Combating and Preventing Mulch Fires


Thousands of mulch fires are reported annually in every state. The Johnson City (TN) Fire Department has its share of mulch fires, averaging 100 per year. Mulch fires occur year-round but primarily in the summer when there is little rainfall. As a result, vegetation and landscaping materials become dried out, allowing for easy ignition. Mulch fires have caused extensive damage to structures and woodlands in and around Johnson City.

Our department is frequently called out to extinguish smoldering fires in bark mulch. Often, this burning mulch is up against the side of a residential or commercial structure, where it is likely to be unnoticed. This burning/smoldering mulch may eventually ignite the underneath of the siding and then spread into the structural components of the building and cause extensive damage.

Factors such as below-average rainfall, extremely dry conditions, warm temperatures, and abnormal winds increase the risk of serious damage from mulch fires. There has been a significant increase in mulch fires in northeast Tennessee over the past several years because of drought-like conditions. Another key factor in the increase of mulch fires has been the prohibitions on smoking indoors enacted by state/local governments and private businesses. Cigarette and cigar smokers often discard lighted smoking materials, including matches, into the landscaped areas as they enter buildings, which has been the cause of ignition for many mulch fires.


At 6:43 p.m. on April 18, 2005, the Johnson City Fire Department responded a full-alarm assignment of three engines, two ladders, and a chief officer to a structure fire at the Ramada Inn located at 2606 North Roan Street. The first-arriving company observed smoke issuing from the roofline of a two-story hotel building of ordinary construction. On further investigation, personnel discovered that the synthetic stucco exterior wall of a support column was turning brown (photo 1). Fire had charred the interior wood frame of the column and was starting to extend into the void space under the roof (photos 2, 3). The deep and extensive charring indicated the fire had been “working” for an extended period.

(1-3) Photos by Assistant Fire Marshal Lori Ratliff, Johnson City (TN) Fire Department.





Firefighters were on the scene for 3½ hours. Responding personnel prevented the fire from extending into the open void space under the roof, thus preventing major damage to the business. Investigation revealed that the fire started in the mulch at the base of the column; a discarded cigarette was the ignition source. Property damage was estimated at $10,000.


Another example of burning mulch extending into structural components occurred on March 27, 2007. The Johnson City Fire Department responded to a reported house fire in the northern part of the city. On arrival, the first-arriving unit found smoke and flame issuing from around an exterior wall of a single-family, two-story wood-frame dwelling. The fire was quickly extinguished but required extensive overhaul (photos 4-6). Investigation revealed heavy charring of the wooden stud walls. The fire cause was undetermined, but the area of origin was in the landscaping mulch that was placed up against the exterior wall. Damage to the structure was estimated at $8,000.

(4-6) Photos by Assistant Fire Marshal Mike Hughes, Johnson City (TN) Fire Department.







Mulch stored in huge piles, 10 to 20 feet deep, has also been known to build up enough heat from internal decomposition to start a fire. A huge mulch fire near the tiny Texas town of Helotes burned for three months before it died out in March 2007; authorities say spontaneous combustion may have caused it. Spontaneous combustion occurs when a substance generates enough heat to ignite without an outside source. The fire triangle consists of heat, fuel, and oxygen. Ignition of fire requires not only a combustible material but also oxygen, generally from air, and usually an energy source to provide ignition, although certain materials other than mulch can also spontaneously ignite. Since mulch is a wood product and is always decomposing, a large enough pile of it can build up enough heat and spontaneously combust, as happened in Helotes.

On October 13, 2006, trees in Buffalo, New York, suffered immense damage from an early snowstorm. Twenty-four inches of heavy wet snow hit the area (as much as one inch an hour) when a majority of the trees still had leaves on them. The combination of the wet snow and the leaves caused the branches to snap and break off. Residents described the noise of the tree limbs breaking off as similar to that of shotgun fire. President Bush declared the region a federal disaster area. Some 350,000 residents lost electrical power for between several days and two weeks. The Federal Emergency Management Agency (FEMA) authorized at least $5 million for cleanup efforts in Erie County and western New York state.

An “urban logging” operation commenced. The National Guard and many tree-trimming companies assisted with brush/tree limb removal and cleanup. The loads of brush and tree limbs were taken to strategically located dropoff sites, where the remnants were converted into mulch. The resulting large piles of mulch generated intense heat and smoldered continuously. The heat produced from one mulch pile burned a hole in the pavement underneath it (photo 7).

