"The fire was caused by self-heating through decomposition of organic materials contained within a plastic flowerpot," the Aug. 25 letter from State Farm Insurance Co. said.
Or, in layman's terms, spontaneous combustion.
Duncan, whose home is a few miles south of Paragould, said the flowerpot had contained dead, decomposing flowers and potting soil that his wife had planted in the summer of 2009. Paragould is about 150 miles northeast of Little Rock.
"She had intended on repotting (the flowers)," Duncan said. But they sat on the porch, unwatered, and eventually died.
He said it was clear where the July 25 fire had begun, because the burning flowerpot and plants charred a hole in the porch and they fell to the ground several feet below.
Still, Duncan said he was surprised at the conclusion contained in the letter. Duncan provided The Associated Press with a copy.
Fortunately, no one was injured in the blaze and Duncan's father-in-law was able to put it out with a garden hose even before firefighters from a nearby volunteer fire department arrived.
But it still caused some damage.
Duncan, 51, CEO of Craighead Electric Cooperative, said the blaze charred decking around the hole where the flowerpot had been, and caught the home's vinyl siding on fire. He said the heat broke a sidelight window next to the front door, and his air-conditioning system sucked in smoke from the fire.
"The house was full of smoke," he said.
The smoke damage inside the 15-year-old home, Duncan said, meant his family had to repaint the entire interior of the 2,200-square-foot home and replace the carpeting, in addition to replacing the vinyl siding on the front of the house and the wooden decking of the porch.
Duncan said that, since the fire, he had begun spreading the word about the potential fire hazards of dead plants.
A fire marshal in nearby Jonesboro, Jason Wills, said such an occurrence was rare.
"Spontaneous combustion is something where you have to have a lot of variables come together and it has to be just right," Wills told Jonesboro television station KAIT. "It's something that does happen, but this is the first one in our area that I'm aware of."