This interesting study, out of U Nevada in 2008 by Quarles and Smith, tested the combustibility of different types of mulches. The different types of mulch were shredded rubber (not wood obviously), pine needles, shredded Western red cedar, medium pine bark nuggets, Tahoe chips, Tahoe chips with fire retardant, Tahoe chips in a single thin layer (80-100% coverage), and composted wood chips (through a proprietary compost process).
The results were quite disappointing: “All of the mulches evaluated were combustible under the test conditions of dry, hot and windy weather and more than 2½ months of outdoor exposure.”
- The rubber was by far the worst, bursting in 3-ft flames burning at over 600°F.
- The fire retardant treated chips did not do much better than the original ones: " The retardant delayed fire spread for approximately five to 10 minutes, after which the rate of spread was similar to the untreated chips."
- “The most rapid rate of fire spread came from shredded western red cedar” (at almost 50 ft/mn)
- “The lowest temperature values were produced by the Tahoe chips, single layer treatment. They also produced relatively low flame heights and rates of fire spread.” However, given the fact that they were spread as a single thin layer, without perfect coverage, their usefulness as a weed blocker would be low.
- “Composted wood chips demonstrated the slowest fire spread rate of the eight mulch treatments evaluated, less than 0.3 feet per minute.” This result, of course, is only good as relatively to the others—the fire still progressed successfully, assumingly towards the structure on the other side of these chips… The author carefully adds that " It is not known if the performance of the composted wood chips is specific to [the provider of these chips] or if composted wood chips from other sources would perform in a similar manner."
As an anecdote, it may be interesting to note that the author of the present summary witnessed, two weeks on a row, on cold May mornings, spontaneous combustion of redwood mulch, approximately one year after spreading, the second time in the presence of a fire inspector.
The conclusion of the study is dire: it is hard to see in which conditions the performance of any of these mulches would be satisfactory.
Download the study here:
Addendum: the Butler et al 2018 study
This interesting study (Wind-Driven Fire Spread to a Structure from Fences and Mulch) focuses primarily on fences, but spends considerable time studying mulches as well. It is long, thorough and broad—so it is not possible to summarize all of its conclusions. A few of them, however, can be directly mentioned:
- “Fire spreads easily across the fine overlapping particulates of a mulch bed.” [this was true regardless of the type of mulch, some mulches being faster propagation than others]
- “… fences combined with mulch were more hazardous than either the fence or the mulch bed separately. Adding fine combustible materials to the base of a fence promoted fire spread along the base of the fence, allowing the combination of fence and mulch to act as a wick transporting fire along the entire length of the fence.” In other words, mulch in combination with fences make both worse as fire propagators.
Download the study here: