Leaving mulch in mounds does it start fire

Another quick question here. I was talking to a neighbor today and she mentioned she sees this smoke coming out of mulch piles concerns her that might start fire. I am under the impression that the organic material is breaking down in that pile and that creates the heat, heat is coming out but when it is cold outside the smoke shows. we do always spread the mulch piles but I did keep some in the yard where neighbors can not see and the mulch breaks down much nicer.

Does the fire start in the piles? With hay stored in the barn, if there is moisture in the bales I heard it starts the fire.

Thank you.

you need a really giant pile to get enough heat to start a fire. generally what you see is steam, not smoke, and you won’t see it much during a hot day - it’s just not all that hot

Hello Jhansi!

It might look like smoke but likely all you are seeing is water vapor.  You should be able to tell by smelling it.


Your heat is going to come from nitrogen in your compost or mulch pile.   If you are getting a lot of water vapor coming off your pile you can add more carbon to the pile and cool it down.  

Nitrogen would be from manures, kitchen scraps or green vegetation.   These high nutrient items will cause the pile to get warm and break down quickly.   It will release water vapor and often a bit of a compost smell.   As the water vapor meets the cool air it will condense and become visible.   If this is bothering your neighbors, do it out of sight or add more carbon.

Carbon will come from wood, dead leaves or brown grass.   It will break down more slowly and keep your pile at lower temperatures.   You will have less loss of water vapor and less smell.  

Another excellent benefit of using more carbon and a cooler, slower compost pile is you will get more compost and more nutrients for your garden.

Update:

So neighbor complained to fire dept and he came to the property today. He wanted wood chips no thicker than 3 inches.
I had to scrape up bunch of chips and the bottom is so good very much composted

Is it even reasonable to think that the chips can be only 3 inches deep. Deep mulch gardening?

He wants me to disk in the mulch in the pasture.

Thank you

Disking the mulch into the topsoil would be the worst thing you could do. That's really poor advice. It seems that your fire officials are as badly informed as your neighbour. Without a source of nitrogen in the pile, wood chips rarely heat up beyond about 60 degrees...I have a couple of 3-5 m3 piles that have a high proportion of wet, starchy cordyline pith in them and they're about that hot.

A 3-inch layer is deep enough to do a lot of good, so if that is the necessary compromise that allows you to move forward, it's not so bad.

Was the 3 inch and disking a code/ordinance or just a recommendation?  I’ve had chip piles get hot to the touch, maybe 120-130F, but no where near kindling/auto ignition temps of 450F+

Three inches would be all right. Depending on the tenacity of some weeds, you might want to consider a smother layer underneath the chips. I would recommend against disking them in due to nitrogen tie-up.

I keep my wood chips like this. They steam for a few days after delivery and then settle down. I've felt like I was at risk of burning myself by plunging my hand into the pile and I've measured the temps above 140F/60C, but there's never been any risk of fire. I would not comply with the neighbor's request/demand.

He can recommend what ever he likes, but he can't enforce that 3" silliness.  I have piles of wood chips 7 or 8' deep and will let them sit that way for a couple years.  At 3", they make good mulch, but they will take forever to break down into the excellent soil they can be.  I would ignore their "advice".

We have piled hardwood mulch with no issues. I have heard something about pine straw (pine needles) falling out of favor because of flammability, which makes more sense. I do use some on my strawberries.

There was a recent fire in Asheville where piles in a mulch yard caught fire (started by machinery that caught fire initially). These piles were sitting there for many years, by the way. They did burn like crazy, for over a week, despite firefighting efforts and even rain. They would put the fire out, and then they would reignite. So once on fire, it looks to be a challenging one to control. But the piles didn't catch fire on their own.

Here is an article on Combustibility of landscape mulches

Agree with everyone that the "smoke" is water vapor.  I see that all the time on the side of the highway.

I have had mounds of wood chips 4 ft high without any issue.   If the suggestion is a problem, I suspect your community has public buildings and parks with deeper mulch on their grounds.

I have mounds of wood chips (river red gum eucalyptus, oak, cotton wood) around 1 meter high. In a dry hot summer aridity where just looking around thinking about fire can start one... Nothing happens.

I would get a compost thermometer and put in the pile. Look up the ignition temps of whatever wood you have, and it will likely show the chips are at least 200F/110C below that. I’d tell the misinformed firefighter if they are going to enforce that level of illogical risk intolerance, they should be checking every home for all-metal fire resistant fuel lockers and pulling cigarettes from mouths, stomping them out, and then placing in a metal can.

I have compiled small mountains of woodchips (100s of cubic yards) for years and have never seen them reach temps above 150F, and rarely above 135F. If they get hotter than 160F, some rich nitrogen source must have been mixed in. An outside strike source is always going to be a bigger risk than spontaneous combustion, so I’d tell the fire guys to go enforce laws and teach the public about cigarette safety, and read some articles on composting and mulching from people not selling plastic or dyed-poison soaked “mulch”.

