Landscape Mulches Prove To Be Hot Topic

By P.J. Heller

   Warnings are being sounded about the flammability and combustibility of various landscape mulches used in areas prone to wildfires, which could lead in the future to a rating system for the ground-covering products.
   “Unfortunately, some mulches are easily ignited, burn rapidly and represent a threat to homes,” says a study conducted by two researchers from Nevada and California.
   “There weren’t any huge surprises,” said Steve Quarels, a cooperative extension advisor at the University of California. “If you have a combustible mulch, it definitely has the potential to burn. We pretty much showed that any of combustible mulches will burn . . .”
   “Some of them [mulches] were a much greater threat than others,” added Ed Smith, a natural resource specialist with the University of Nevada-Reno Cooperative Extension.
   Smith and Quarels conducted the study in the wake of the devastating 2007 Angora wildfire in the South Lake Tahoe area. That fire burned more than 3,000 acres and destroyed 242 homes and 67 outbuildings.
   “Clearly, fewer houses would have burned had they had more effective defensible space, better access for firefighters and contained less flammable material,” noted a report that was published after the blaze.
   The findings by Smith and Quarel, however, were disputed by the Mulch and Soil Council, which contended there were major flaws in the way the study was conducted.
   “I am a little surprise that the demonstration worked so hard to disadvantage the mulch in the testing,” said Robert C. LaGasse, executive director of the council, describing it as “a purposefully exaggerated scenario.”
   Smith and Quarels set out to learn what role, if any, mulch played in spreading the fire under the extreme fire conditions.
   “What we attempted to do was to set up a project to better mimic extreme fire conditions in Nevada,” Smith explained.
   To do that, he and Quarels evaluated eight landscape mulch treatments — all except one at a depth of 2 to 3 inches and with one product treated with a spray-on fire retardant. Each 8-foot diameter mulch plot was replicated three times.
   The plots were set up in Carson City in May 2008, then allowed to dry out until mid-August, typically the height of the fire season in northern Nevada.
   At that time, they were ignited with a drip torch. Fans, used by the Carson City Fire Department to normally clear smoke from buildings, were turned on to simulate the windy conditions that help push wildfires.
   In addition to Smith, Quarels and officials from the fire department, others attending the test included representatives from the Nevada Tahoe Conservation District.
   The day of the actual test, weather conditions were similar to those encountered during a wildfire: temperatures hovering near 100 degrees, low relative humidity of 6 to 13 percent and winds of up to 10 mph.
   The study looked at the fire’s rate of spread, its temperature and the maximum flame height.
   Mulches being evaluated were “gorilla hair,” or shredded western red cedar; a shredded rubber produced from recycled material; medium pine and fire bark nuggets; a partially composted wood chip from a local Carson City company; pine needles, and “Tahoe chips,” primarily Jeffrey pine, white fir and manzanita wood chips, the by-product of fuel-reduction efforts in the Lake Tahoe Basin. The pine needles also came from the Lake Tahoe Basin while the other mulches were all available commercially.
   All the mulches were applied at a depth of 2 to 3 inches, with one plot of the Tahoe chips applied at just a single layer. One of the 2- to 3-inch layers of Tahoe chips was treated with the spray-on fire retardant.
   “They all burned,” Smith reported. “All of them are of concern if you have them immediately adjacent to a wood-sided house.”

LaGasse, however, contended that the study did not reflect real world conditions, with the plots allowed to dry out for months.

