But the combination of moisture and heat can also cause mulch fires, according to Capt. Jim Hailey, fire division commander for the Sikeston Department of Public Safety.
Several mulch fires were extinguished over the last couple of weeks, Hailey confirmed, although "not any more than normal. We're having just about the same number we usually have. We usually have some mulch fires -- especially when you have humid weather and then it gets hot."
He explained the conditions that cause mulch to spontaneously combust is mulch that is dry on top and wet underneath. "The natural decaying process of the wood is speeded up by this. Then the weight of the mulch will cause it to combust," Hailey said. "It has been wet and hot, and that will do it."
This is the time of the year when a lot of new mulch is being put down, and new mulch is typically wetter to start with, he said. Hailey said he has noticed when purchasing mulch from a big pile that "whenever you get a big scoop of it will be smoking like crazy."
Mulch fires can and do happen year round, however.
Hailey recalled one mulch fire that ignited in the middle of winter at a business that stores and bags bulk mulch to wholesale to retailers. That fire took DPS firefighters a day and a half to completely put out.
Extinguishing mulch fires is "labor intensive," according to Hailey. "You have to keep the water on it, let it soak down into it."
He added that companies that store and bag mulch are usually "very diligent" about making sure mulch doesn't sit long enough to spontaneously combust. In this instance, normal operations were temporarily disrupted so "it sat there too long, caught on fire," Hailey said. "They weren't able to mix it and get it out fast enough and it caused it to combust."
Mulch around homes usually isn't at risk for spontaneously combusting, Hailey said, as shallow mulch beds usually stay dry and don't have the pressure from the weight of the mulch above.
"It has to be deep to catch on fire," he said. "What you normally put in your yard, three or four inches, is not enough to cause it. It has to be fairly deep."
Nearly all the mulch fire calls DPS responds to have been at the city's Sports Complex and parks where "they lay it deep enough for people to play on," Hailey said.