From car crashes to spontaneous combustion: 5 surprising ways wildfires start

From car crashes to spontaneous combustion, some wildfire causes are stranger than they sound.

A single-car crash in the hills above Burbank, California, may have ignited a 40-acre wildfire Saturday, prompting evacuation warnings, closing hiking trails and canceling a concert before firefighters stopped the flames from spreading.

Fire officials said the vehicle crashed into brush near Wildwood Canyon, where the Walnut Fire quickly spread across the hillside. No buildings burned, and evacuation warnings were lifted later that evening.

It sounds like a freak accident, but ordinary human activity starts most wildfires in the United States. About 85% of wildfires are caused by people each year, according to the National Interagency Fire Center, and most are preventable.

“Humans are incredibly predictable,” Cal Fire Battalion Chief David Acuna told CalMatters. “They drag chains and they leave campfires to burn or they have a flat tire but think they can just make it to the next exit.”

Here are five surprisingly easy ways a wildfire can begin.

Cars, flat tires and dragging metal

The Walnut Fire is only the latest example of a vehicle problem potentially turning into a wildfire. In 2024, a driver traveled more than 4 miles through Sonoma County, California, without a front tire. The metal wheel scraping against the pavement threw sparks into dry vegetation and ignited the Flora Fire, investigators said.

A flat tire also started the devastating Carr Fire near Redding, California, in 2018. The rim of a trailer wheel scraped along the road, throwing sparks into dry vegetation. The fire eventually burned nearly 230,000 acres and killed eight people.

Dragging tow chains can create the same dangerous shower of sparks.

Lawnmowers

Even mowing the lawn can start a wildfire under the wrong conditions.

Investigators determined that improper use of a lawnmower in dry grass sparked California’s French Fire on July 4, 2024. The fire burned 908 acres near Mariposa.

Metal mower blades can strike rocks and create sparks, which is why fire officials warn against using lawnmowers on dry weeds or grass during dangerous fire conditions.

Animals

Sometimes the ignition source isn’t human at all.

A small wildfire in Colorado’s Waterton Canyon in 2022 was traced to rodents that chewed wiring inside an electrical box. The damaged equipment shorted out and sparked, igniting dry grass below, according to West Metro Fire Rescue.

Rodents damaging electrical equipment are one example of how wildlife and power infrastructure can combine to create an unexpected fire risk.

Gender reveal parties

Two of the country’s most notorious human-caused wildfires began with attempts to announce a baby’s sex.

A smoke-generating pyrotechnic device used during a 2020 gender reveal ignited dry grass in Southern California and started the El Dorado Fire. The blaze killed firefighter Charles Morton and burned more than 22,000 acres.

Three years earlier, an off-duty Border Patrol agent in Arizona fired a rifle at a target packed with explosive Tannerite during another gender reveal. The explosion sparked the Sawmill Fire, which burned more than 45,000 acres and caused more than $8 million in damage.

Hay and spontaneous combustion

Fire doesn’t always need an outside spark.

Hay that is baled while too wet can generate heat as microorganisms break down the plant material. Because tightly packed hay traps that heat, temperatures inside a bale can continue climbing until fire becomes possible.

Below 130 F, hay carries little fire risk. By about 150 F, the risk becomes moderate, and between 175 and 190 F, fire may be imminent, according to the University of Minnesota Extension.

The strange ignition sources all have something in common: under dry conditions, it can take very little heat or a single spark to start a wildfire.

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