Flying flames apparently take a likin’ to some mulches a lot more than others. For those that are interested, a 2007 study(1) on ignition rates and flame heights came to these love affair conclusions and some recommended physical separation from flammable structures:
- Love at first landing: Straw and pine needles caught fire the fastest – less than five seconds. Keep at least 15 feet away.
- Totally infatuated: Wood chips and bark nuggets had few fire-proofing characteristics; 15–30 feet separation.
- Can be dangerously flirtatious – keep several feet away.
- Green, closely-mowed sod can provide excellent fire-proofing. However, when grown more than four inches or dry, it becomes as flammable as pine needles and wheat straw.
- Dense, finely ground/screened materials such as garden compost and shredded bark had strong fire-proofing characteristics, however, with enough time could possibly cause other materials to ignite.
- But, flames can’t stand inorganic mulches! Decomposed granite, gravel and rocks are the motherlode for superior fire-proofing, especially for cozying up to flammable structures. Only concern is regularly removing flammable, windblown debris.
Since this was an Arizona study, we asked local landscaping expert Greg Rubin (2) for his opinion. Here’s what he said:
“These results seem very consistent with our experience and measurements. Except that when the mulch is consolidated with overhead watering (within months) or naturally (years), the flame height drops to around ~2″ (consolidation limits oxygenation). A local fire marshall ran ignition tests on our gorilla hair and came back asking, ‘What kind of fire retardant are you putting in this stuff?’. Of course, we can never guarantee a yard or home won’t burn in a firestorm, but at least these results so far have been pretty good.”
We’d never heard of gorilla hair – maybe you all are familiar with it. For those who aren’t, it’s finely-shredded redwood and western cedar tree bark, that looks remarkably like the backs of Jane Goodall’s best friends.(3)
Let’s hope none of us ever have to deal with any romance between flying flames and our mulch!
- Check out the full study here: https://www.firesafemarin.org/images/documents/resources/az1440.pdf
- Mr. Rubin, the 2018 San Diego Horticulturist of the Year, recently completed a five-year Navy research project on fire-resistant native landscapes. He has published two popular books on California native landscaping and his company has installed over 700+ landscapes.