Is mulch a fire hazard?

Larry Steward puts mulch to an ignition test. Some mulches are more combustible than others.

Larry Steward, assistant professor of horticulture at The Ohio State University Agricultural Technical Institute (Ohio State ATI) enlisted the aid of some colleagues and put 13 organic mulches to ignition tests. They tested the mulches under natural field conditions for their ease of ignition from cigarettes, matches and a propane torch. The tested mulch was 2 to 4 in. deep, which eliminated the chance of spontaneous combustion.

Some of the mulches, like cocoa hulls, never ignited under any of the circumstances, while others, like ground rubber, had flames within a minute and were a struggle to extinguish.

“It’s rare that you would have a fire in mulch that is immediate,” Steward said. “Most creep along under the mulch and then ignite, with rubber as an exception. It spreads fast and can’t be contained. What’s scary is that rubber’s commonly used in playgrounds.”

Overall, mulches high in oils, such as pinebark and shredded cypress bark, were easiest to ignite, Steward said. Fresh wood mulches like the currently popular recycled pallets, also were easily ignited, and shredded materials tended to go up in flames more quickly than cubed.

“Overall, cocoa shells, the medium pine bark nuggets and hardwood bark are the three organic mulches that I would recommend for most locations, especially where there is a real chance of someone flicking a cigarette," advised Steward.

For more details about mulch ignition and the specific mulches tested by Steward, call 330/263-3780 or email ct-oardc@osu.edu< and ask for a copy of “Is Your Landscape Going up in Smoke?”