The Fire-Resistant Benefits of using Compost as Mulch in your Landscape

compost as mulch

Compost as mulch – it won’t catch fire.

By Vesta Copestakes

Back when I was a professional gardener, I had clients who insisted on having me lay down landscape fabric then putting bark on top of it to discourage weeds. I hated this practice and tried to talk them out of it. Weeds come anyway because seeds find any crevice to tuck themselves into and bark has plenty of crevices.

But the worse part is that once you have landscape fabric with bark on top of soil, you are forced to fertilize with chemicals instead of rich, organic matter. This is not conducive to soil full of worms. Birds will not forage and scratch at the surface to find tasty insect treats. It’s simply doesn’t do what you want it to do and hinders a healthy garden.

BUT – I learned something else last October after the fires when I attended a fire summary put on by our local fire departments and CalFire.

BARK catches fire and flies in the wind as embers. When it falls it starts another fire.

When I bring this up people say, OK – that’s the big chunks of Redwood bark. Redwood doesn’t catch fire. Excuse me. EVERYTHING catches fire if it’s hot enough! Well, then, what about Redwood shredded bark? Does it have air in that fluff? YES. Is it dry? YES. Then it’s flammable and capable of flying in wind. Heat from fire will carry it up to a brand new location where – if it lands on dry leaves…dry grass…anything easily flammable – poof…fire!

What about Wood Chip for paths? Is it WOOD? Then it’s capable of catching fire and flying. If you want water permeable path material – use gravel. We have plenty of local quarries who carry fine gravel in colors. Take your pick.

Compost decomposed

Compost is thoroughly decomposed

Compost? NO – it won’t catch fire, In fact, it’s more like soil that way. It’s decomposed, tends to be moist, compact with tiny air pockets but dense enough it won’t catch fire. If you are worried – water your garden, It will hold the moisture for a long time.

You want something to cover your garden beds that is ALL GOOD and NO BAD? Then use COMPOST for mulch.

One warning – if you get it out of a bag and it’s cool, then lay it on without hesitation. If you get it in bulk and you can feel the heat on it? Let it cool in your yard for a while, or when you lay it down, leave at least 6 inches between the compost and plants and water it in.

Local garden centers have lots of different kinds of compost, some with animal manure, some without. If you get it with manure (usually higher in nutrients), make sure is free of any residue from weed killers like glyphosate or clopyralids. They are broadleaf herbicides that have a long life (and yes they make it all the way through an animal’s digestive system and survive the high heat of composting) and can impact your plants.

The benefits of compost as mulch not only increase organic matter and nutrients in your soil, it also protect your home, and your neighbor’s homes, from fire. And when winter rains start, the rain leaches those nutrients into the soil while it protects soil from the pounding of raindrops falling thousands of feet from the sky to hit like tiny bombs on your soil surface.

Ah – the MANY benefits of COMPOST!

Here’s a VERY interesting experiment on the Combustibility of Landscape Mulches with plenty of advice on what not to use and how and where to use mulches. here’s  just one chart from the tests of common garden mulches and their combustibility factors:

Key Findings: presents the evaluation results and the key findings are described below.

– All of the mulches evaluated were combustible under the test conditions of dry, hot and windy weather and more than 2½ months of outdoor exposure.

– The mulch treatments varied considerably in terms of flame height, speed at which fire spread and temperature measured above the mulch bed.With the exception of the composted wood chips, all of the mulch treatments demonstrated active flaming combustion. Composted wood chips produced only incidental flaming with smoldering as the primary form of combustion. It is not known if the performance of the composted wood chips is specific to the Fertile Mulch product produced by Full Circle Compost, Inc. and evaluated in this project or if composted wood chips from other sources would perform in a similar manner

– Based on cumulative values for the three combustion characteristics, shredded rubber, pine needles and shredded western red cedar demonstrated the most hazardous fire behavior.

– Composted wood chips and Tahoe chips, single layer demonstrated the least hazardous fire behavior based on the factors measured in this evaluation.

– Shredded rubber mulch burned at the hottest average maximum temperature (in excess of 630° F measured at four inches above the mulch bed) and produced the greatest flame heights which averaged over 3 feet. It ignited easily and burned intensely for a prolonged period (Figure 4).

– Pine needles were second only to shredded rubber mulch in terms of the cumulative value of combustion characteristics.

Mulch treatments evaluated

Mulch treatments evaluated

Fire potential of mulches tested

Fire potential of mulches tested

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