which ones offer some fire resistance and which are more flammable will serve you well.
Hurry to start the fire
Plants that contain essential oils, resins, wax or gummy sap are some of the quickest to catch fire, even if they are well watered and cared for. These plants include acacia, bamboo, eucalyptus, Japanese honeysuckle, rosemary, Scotch broom, and the gas plant, which gets its name from the flammable vapor that its flowers and leaves produce.
Trees with peeling, papery bark, such as river birch, are generally more flammable than those without. Evergreen shrubs and trees with fine needles, such as cedar, cypress, spruce, juniper, pine and spruce, contain volatile sap and resins. Their fallen needles, left to dry on the ground — or on a roof — increase the fire risk. Redwood, a notable exception, is considered fire-resistant due to the tannic acid in its bark.
Many grasses, such as the buffalo, molasses and guinea species that sparked the Hawaiian fires, as well as fountain and feather grasses, are highly flammable. Its flammability increases when left to dry during winter or during periods of drought. Excessive dry heat causes moisture to evaporate from and from the soil and many other plant species, essentially turning it into fire.
Original vs. non-original
As a group, “native plants are not necessarily less flammable” than introduced species, Steinberg said.
But non-native invasive plants often pose greater fire risks because they spread easily, are usually left undisturbed by wildlife, outcompete native plants, and often tolerate heat, drought and heavy rain well. They can quickly cover fields, acres, and even miles of land, where a spark, like lightning, can set it on fire.
For the best fire resistance, choose deciduous trees such as ash, redapple, dogwood, locust, maple, and oak, over evergreen trees with fine needles. Succulents with water-filled leaves, such as ice plants and sedums, are slow-burning, as are some ground covers, such as ajuga and creeping phlox.
What to look for in plants
The Washington State University Extension Service has published valuable guidelines that identify these general plant characteristics as fire resistant:
- High moisture content in the leaves (they ignite and burn more slowly)
- Little or no seasonal accumulation of dead plants
- Open branching habits (they provide less fuel for fires)
- Fewer overall branches and leaves (again, less fuel for fires)
- Slow growth, so less pruning is required (to maintain the open structure as above)
- Non-resinous materials found on the plant (i.e. stems, leaves, or needles that are not resinous, oily, or waxy)