That is a very common cause of fires. Many people assume that its OK to stub out a cigarette (or match) in a planter pot because its dirt. In fact, most potting soils are peat based or other organic material - stuff that burns easily when dry.

Yes, the apartment fire in south Calgary last month was started in a planter pot. They can spontaneously combust, but that is less common since the media actually needs to be moist for that to happen. The heat is gernerated by the decay process, much like compost, then as the heat builds, it dries out parts of the media and starts it smoldering. It is hard to create the heat needed to start combustion that way, typically it is a discarded cigarette that gets them going.

I saw the same type of thing when I was living in an apartment downtown Calgary. I was on the 23rd floor, and somebody put out a cigarette in a planter box on the balcony above me. Every single planter box on the entire balcony caught on fire - good thing the balcony was concrete. Nothing like loading up your animals and hitting the stairs for a 23 floor exercise session at two in the morning!