Why Landscaping Mulch Can Spontaneously Combust—and How to Prevent It

What Causes Mulch Fires? 

There are two beliefs about mulch fires out there. The first is that mulch can spontaneously combust causing fires. The second—and most widely believed—is that mulch catches fire due to improperly discarded smoking material.

Spontaneous mulch combustion is believed to happen when heat builds up within a thick layer of mulch, say six inches or more. Theoretically, if enough heat builds within that layer, it can start to smolder and cause a fire. However, there isn’t any research out there that proves that mulch spontaneously combusts.

The latter theory of mulch fires being caused by improperly disposing of flammable material, like cigarettes, is more plausible, said Michele Steinberg, Wildfire Division Director of the National Fire Protection Association. 

“Mulch may not burst into flame at the touch of a cigarette, but it can and does smolder, and fire can creep along under mulch beds and erupt long after the initial ignition as humidity drops and wind picks up,” she said.