Every year, we get calls from restaurants and retailers (usually fast food restaurants) wanting to put a retardant on their landscape mulch. Because here's what happens - customers come in, eat, and the first thing they do when they leave is fire up a cigarette. Then they flick the still-smoldering butt into the landscape mulch that surrounds the building, and sometimes it starts a small fire.
We usually recommend Lumber Guard XT, a clear Class A retardant for exterior use. It is non-toxic and non-hazardous, with an excellent coverage rate. Just bag or wrap your plants to protect them, put the Lumber Guard XT in a standard bug sprayer, and soak down the mulch. You can take a garden rake to flip the mulch, and give it another spray. You don't have to get every inch of the mulch, and it will still keep it from spreading.
Of course, every time you put down fresh mulch, you will have to apply the retardant, but it doesn't take much time at all, and seriously reduces your fire risk. The Lumber Guard is water based, and has polymers that keep it from 'weathering out' and leaching into the plants.