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Master Gardener: Mulch combustibility


April 19, 2012 at 11:01 am

Q I live in an area that is somewhat fire-prone. Is it OK to use wood chips on the planting beds next to my house? They won’t catch fire, will they?

A For years, Master Gardeners have been singing the praises of mulch, especially organic mulches such as wood chips, pine needles and compost. Mulch does so much good in our gardens that it’s hard to think of it as causing problems.

However, a recent study conducted by the UC Cooperative Extension in association with several other groups has determined that many common organic mulches are combustible and can pose a real fire danger if used too close to buildings, especially in fire-prone areas.

In the study, eight common mulch materials were applied in a variety of ways, exposed to the elements for 2½ months, then ignited. Researchers compared flame height, rate of spread and maximum temperature of the fires to determine their combustibility.

The mulches were tested on a hot, dry, windy day — typical weather during the fire season in California.

The mulch materials tested included shredded rubber, pine needles, shredded Western Red Cedar, medium pine bark nuggets, Tahoe chips (a mixture of dried wood chips, bark and pine needles), Tahoe chips with fire retardant, Tahoe chips in a single layer and composted wood chips.

The combustion characteristics of the mulches varied. Some burned hotter or with taller flames, some spread more rapidly, some smoldered rather than flamed.

However, they were all combustible and, therefore, not suitable for use within five feet of structures.

The study’s authors have some recommendations for property owners who want to protect their buildings in fire-prone areas:

  • Maintain a noncombustible, ignition-resistant area next to the structure. Do not use any combustible materials within five feet of structures.
  • Within those five feet, use only noncombustible rock, gravel, concrete, pavers or ignition-resistant plant materials such as irrigated lawn and flowers. If you use plants in that area, keep them well-maintained — watered, free of dead plant material, pruned appropriately — and keep lawns mowed.
  • From five to 30 feet from structures, it’s acceptable to use medium pine bark nuggets, Tahoe chips with or without fire retardant, and composted wood chips because these had the least hazardous combustion characteristics of the mulches tested.


Separate areas mulched with these materials with noncombustible and ignition-resistant materials such as concrete, gravel, rock and lawn. Of the eight mulches tested, composted wood chips demonstrated the least hazardous fire behavior overall. Composted wood chips smoldered rather than flamed. This smoldering could go unnoticed during a wildfire and might be missed by firefighters. Do not use composted wood chips within five feet of structures.


Fire retardant on wood chips provided five to 10 minutes of fire suppression. After that, the behavior of fire retardant-treated chips was identical to untreated chips.


Although using chips on an irrigated flower bed may reduce ignitability, don’t count on having water pressure to wet down flower beds during a fire.  Drip irrigation in flower beds typically does not wet the entire area. Even irrigated flower beds, if topped with combustible mulches, can pose a hazard.


For more information about minimizing the risk of fire through appropriate landscaping techniques, see “Landscaping Tips to Help Defend Your Home from Wildfire,” by Pamela Geisel and Donna Seaver, UC Statewide Master Gardener Program, and “Home Landscaping for Fire”, by Glenn Nader, both downloadable for free at


Earlene Millier is a Contra Costa Master Gardener.