The right mulch could help save your house and garden

Summer is prime fire season in California. It’s also when our gardens really benefit from moisture-saving mulch.

Use the right mulch in the right place – including fire-prone landscapes. What you put on the ground makes a big difference for your plants as well as fire retardation.

What’s the best choice for maintaining moisture and fire safety, too?

– Start with composted mulch. Its relatively high moisture and nutritional content benefit soil, but it’s less flammable than other organic mulches.

“Research indicates that composted mulch is less flammable,” explains Bruno Pitton, UC Cooperative Extension Environmental Horticulture Advisor for Placer and Nevada counties. “The inference is that composted mulch has less carbon to burn and, with a smaller particle size, there’s less oxygen (to feed flames).”

In addition, fine-particle, moist mulch can act as a buffer to embers. According to Cal Fire, ember ignition is the leading cause of home loss during wildfires, with an estimated 90% of homes destroyed indirectly by wind-borne embers. That’s true in fire-prone areas that border wildlands as well as suburban wildfires.

“Composted mulch is less likely to ignite,” says Pitton. “When it does, it smolders; it doesn’t flame.”

Organic mulch – such as wood chips, dried leaves, straw or compost – helps soil retain moisture as well as suppresses weeds. It naturally regulates soil temperature, keeping plant roots comfortable, while slowly adding nutrients.

In fire-prone areas, taking that extra step to compost the mulch before applying it around plants can be the difference between fueling a blaze and possibly stopping one.

– Size matters; smaller is better. Some commercial mulches such as shredded redwood bark or “gorilla hair” can be problematic; it burns easily when fresh. Choose smaller wood chips (not shreds) instead of large pieces of bark.

“All plants are going to burn if it gets hot enough,” Pitton explains. “The main thing that lowers flammability is moisture content.”

That goes for plants as well as mulch. “Flat thin leaves are more flammable; they dry out really fast,” he adds. “And dried leaves are really flammable. So are pine needles. Think about a campfire – what do you use for kindling?”

Pine needles aren’t recommended for composting; they’re slow to break down. But dried leaves make excellent compost fodder, Pitton notes. “Instead of using your dried leaves for mulch, compost them. Then, use the compost for mulch.”

Small bark chips offer positives for fire-resistant mulch; the qualities that help bark protect evergreen trees also makes bark a good mulch. Bark chips hold more water than dried leaves, etc., while also containing hydrophobic compounds that are resistant to water penetration.

“They break down slowly,” Pitton says. “Use the fine size; it has a lower risk of ignition than coarse bark. Composted bark would be OK, too.”

Why small? “Large particles hold a lot of oxygen between them. Coarse bark has plenty of room for embers to catch. And worse, they can become flaming.”

– Mixed chips work, just not too thick. Arborist wood chips – a mix of wood varieties and sizes, chipped from downed or trimmed neighborhood trees – offer the positives of other organic mulches. (That’s the kind of wood chips available from local tree trimmers.)

Pitton has experimented with arborist wood chip mulch in his own home landscape and found that regular irrigation greatly speeds its break down. In his experiment, arborist wood chips decomposed 16% in 100 days. By comparison, nature’s wood mulch on the forest floor decomposes only 10% in two years.

Cal Fire recommends keeping wood chips or other mulch no more than 3 inches deep. “If (the mulch) does catch, there’s not too much fuel,” Pitton says.

– Composted arborist wood chips are best. That’s what Pitton recommends and uses. “You can compost the chips yourself and apply it yourself. In summer, give it a little irrigation just like a compost pile (to keep it from drying out).”

When applying mulch, allow space (6 inches to 2 feet) around the trunks of trees or shrubs. Keeping mulch away from trunks not only helps prevent crown rot, but offers it some extra fire protection, just in case. “Even if the mulch only smolders, it’s hot enough to kill the plant.”

– Use hardscape within 5 feet of your house or other buildings. As part of Cal Fire’s recommendations for defensible space, use gravel, rock, pavers, concrete or other hardscape around structures – no bark or organic mulches.