A house fire in Seattle’s Phinney Ridge neighborhood on Saturday is the third this month triggered spontaneously. Oily rags and compost have been the culprits.

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As if Western Washington residents unaccustomed to sunshine and hot weather didn’t have enough to deal with — sunglasses? sunscreen? water bottle? — add another hazard to the list: spontaneous combustion.

Seattle Fire investigators say a Saturday blaze that displaced seven people from a three-story complex on Phinney Ridge was sparked by compost, soil and vegetation in a flower pot.

While oily or solvent-soaked rags are the usual culprits in fires that start on their own, organic matter like compost and beauty bark can also smolder spontaneously, Seattle Fire Department spokeswoman Kellie Randall said in an email.

And dry, hot weather elevates the risk for spontaneous combustion fires of all types, she added.

No one was injured in the Phinney Ridge fire, which was reported shortly before 7 p.m. But residents of the multifamily structure were at least temporarily driven out and total damage was estimated at $60,000.

Spontaneous combustion occurs when a material generates enough heat through chemical or biological reactions to reach its ignition temperature, without being exposed to an external source of heat or fire.

Fires that spark spontaneously in organic material, like grass clippings, compost and mulch are usually initiated by heat from microbial processes, like decomposition, says a 2008 article in the journal BioCycle. But as the heat builds, the microbes are killed and chemical processes, like oxidation take over. Moisture can accelerate the process.

Saturday’s fire is the third in Seattle this month attributed to spontaneous combustion, Randall pointed out.

A residential fire in the Maple Leaf neighborhood Thursday that caused $310,000 in damage to two houses started in a pile of rags used for oil staining. The trigger was the same for the fire that struck the Salmon Bay Marina and caused $4 million in damage.

The National Fire Protection Association estimates that spontaneous combustion is responsible for more than 14,000 fires in the U.S. every year. More than a third are caused by oil rags. More than a quarter were traced to organic material, like vegetation or compost.

The Seattle Fire Department offers several general tips for residents who want to protect their property:

• Clear leaves, debris and dead vegetation from roofs, gutters, porches and decks.

• Keep wood piles and propane tanks away from structures.

• Keep vegetation, garden beds and potted plants watered.

• Keep lawn watered, or if it is brown, keep it cut short.

Sunny, May weather was also implicated in one of the region’s more unusual home fires in recent history. In 2009, fire investigators in Bellevue blamed a backyard blaze on a dog’s glass water bowl.

As the sun’s rays passed through the water-filled bowl, they were refracted and concentrated — as if by a magnifying glass. The intense beam bored into the wooden deck and started a fire that spread to the kitchen.