Laura Christman: The combustibility of mulch sparks concern

When it comes to the wonders of mulch, gardeners like to lay it on thick. The protective layer of bark chips or other materials prevents weeds from popping up. It saves water by keeping the ground cooler and slowing evaporation. It adds organic matter to the soil. And it makes garden beds look tidy. Mulch is the pretty protector of the garden.

Then again, it might set your house on fire.

The University of California recently announced results of a study demonstrating organic mulch's darker side — a willingness to ignite. The study involved several mulches, most of which flamed readily and one that smoldered dangerously.

"I think our message is: You can still use mulch, but you need to be careful where you use it. Near the home you might be better off with something else," said Steve Quarles, a UC Cooperative Extenstion advisor who has researched wood performance with the Center for Fire Research and Outreach at UC Berkeley.

Perhaps his name is familiar. Quarles has given talks in Redding about making homes safer from wildfires. There's nothing like crackling flames to make a point, so Quarles often torches sections of siding and such during his presentations. It's not much of a surprise that he would be involved in a study that called for igniting mulch.

Quarles and University of Nevada Cooperative Extension natural resource specialist Ed Smith tested composted wood chips, pine bark nuggets, pine needles, shredded rubber, shredded cedar, biomass chips and biomass chips sprayed with a fire retardant. All the mulches in the study fell under the "organic mulch" label.

The mulches were left out in the summer sun in Carson City, Nev., for 12 weeks so they were nicely sunbaked, just like mulch would be in a Redding-area landscape.

When it came time to add fire, the composted wood chips smoldered and the rest of the mulches flamed. Some burned more eagerly than others. Fire spread the fastest in the shredded rubber, pine needles and shredded cedar.

The results showing the combustibility of mulch weren't unexpected, Quarles said. Fire best jumps to life when it has small pieces close together to feed on. That pretty much describes mulch.

"Think about how you might start a campfire or a fire in your home," Quarles said. "You start with small things and put big things on later."

But people don't think, as they are spreading mulch on garden beds, that they are surrounding their home with a layer of fire starter.

UC Cooperative Extenstion's mulch warning meshes with a message about wildfires that fire officials have been trying to get across to the general public: It's not a wall of flames that's the most likely danger, it's embers.

Flaming pieces of branches, siding, shingles and other debris get lifted into the sky in a wildfire. Driven by winds, they go long distances — sometimes more than a mile. If they land on something flammable, they spread the fire.

"If you have conditions around your house where embers can't ignite things, the home is that much more able to survive," Quarles said.

He'd like people who live in wildfire country to view bark and other organic mulch the same way they would a stack of firewood, wooden play structure or pile of lumber. Don't put it right next to the house.

Quarles recommends organic mulches not be used within five feet of a house in a wildfire-prone area. Nonorganic mulches, such as stones, gravel and brick chips, are a better choice in that location, he said.

That doesn't mean you have to swear off organic mulch completely. It just needs to be farther away from the house.

The goal with landscaping is to prevent a fire from easily moving to the home by jumping from one flammable plant or material to the next. There shouldn't be a continuous line of ignitable things leading to the home, Quarles explained.

He acknowledged that messages from fire officials, who like lawns and other moisture-loving plants near homes, can conflict with advice gardeners get about making home landscapes more water efficient and natural.

"There are conflicting issues Things that might be good from a moisture perspective may be completely different from making something more fire safe," he said.

There is room for interpretation. It's not a choice between a nice landscape and a moonscape. What's important, Quarles said, is to put plants in the right location, keep them hydrated and trim out dead branches or dry foliage.

"You need to space them accordingly and maintain them accordingly," he said.

Laura Christman's column runs every other Saturday in the Home and Garden section. Contact her at or 225-8222.