The battles against big wildfires in the Southwest and other parts of the country this summer highlight the importance for homeowners in fire-prone areas to consider the combustibility of mulch materials used in their landscapes, especially within five feet of a dwelling. Mulches used in this near-home zone should always be non-combustible or of low combustibility.
Recently the California Agriculture and Natural Resources Department released information on the fire safety of various types of commonly used mulches. In a research study, eight different types of organic mulches were tested, including composted wood chips, medium-sized pine bark nuggets, pine needles, shredded rubber, shredded western red cedar, and either two or three inches or a single layer of Tahoe chips (a composite made of plant materials) with and without fire retardant. After exposing the mulches to summer weather for 12 weeks, each type of mulch was ignited and evaluated for flame height, rate of fire spread, and temperature above the mulch bed. All of the mulch materials began flaming readily, with the exception of the composted wood chips, which smoldered but produced few flames. The fire spread fastest in shredded rubber, pine needles, and shredded western cedar.
The researchers recommend that no organic mulches be used within five feet of a home in fire-prone areas, instead using non-combustibles like rocks and pavers or well-irrigated plants with low-combustibility such as turf or flowers. Highly combustible mulches should be located at least 30 feet from a structure.
To read the entire study and its recommendations, as well as other helpful information on landscaping for fire safety, go to: Center for Fire Research and Outreach.