Mulches Don’t Ignite Spontaneously!

Here’s yet another urban garden myth worth quashing: the one that says bark and wood chip mulches should be avoided because they can spontaneously burst into flame. Usually the legend tells a story of a house or garage that burned to the ground fire when a landscape mulch caught fire all on its own.

Newspapers often vehicle stories about a fire supposedly caused by self-igniting mulch in the hours that follow the incident, but by the time the final report on the fire comes out, the story is old news and the public is never informed of the real cause (whatever it was), thus helping to sustain the urban legend.

A Bit of Truth, A Lot of Myth

So, let’s set the record straight.

Bark and wood chip mulches can indeed catch fire, as can other organic mulches. After all, they are derived from combustible materials. It’s just that they don’t catch fire spontaneously, at least, not when used in home landscapes.

Organic mulches can spontaeously ignite, however, when stored in large piles, as can peat moss, compost, wood residues, and all sorts of organic wastes. In piles (and the piles have to be huge, like 10 to 20 feet high!) that are not turned regularly, enough heat can build up from internal decomposition to start a fire. The danger point is when the temperature is above 180?C (82?C). This is a known risk and sawmills, peat harvesters, and mulch and compost manufacturers carefully monitor their piles for signs of any abnormal heat buildup.

But thin layers of mulch used in home gardens, rarely any thicker than 3 to 6 inches, can’t ignite on their own. They simply do not produce enough heat, not even black mulches fully exposed to the sun.

What Causes Mulch Fires

Organic mulches can catch fire if lit, though… and the cause is most commonly a discarded cigarette butt or match. Other causes are inadequate disposable of charcoal briquettes from barbecues or wood fires or campfires lit near a mulched bed.

In many areas, fires in landscaping mulch are much more common in municipal plantings at road intersections than in home gardens or commercial landscapes. Typically a driver waiting at a red light tosses a cigarette or match out of the window onto a landscaped median strip or other roadside planting and a fire starts.

Clearly some mulches burn more readily than others, but studies about the flammabilty of mulches give widely varying results, depending on the source of ignition (cigar butts, lit match, spark from a fire, etc.), the conditions under which the mulch is used, and apparently other as yet undefined factors, so it is difficult to make any clear recommendations. At least most inorganic mulches (various stones and rocks, notably) are not flammable, the biggest exception being shredded rubber mulches which can burn.

Safety With Mulches

Here are some tips on how to keep your mulch safe from fire.

1. Properly discard cigarette butts and make sure they’re completely extinguished before walking away;
2. Never throw a cigarette butt from a the window of a vehicle;
3. Provide receptacles where smokers can place cigarette butts and used matches;
4. Keep mulch moist during periods of heat and drought;

5. 

   Leave a space 18 inches (45 cm) wide clear of any flammable mulch at the base of combustible building exteriors such as wood or vinyl siding;

6. Use nonflammable materials like stones, gravel or mineralized wood mulch as a barrier in the first 18 inches (45 cm) around the foundation of a building;
7. Don’t run electrical wires (Christmas lights especially) through dry organic mulch;
8. Never light a campfire, wood fire or barbecue directly on or right next to a mulched surface;
9. Be sure to carefully extinguish charcoal barbecues, wood fires, and campfires and do not throw their residues onto a mulched surface;
10. If you see mulch smoldering, douse it with water if possible and call the fire department.