Mulch and Wildfire: Bark, Rubber, Rock and What to Use Near a House

Does mulch really catch fire from embers?

Yes. MSU Extension's MontGuide on fire-resistant plants for Montana warns that sparks and firebrands from a wildfire can ignite bark mulch and endanger a home, and advises keeping wood mulch moist during a fire in your area or replacing it with rock mulch. MSU's later WUI landscaping MontGuide repeats the point: embers can ignite mulch, so if mulch is used it should be kept moist, or replaced with rocks as a preventive measure.

The reason is location. Embers are the main way homes ignite in wildfires, according to NFPA, and they collect where wind eddies drop them: against walls, in corners and along the base of the house. That is exactly where most mulch beds are. Our guide to how homes ignite explains the ember mechanism in more detail.

Which mulches burn worst?

The clearest test comes from Carson City, Nevada. In 2008 the Carson City Fire Department, the Nevada Tahoe Conservation District and the University of Nevada and University of California cooperative extensions set out eight mulch treatments in 8-foot plots, let them weather outdoors for 79 days, then ignited them on a hot, dry August afternoon with fans blowing about 10 to 15 miles per hour. The results were published as The Combustibility of Landscape Mulches by Stephen Quarles and Ed Smith.

All eight mulches burned. Based on combined flame height, rate of spread and temperature, shredded rubber, pine needles and shredded western red cedar showed the most hazardous fire behavior. Shredded rubber burned hottest, more than 630 degrees Fahrenheit measured 4 inches above the bed, with flame heights averaging over 3 feet. Pine needles came second overall. Shredded cedar (the stringy product often sold as gorilla hair) spread fastest, at an average of 47.9 feet per minute, and threw embers that ignited neighboring plots.

Which mulches burned least?

Composted wood chips showed the least hazardous behavior. They mostly smoldered rather than flamed and spread fire at less than 0.3 feet per minute. A single thin layer of wood chips also did well. Medium pine bark nuggets, a common bed mulch, fell in the middle: moderate flame height and temperature, with a low rate of spread.

The authors attach two warnings to the good results. Smoldering compost can still ignite siding or debris it touches, and because smoldering is hard to see it may go unnoticed by firefighters. And a spray-on fire retardant tested on wood chips only delayed fire spread for about five to ten minutes, after which the chips burned like untreated chips. Treated mulch is still combustible mulch.

What should go within 5 feet of the house?

No combustible mulch. The Carson City authors concluded that because every mulch tested was combustible, none is recommended within 5 feet of the house and other structures, and recommended rock, gravel, concrete and pavers there instead. The IBHS Wildfire Prepared Home checklist is explicit: in the first 5 feet, remove combustible groundcover such as wood or rubber mulch, pine needles, artificial turf and exposed weed cloth, and use only noncombustible materials such as gravel, pavers, river rock, decomposed granite, stepping stones or concrete.

Note the weed cloth. Landscape fabric showing at the edges of a gravel bed is itself a fuel, so a rock band needs the fabric buried completely or left out. Our zone 0 guide covers the rest of that first 5 feet, and our guide to patios, paths and rock as firebreaks covers rock and paver choices.

What about the pine needles that fall into beds on their own?

On a lot with ponderosa pine, the mulch problem refills itself every year. The Colorado State Forest Service's Home Ignition Zone guide notes that long pine needles do not compact as readily as shorter needles and that fire in them can spread rapidly, especially in wind, which matches the Carson City result that ranked pine needles second worst of the eight mulches tested.

CSFS's checklist says to rake and dispose of pine needles, dead leaves, mulch and other organic debris within 5 feet of all decks and structures, and adds that farther than 5 feet from structures, raking will not significantly reduce the likelihood of ignition and can harm trees. So the needle job is concentrated: the rock band against the house, the deck edges, the roof and the gutters (our roof and gutter guide covers that part), done more than once a year on a pine lot, rather than raking the whole property bare.

Gravel and rock make that job easier than bark does, because needles sit on top of rock and blow or rake off it, while in bark they knit into the mulch and become part of it. Missoula County's Home Upgrade Hub gives Missoula homeowners the same direction: replace wood mulch with noncombustible options such as crushed stone or gravel.

Can I still use bark mulch farther from the house?

Yes, with limits. For the area 5 to 30 feet from the house, the Carson City study rated medium pine bark nuggets, wood chips and composted wood chips as the better choices, and said to avoid using them in a widespread or continuous way: separate mulched beds with noncombustible or ignition-resistant breaks such as concrete, gravel, rock or lawn. The Colorado State Forest Service gives similar advice for chips, recommending gaps of at least 3 feet between chip accumulations, or breaking the layer with nonflammable material.

Beyond 30 feet, the study says the most hazardous materials (shredded rubber, pine needles and shredded cedar) can be used. On most western Montana lots that means pine needles can stay where they fall in the outer zone, as long as they are not piled against a tree trunk, fence or outbuilding.

Does watering mulch make it safe?

It helps, but the study's authors caution against relying on it. Irrigating wood and bark mulch in a flower bed may make it harder to ignite, but water supply and pressure may be limited during a wildfire, hot windy weather dries mulch quickly between cycles, and drip irrigation typically does not wet the whole bed, so dry patches remain. MSU's advice to keep wood mulch moist is good practice in a fire season; it is not a substitute for keeping mulch out of the first 5 feet.

What about wood chips from my own tree work?

Chips from thinning and pruning are useful, but where they go matters. The Colorado State Forest Service's Home Ignition Zone checklist says any accumulation of slash, chipped or otherwise, should be 30 feet or more from the home, and that continuous areas of chips should be avoided. On our own fire mitigation jobs, chips that stay on the property go out away from the building in a thin, broken layer; they are never banked against the house.

Our guides to chipping versus hauling slash and to firewood and wood chips after tree removal cover what else can be done with the material.

How do you switch a bed from bark to rock without hurting the plants?

Start by deciding whether the plants should stay at all. If the bed is within 5 feet of the house, the IBHS checklist calls for removing the vegetation as well, so the change is usually a new rock band with the planting moved outward. If the bed is farther out, the plants can stay and only the mulch changes.

Rake the bark out completely, including the broken-down layer at the bottom, and haul it away or move it well out from the house. Bury or remove any exposed fabric. Lay rock or gravel at a depth that suppresses weeds, keeping it a few inches back from the base of any shrub stem. Rock holds heat in summer, so check soil moisture on the sunny side of the house for the first season and adjust drip emitters if plants show stress. MSU's plant MontGuide notes that wood mulch around the base of plants moderates temperature; that trade-off is the reason to move tender plants out of the hottest rock strip rather than leaving them in it.

What about rubber mulch around play areas?

The Carson City test found shredded rubber to be the most hazardous mulch overall and recommended it only for areas more than 30 feet from the house. If a play area sits closer than that, it is worth talking to the equipment maker about alternatives. That is a safety-surfacing question we do not answer; we can tell you where on the lot a play area would sit outside the zones that matter most.

Mulch is one of the cheapest and fastest changes in the whole Montana Wildfire & Defensible Space Guide, and it is often the first thing we do on a property. HJ Property Care & Tree Service handles bed conversions, rock and gravel installation and the landscaping and design work around them across Missoula and the Bitterroot Valley. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

More resources