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Mulch on Fire

I read a story in my local newspaper about a house that burned down and the garden mulch was listed as a possible cause. Is mulch really a fire hazard?!

Question: I read a story in my local newspaper about a house that burned down and the garden mulch was listed as a possible cause. Is mulch really a fire hazard?!

Answer: Mulch is flammable and once it catches fire, that fire can be very persistent. If the mulch is touching the side of a building, the fire can easily spread to the building. Mulch fires are most likely in dry, hot weather. The Virginia Department of Forestry also cites "non-smoking" public buildings as at higher risk; if people discard cigarette butts in the mulch before entering the building, that mulch may ignite.

Some kinds of mulch are more combustible than others. In a study by The Ohio State University Agricultural Technical Institute, cocoa hulls never ignited while rubber mulch showed flames in less than a minute. The researchers noted that with most mulches, fire smoldered under the surface of the mulch for some time before flames appeared. Shredded mulch ignited quicker than chunky mulch. The researchers cited cocoa shells, medium pine bark nuggets and hardwood barks as the safest choices, especially in sites frequented by smokers.

Here are tips from the Virginia Department of Forestry for reducing the potential for a fire in mulch:

  • Be aware of this danger.
  • Provide a minimum of an 18-inch clearance between landscaping mulch beds and combustible building materials. Ensure proper clearance to electric devices, such as decorative lights, by following the manufacturer’s instructions.
  • Recognize when it gets hot and has been dry for an extended time, which enables these fires to start more readily.
  • If you smoke, use the provided receptacles to discard your smoking material and matches.
  • If you see anything smoking in a landscaped bed, put it out if you can and report it to someone inside the building. If the burning material is not thoroughly wet or removed, it may re-ignite.
  • Grounds maintenance crews should be aware of the conditions that are favorable for mulch fires and increase surveillance of mulch beds in the afternoon when fires are more likely to occur.
  • Provide proper receptacles for smoking materials at all entrances to public buildings and in designated smoking areas. Do not use mulch in or near these areas.
  • Keep landscaping mulch beds moist if possible.

If you live in an area susceptible to wild fires, you might also consider using gravel mulch instead of wood mulch and planting fire-resistant plants. Mulch can also spontaneously combust, but this generally happens when the mulch is stored in a large pile (10 to 20 feet deep). As the mulch goes through its natural decompostition process, it releases heat. In a large pile, enough heat can build up to ignite without an outside source of flame.

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