Mulch on Fire

mulch

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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11 thoughts on “Mulch on Fire

  1. No doubt about mulch fires being hard to put out. We get them started here in Kansas from prairie fires; intentional or otherwise. And rock landscaping helps but may not prevent the problem. I had some fire in buffalograss around my house last year that somehow (chinks between landscaping blocks? a tall weed?) climbed to the other side of a 3 foot landscape block wall and started again in the mulch of a bed.

  2. Hello!

    As a former Volunteer Fire Fighter, I can definately attest to the ability of mulch to ignite. Even with out an ignition source it still can catch fire. Especially piles of mulch. They get very hot inside the middle of the pile and if not turned over on a regular basis it can catch fire. While I was member of my local Volunteer Fire company we were called out to a farm, with a mulch pile on fire. We had to use rakes, shovels, and eventually a front-end loader to tear the pile apart and make sure the fire was out and there were no more “hot spots” buring. Even a pile that is moist on top can retain a significant amount of heat to ignite the inner most mulch, as long as it is dry enough.

    If you ever go past a Garden Center with a large pile(s) of mulch, on a cold day you will most likely see some steam comming from the pile(s) of mulch. That is how hot it gets inside the piles of mulch. If I remember corectly it can get well over 120 degrees F inside a pile of mulch that is properly turned.

  3. Good article; I forwarded to my facebook page – I’m a landscape designer and hadn’t given that much thought on mulch and fire so the statistics are very interesting!
    MJ Martin
    MJ Martin, Landscape Designer

    • A few years back, I had a contractor throw his cigarette into some wood mulch I had on the side of my yard – under pine trees. While it was a wet area, the mulch slowly burned and I had a smoldering fire in a 20 x 20 foot area. Took quite a while to put the fire out. Fortunately it was about 20 feet from the house. Also compost gets hot – and mulch is compost. Coming from a farm, I’ve seen barns blow up from moist hay in them. Be careful.

  4. Interesting but not totally surprising… but the topic does raise a closely related question – what is the risk of spontaneous combustion in some mulches? Anyone aware of any studies that may have been done in this regard?

  5. The mulch that is most likely to catch fire is pine needles. Why? Because the “strategy” of pine trees is to shed needles that are highly flammable so that competing trees–like deciduous hardwoods–would be killed in fires caused by lightening strikes. A pine tree’s thick bark protects them from moderate fires and many plants associated with pine forests, such as grasses, are fire-tolerant. So pine needles do make an attractive and cost-effective mulch in some parts of the country, but they are HIGHLY flammable. So don’t put them within about 10′ of any wooden structures.

    • This is incorrect. Plants generally don’t attack other plants or have strategies for competition. General forest succession is due to the adaptation of plant species. When trees get larger they cast more and more shade as they get older. In turn this creates changes in ecology (moisture levels, soil changes, shade, etc..) understory plants then begin growing and at the same time changing the ecosystem again. The reason pine needles are so flammable is because pine trees produce turpentine. This turpentine (which is extremely flammable) is a defense mechanism many trees use against (specifically) bark beetles. Bark beetles and many other insect pests generally will only attack previously un-healthy or injured plants. Weaker un-healthy trees produce far less turpentine sap which allows the insect to penetrate the defense mechanism. For more information, search Ambrosia Bark Beetle. Generally speaking pine forests do not offer much diversity between species due to the soils acidic nature therefore conifers and deciduous trees do not have head to head competitions. Yes, pine needles are flammable, but not for the reason of competing with deciduous trees.

  6. In additon to mulches, I have a smoking firend who used her porch pots to put out her cigaretes before entering the house. Sphagnum peat moss and charcoal did the same thing. Fortunately, they family noticed before the wood siding went up.

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