Millions of bags of it are sold at home and garden stores nationwide, but firefighters said a porch fire was sparked by a common household product: potting soil.

Firefighters said a fire on Washington Avenue in Wheeling last month started because a potted plant became too hot, and the blaze caused much damage to the house's porch.

Though it is rare for potting soil to catch fire, officials said it can ignite with the right combination of high humidity, extreme heat and dry soil.

Wheeling Fire Department's assistant chief said the mixture of the soil, shredded bark, peat moss and Styrofoam balls can be flammable in those conditions.

"Back to the old rule of spontaneous combustion: The organic and inorganic material that is put in potting soil now combined with some fertilizers that some soils have. And fertilizer is an oxidizer, which would intensify spontaneous combustion," said Assistant Chief Ed Geisel.

There are no fire hazard warning labels on potting soil bags, but firefighters said it's happened before, and it could happen again.

"I've been on (the fire department force) 33 years and within the past four to five years I've seen more instances," Geisel said. "Most of them are small fires. They are contained really quickly because homeowners or passerby-ers see it, but this particular one got a little further along before anyone noticed or we were able to get there."

Geisel said the best way to prevent a fire is to simply keep potted plants hydrated.

"That's one of the main things for safety factors is, if you do have potted plants, make sure you water them and there is plenty of moisture and (if there's) any dead vegetation, pull it and clean it," Geisel said.

The soil could also ignite when people put cigarette butts out in potted plants, he said. Even though that wasn't the case in the Wheeling fire, Geisel said most similar fires nationwide were caused by people using their potted plants as ashtrays.