(7) Photo by Dr. Joseph A. LaNasa.

Once combustion has begun in a mulch pile, the smoldering fire can become deep-seated and spread rapidly. The larger and taller the mulch pile, the greater the fire hazard and the more difficult a fire in such a pile will be to control. The dimensions of a mulch pile should be no more than 25 feet high, 150 feet wide, and 250 feet long. It is much better to have several smaller piles than one large pile.


Larry Steward, assistant professor of horticulture at The Ohio State University Agricultural Technical Institute (Ohio State ATI) and colleagues T. Davis Sydnor and Bert Bishop researched the combustibility of various mulches, putting 13 organic mulches to ignition tests. These mulches were tested under natural field conditions for their ease of ignition from cigarettes, matches, and a propane torch. The tested mulch was only two to four inches deep, which eliminated the chance of spontaneous combustion.

According to this research, there are definite differences in the ease of ignition among commonly used mulches. The results demonstrated that landscapers do not have to resort to using inorganic materials such as brick chips and gravel as ignition-resistant mulches. The ease of ignition of these landscape mulches was also evaluated. The researchers found that mulches have different ignition potentials based on several factors, including the exposure time to heat and to the ignition source. Some mulches, such as those consisting of cocoa hulls, never ignited under any circumstances, whereas others, such as ground rubber, produced flames within a minute and were difficult to extinguish. Steward noted that it would be rare for mulch to ignite that quickly.

In most mulch fires, the smoldering tunnels under the surface of the mulch and then breaks into open flame. In rubber mulch, the fire spreads fast and can’t be contained. What’s disturbing is that rubber mulch is commonly used in playgrounds.

According to the research, overall, mulches high in oils, such as pine bark and shredded cypress bark, were easiest to ignite. Fresh wood mulches such as the currently popular recycled pallets were also easily ignited, and shredded materials tended to go up in flames more quickly than cubed materials. Overall, cocoa shells, medium pine bark nuggets, and hardwood bark are the three organic mulches recommended for most locations, especially, Steward noted, where there is a real chance of someone flicking a cigarette into the mulch.

Many people who live and work in Johnson City and other areas take great pride in landscape decorating. Many residents beautify their homes and businesses with scenic floral arrangements. The outward appearance of their buildings and homes is very important to them. Commonly applied after shrubs and ground covers are planted in the landscape, mulches are chosen for aesthetic appeal, color, price, organic content, nutrient content, weed reduction, dust abatement, and soil moisture retention. Regardless of which mulch is used, taking precautions can prevent mulch fires.

In the general provisions of National Fire Protection Association 1, Fire Prevention Code (2006 ed.), Section 19.1.2 states, “No person owning or having control of any property shall allow any combustible waste material to accumulate in any area or in any manner that creates a fire hazard to life or property.”


To reduce the potential for a fire in landscaping mulch, have citizens and businesses follow these guidelines:

  • Recognize that when the weather is hot and there has been little or no rain for an extended time, mulch fires can start more readily.
  • When entering a business, if you see anything smoldering in a landscaped area, put it out if you can and report it to someone inside the building. If the burning material is not thoroughly wetted down or removed, it may reignite.
  • 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 approved receptacles for smoking materials at all entrances to public buildings and in designated smoking areas. Do not use mulch in or near these areas.
  • Provide a minimum 18-inch clearance between landscaped mulch beds and combustible building materials.
  • Provide proper clearance for electric devices such as decorative lights by following the manufacturer’s instructions.
  • Keep landscaped mulch beds moist, if possible.
  • Use noncombustible mulch such as rock or pea gravel around the gas meter and next to the combustible portions of the structure.
  • Use only the manufacturer’s recommended size/wattage for yard light bulbs.
  • Use only electrical devices and cords listed for outdoor use, and follow the manufacturer’s specifications.
  • Consider replacing landscaping mulch with decorative stone.


Friedman, Raymond, Principles of Fire Protection Chemistry (second edition), National Fire Protection Association, Quincy, MA, 1989.

Brewer, Melissa, “Mulch Ignition Is a Hot Landscape Topic,” Ohio State University Extension, April, 6, 2004,

MARK J. FINUCANE, a 21-year veteran of the fire service, is an administrative assistant chief with the Johnson City (TN) Fire Department. He is a state-certified firefighter II, fire officer I, safety officer, and instructor I and has a bachelor of science degree from East Tennessee State University.

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