Another evaluation of mulch combustibility was performed in 2008 by the University of California Cooperative Extension and the University of Nevada Cooperative Extension. This study resulted in recommendations for mulch use in wildfire hazard areas.
The study defines mulch as any material that is used to cover the soil surface for a variety of purposes. Mulches can be classified as organic or inorganic. Organic mulches usually come from plant materials and include pine needles, pine bark nuggets, shredded western cedar, and even ground or shredded rubber. Inorganic mulches consist of rock, gravel, and brick chips. These inorganic mulches do not burn and are safe to use in any setting.
Eight mulch treatments were evaluated for three characteristics: flame height, rate of fire spread, and temperature. On the test day, the National Fire Danger Rating System value was Extreme. All eight mulches were found to be combustible but varied considerably in the three areas measured.
- Composted wood chips showed the slowest spread rate and the shortest average flame length, usually smoldering.
- Shredded rubber, pine needles, and shredded western red cedar showed the greatest potential for all three characteristics.
- Shredded rubber burned at the hottest average temperature (in excess of 630 degrees F at a height of four inches) and produced the greatest flame length at over three feet.
- Shredded western red cedar had the most rapid rate of spread, traveling at an average rate of 47.9 feet per minute. It also produced embers that moved beyond the plot perimeter and ignited adjacent mulch plots.