Can Mulch Catch Fire?

Date:9/4/2018
Author:Terry B.
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Photo by Tirza Van Dijk on Unsplash

How easily can your mulch catch fire, and do you need to be concerned? This may not be a worry if your gardens get regular watering, but it most definitely is if you live in a hot, dry climate. Or you use a fire pit, or people smoke outside your home.

In 2010, the Massachusetts Department of Fire Services reported that over a 5-year period, 184 fires "began with mulch and spread to buildings."

How easily can mulch ignite?

A 2003 study in the Journal of Arboriculture looked at several types of common mulches and how quickly cigarettes, matches, and a propane torch could cause them to catch fire. Mulch materials were shredded pine bark, shredded hardwood, shredded cypress, composted yard waste, pine bark nuggets, pine needles, ground recycled pallets, cocoa bean shells, oat straw, grass sod (probably to replicate lawn), brick chips and ground rubber tires.

Cigarette test: The researchers found that tossed cigarettes most often ignited mulch made from ground pallets and (surprisingly) composted yard waste—probably because the cigarettes smoldered in the compost. These mulches didn’t ignite: pine needles, ground rubber, shredded hardwood bark, large pine bark nuggets, cocoa shells, and brick chips.

The match tests weren't statistically significant and weren't reported.

Propane torch test: Unsurprisingly, a propane torch ignited all mulches except sod and brick. The quickest to flame were ground rubber, pine needles, and straw. Least likely to burn: cocoa shells and compost. Hardwood and pine bark mulches fell in the middle.

When the torch ignited ground rubber, "flames often spread rapidly and were extinguished with difficulty." For this reason, the study's authors were not in favor of using ground rubber on playgrounds.

Overall, the most fire-resistant mulches were brick chips (which never ignited), followed by cocoa shells, sod, large pine bark nuggets, and shredded hardwood. The most flammable was rubber mulch. 

Drought test: A 2007 mulch flammability study compared shredded cypress, large and small pine bark nuggets and pine needles in simulated drought conditions. Researchers ignited the mulches using a burning gasoline mixture. In this situation, pine needles ignited the fastest and both pine needles and large pine bark nuggets burned faster. Of course, most things will ignite if exposed to burning gas, but if you live in an area where wildfires are a concern, you might want to avoid pine straw and large bark nuggets.

These studies confirm that many types of mulch can catch fire if conditions are right. So, what's a concerned gardener to do? It's okay to use mulch. Just follow these guidelines:

  • If you or your visitors smoke, don't toss cigarette butts into mulch (or on the ground, for that matter)
  • Keep fire pits, outdoor candles, and fireworks well away from mulched garden beds
  • Water beds regularly, especially during a drought
  • Use a stone mulch if you live in a hot, dry climate
  • Consider using a living mulch
  • Avoid rubber mulches
  • Use stone or nonflammable material next to your house

 

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