   “The big advantage of mulch is its water holding capacity around plants, but that most definitely does not mean you don't water plants for four months,” he said, adding that the study did not indicate what the moisture content was at the time of the test.
   “In a home garden situation, the moisture content should not be excessively low or the plants will die,” he said. “So it appears the test was based on a purposefully exaggerated scenario, and under such extreme conditions, any organic material — plants, lawns, etc. — is likely to combust given a sufficient ignition source.”
   Smith said some of the results — such as how easily the pine needles ignited, how fast they burned and the fact there was no residual — came as no surprise. Other findings, however, were unexpected and more vexing.
   “I think the rubber product generated the most concern,” Smith said, noting that the product was relatively new in the area.
   Not only did the rubber burn extremely hot — in excess of 1,000 degrees, Smith estimated — but it gave off clouds of black toxic smoke. The fire department had cordoned off the area downwind of the test site because of concerns about the smoke and toxicity of the smoke coming from the burning rubber.
   “What was amazing was the percent of consumption,” Smith said. “The intensity off of that rubber was incredible but it only burned the top half inch of the 2- to 3-inch depth of the rubber mulch.
   “It would have kept burning,” he added, comparing it to tire fire, which is hard to extinguish and long burning.
   The shredded rubber, which is resilient when walk upon, is popular around schools and playgrounds and is gaining in popularity for home use, coming in various colors and promoted as a “green” product since it is made from recycled materials.
   Another concern was the gorilla hair, or shredded red cedar, a popular mulch found in the Tahoe Basin. It not only burned once, but twice.
   When it was first ignited, Smith said the top layer of the material burned “straight across like flash paper, whoosh.
   “It probably had one of the fastest rates of spread, probably comparable to pine needles,” he said. “Then it came back and burned the rest. There was 100 percent consumption.”
   Smith said the gorilla hair posed other problems for homeowners.
   “It’s very problematic from a fire standpoint because it’s very hard to remove pine needles from that type of mulch,” he explained. “You can’t rake on it without removing the mulch. It’s hard to separate the pine needles from the gorilla hair. It’s a high maintenance material and one that ignited very easily and burned very rapidly. That was quickly a poor choice.”
   Another issue arose with the medium pine bark, which Smith said was considered one of the better choices for organic mulch.
   “It burned very hot, like charcoal briquettes,” he said. The radiant heat coming off the pine bark nuggets was so intense that it shut down the fans, tripping switches on the fans designed to keep them from overheating.
   Even the Tahoe chips treated with a flame-retardant substance applied by a factory representative didn’t fare well. The retardant provided about a five- to 10-minute margin before the mulch burned similar to the untreated plot.
   “After five to 10 minutes, it took off and didn’t look any different with or without the retardant,” Smith said.
   “The big mystery was the partially composted mulch,” he noted, explaining that it produced little if any flaming combustion but was burning underneath. The top of the mulch gave little indication that it was actually burning.
   “You have this glowing combustion underneath and it moved very slowly,” Smith recalled. “It also consumed very little of the material; about 95 percent was still left after 20 minutes.”
   Although there was little sign of flame, the composted mulch burned very hot, with temperatures recorded at 500 degrees.
   “Out of all of them (mulches), that may be the most desirable,” Smith said. “However, that temperature is a concern if it was up against a wood fence or a wood post. It’s a little deceptive because you don’t know it’s burning.”
   As far as using mulch around a home, Smith said one of the most important factors was its placement.
   “It’s probably not a good idea to put combustible mulch next to wood-sided homes or to things you do not want to burn,” he said. “Does that mean they’re banned from the landscape? No. On the other hand, I wouldn’t want combustible mulches next to a wood-sided home. You have to use some common sense.”
   Quarels said he was still analyzing the data from the tests but expected that he and Smith would release their recommendations to the public about landscape mulch in the coming months. They also planned to publish a more detailed technical paper about their findings.
   “I think the whole point of the study was to provide guidance to homeowners and other jurisdictional officials” about the combustibility of mulch and how it can spread to a home versus its other chief attribute of erosion control, Quarels said.
   “Given that everything burned, we’re going to be looking at the relative differences in performance,” he said.
   Both men said their findings were applicable to other areas of the country where wildfires posed a threat to homes. They noted that similar tests have been conducted in Florida and Arizona. Some areas, such as California, have code requirements regarding “defensible spaces” that must be around homes in wooded areas.

Smith put the burden squarely on homeowners to lessen the fire threat by doing such things as building up defensible spaces around their homes, cleaning pine needles out of gutters before fire season, pruning trees, placing firewood at least 30 feet away from the home and putting mesh screening over outside vents.

“There’s no excuse for not doing the simple things,” he insisted. “Contrary to popular belief, the most important person in protecting your home from wildfires is you the homeowner, not the fire department. The things that you do before a fire ever occurs is what’s going to make the difference. There’s a misconception that it’s the fire department’s responsibility to protect the home, not the homeowner.

“I’m not talking about fighting the fire. I’m talking about creating defensible spaces, landscaping, screening their vents . . .” he added.

   Quarels and Smith also said that their study could lead to fire ratings for mulches, much like the fire ratings for roof materials.
   “I see that as a possibility,” Quarels said. “I think to the extent possible we would try to come up with something that works that’s useful and conservative. It may be that you have different zones around your house, the same way we talk about vegetation management around your house. You may end up with similar zones for mulch-type products. Near the house it may be more important to have more non-combustible or not-as-combustible material and as you get further away from the house, you can use some different types of products.”
   Smith agreed, noting that it was a bad idea to have a continuous layer of mulch stretching from wildlands to a home. Rather, he said the mulch needs to be broken up with non-combustible materials, such as a sidewalk or a lawn.
   “A lot of it depends on what your house is made of,” he said. “In a worst-case scenario — a wood-shake sided house — I wouldn’t put any organic mulch around the base of my home.”
   Quarels agreed that it was up to homeowners to be responsible for keeping their homes safe from wildfires and that the mulch study was just one part of helping them accomplish that.
   “It’s not going to be any one thing that’s going to do the trick,” he said. “It’s going to be paying attention to a number of things. It’s not a simple solution but we understand the ways that homes ignite. This is one more part to add into the home survival kit.”