Wednesday, May 09, 2012

Mulch - is it really a fire risk?

I've just spent a lovely autumn morning with my girls spreading wheelbarrow, after wheelbarrow, of decayed tree-lopper mulch around the large established trees and shrubs in our front garden.  We also took time to remulch the areas around newer plantings and generally enjoy being in our bush garden.

Whilst doing this, a tradie was working on a new car parking area next door and my girls couldn't resist in engaging him in conversation.  They frankly never stop talking, so I'm glad they found a new target for their endless questions!

The tradie had watched me carting wheelbarrows full of mulch around for about 30 minutes before he said: "You do realise mulch is a fire risk here?"


I've now lost count of the number of times I've been told that mulching in this area will inevitably lead to my property being engulfed in flames and endangering the entirety of the Ranges.

When I question these people, they usually look confused and mutter something about of course it's a fire risk, it's dry wood right... and it's got gum oil in it... and what about the termites?  None of them are ever able to recall a time when they've been aware of a bush fire starting in someones mulched garden bed.

I do recall a fire that allegedly started in someones mulch bed.  I say allegedly, as the mulch was only 4-6 inches (10-15 cm) deep and it's considered near impossible for such a thin layer of mulch to spontaneously go on fire, plus the home owner did not report any fire in his mulch - he reported only a smoke filled garage and then flames bursting out of it's roof.  It was the fire-brigade who later, after finding no obvious signs of ignition, determined it was likely a spontaneous mulch fire.

Now, in the US it's more common to use shredded mulch (pine bark, cypress or hardwood) in garden beds, rather than the nuggets that are often preferred here in Australia.  This is rarely fresh and green, having done most of it's hot composting in the supply yard.  However it is dry and fairly flammable.  So much so, that mulch fires are common in various US counties towards the end of hot, dry Summers, or in dryer than normal Autumns.  It is also not unusual for fires around commercial buildings to break out due to carelessly discarded cigarettes etc.

All that said, the original article from 2007 prompted quite a lot discussion in the local gardening community at the time.  Here's an article from one of the same newspaper's feature bloggers, where they'd contacted some organisations who could be considered to be authorities on mulch, to get their viewpoint.

Interesting that the Morton Arboretum staff (who do some good work on trees in urban landscapes and have some interesting soil science research projects, including compost teas which is worth a look), were described as being "highly  sceptical" that a 4-6 inch layer of mulch could spontaneously catch fire.

Personally, I've had a large 3m high pile of green tree-lopper mulch sit on my drive, between my garage and house, for several months whilst I slowly worked through it.  In fact, the pile was so big I still have some left which has almost fully composted and is making a great soil amendment.

Anyway, this pile of mulch was in pretty much the same environmental conditions as the thin layer of mulch in the frame in the Crystal Lake fire.  The heat generated as it decomposed was so great it felt pretty hot to the touch when you broke it open and some pieces inside did become blackened, but that was more due to decomposition than scorching.  My cats liked the heat and would frequently snuggle on it over the cooler months.

My mulch was spread out over the garden throughout Autumn, Winter, Spring and Summer.  It is always at least 10cm deep and, in the fernery area, was in fact 20cm deep.  Despite some severely hot days and extreme fire risk days throughout summer, I have not once had any concerns about my much spontaneously bursting into flames.

Due to the difference in my experience and that of the obvious documented fires in the US, I did some research and came across this scientific report on the Ease of Ignition of Landscape Mulches.  In summary:

  • composted yard waste and recycled pallets are the most likely to ignite (50% of the time) when a lit cigarette is discarded on them.  Shredded pine bark would ignite a third of the time and shredded cypress bark or oat straw 25% of the time.  Pine bark nuggets, rubber, pine needles, shredded hardwood and coconut shells did not ignite at all, or ignited so rarely that the risk is negligible;
  • the match tests were done after the mulch was in place for 6 months, giving it time to settle in and dry out.  Pine needles, oat straw and rubber sometimes ignited when a lit match was dropped on them, but because the ignitions of each mulch were so random the study was not able to determine any statistical likelihood of one type of mulch being more likely to ignite in this manner than another;
  • using a blowtorch (bet the students loved this!), rubber, pine needles and oat straw all either smoldered or actually produced flames and had to be extinguished.  Shredded hardwood, cypress bark and pine bark, recycled pallets, pine bark nuggets, composted yard waste and coconut shells all took about 15 seconds of exposure before they produced flames, but the small fires were out after 60 seconds.  The pallets and pine bark nuggets did however produce embers at 30 seconds, which were out by 60 seconds, but as it took 15 seconds of applying a flame from a blowtorch for these to go on fire in the first place, I doubt these embers would be much risk.
Conclusion?  Coconut shells, pine bark nuggets in the 2.5-5cm range and shredded hardwood are the most fire resistant mulches under each test.  Shredded pine, cypress, rubber chips (very hard to put out!) and oat straw are the most likely to result in fire when an ignition source is supplied.

So, my preferred mulch of tree-lopper waste, whether fresh or matured, is the safest option, should someone I know wish to flick a cigarette, match or apply a blow torch... (that is not an offer to come and give it a go!).

Next thing to consider for this area I guess is the possibility of ember attack during a bush fire and the mulch igniting.  

The Dandenongs have an extreme bush fire risk because they are a heavily treed area, have minimal exit roads and of course, because fire loves to travel up hill.  We are told that if you have trees on or around your property, it is NOT defensible.

The use of mulch does not increase this risk to us and nor would it for anyone else in the area.

In fact, the use of tree-lopper mulch traps moisture even during the hottest, driest periods.  I know this for a fact because I've checked. :)

This means that, although there is a minimal chance of an ember missing the trees on my property etc and landing on the mulch, said ember would need at least 15 seconds to start smouldering and, due to the moisture content, would likely extinguish itself quickly.

I think we're pretty safe.  Well, from the mulch going on fire anyway!

As for the replying to the tradie's "You do realise mulch is a fire risk here?" question, I smiled at him and said "Really...?".

End of conversation. :)

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