Good thoughts above.

A further thought -- wrap the pile loosely in clear plastic for show and tell, with a chimney to keep the process going, and have the attending official note the massive condensation on the inside of the plastic. Ask: how much gasoline, realistically, would you have to dump in there to actually light this pile on fire?

The council have a mulch storage area beside the community garden and in the 15 years that I have gardened there, there has never ever been a fire.

Some of the mounds have been there for so long that vegetation has grown over them.

The mounds are several meters high and as others have mentioned, they will steam when fresh.

The only times I've seen wood chip piles hit smoke point or ignite they were more than 2 or 3 stories tall. A pallet company a half a mile from me accepts chips for some reason (I don't know why) and stacked them really tall to the point I remarked that it would catch fire when driving past. Sure enough it happened last year and at least one fire truck was there for like 3 or 4 days. Any hot compost is going to let off some steam, but I can't imagine a heap that a typical chipper truck dumps ever being a threat. I once had a windrow of around 12 loads from the street trimming crew sit for years and turn to black gold without any concern.

Thank you all. I had mulch pile sitting over a year on concrete and it became almost powder when I moved it with bunch of red worms at the bottom which I transferred to my garden beds.

We spent all day yesterday scraping mulch in the field to remove the top layer which is probably more than a foot at some places and piling bucket loads on top of hugelkultur beds. Now that needs to be spread out. I have lot of hugelkultur beds in the pasture. The bottom layer is all composted mulch and it doesn’t look like mulch.

The problem here is not fire department or fire fighter. Some years I had wood chips  piles sitting in the lower pasture when fire dept do their fire abatement inspection. I was never cited. I have been doing this since 2015. What feels like the department wants to please the people who complained and his question was I am still going to keep getting mulch. Thad is the concern here of the neighbor she doesn’t want to have me keep spreading wood chips across my property. My goal is to do the whole property and keep topping it off.

Here is what fire fighter said when I asked can I just spread soil on top of the mulch and he said that is how fire started at two places. I really doubt fire starts once organic material is decomposed thad is all the leaves are decomposed.

The field looks beautiful dark brown once we scraped off the top layer. Last week I had the strip outside the fence stripped and throw the composed mulch inside the fence area. That mulch looks like the mulch companies are selling these days composted mulch all natural beautiful

Fire fighter literally put his finger at the edge of the mulch saying this is more than 3 inches after we stripped. I told him at some places it is less because of the machine removing it and now humans have to go with rakes and spread.

University of california agriculture and natural resources website recommends 4 to 6 inches or more deep to suppress weeds. I didn’t go on arguing with fire fighter with all the information where I can clearly see he is there because of the complaint of a neighbor. When he drove in he saw lot of people have mulch piles in their yards. I asked him what is he going to do about that, all of them would be notified. Let’s see. He willl not go inspect everyone’s property especially the person who complained and he can’t even see their property because of the high fence they installed.

Sorry about the ramble. I see the goodness in what I am doing and it is frustrating to see all this stress I have to deal with. It is lot of hard work. Fire fighter asked me why am I doing this as it is lot of hard work. I said I believe in helping the world in a smal way I will grow agaves and they reduce carbon dioxide.

Thanks again.

I'm sure their 3" rule is the absolute safest possible standard for the most combustible mulches on the planet, being that they don't know what kind of mulches people are using, and they have to assume the people who use mulch might not even know what they're using. Natural mulch like people on this site use will probably not combust and we think the fireman is ridiculous. But somewhere, at some time, someone's mulch that was 4-5" must've spontaneously caught fire and hence, that rule.

Point #1 - My husband does all kinds of work on houses, and brought some old wood fencing to a lumberyard that said they take old lumber. When he arrived, he saw they were making it into mulch. Into the machine went fences, pallets, old decking, anything made of wood. Therefore, the people buying it have no idea what stains, paints, or other chemicals may be in their mulch.

Point #2 - I saw thin layers of mulch catch fire once.
We were shocked, and amazed, and wondered if we should call the fire department. It was dyed black mulch in a parking lot, around little curbed, raised areas of trees & flowers. It was in a suburb 45min outside NYC, and it was a hot day, 90-something degrees F. We got out of our car and saw a tiny bit of smoke coming from the mulch, the amount you might see from the tip of a cigarette. We wondered if it was steam, and we poked at it with a stick. There was an ember inside. We had water in our car so we poured some on it. As we walked through the parking lot into the store, we saw it in 2 other places, which is why we thought of telling the fire department; there were no flames, but maybe they should be aware of the possibility. For those other 2 we kicked the mulch around to mix it up, and we saw another person staring down at mulch across the parking lot.

We all know that chemical soaked, dyed black mulch in the sun in a hot, black parking lot on a 90-100 F day is not the same as a compost pile full of natural ingredients in a yard, but the law & the fire department have to 1) assume people are not very bright, and 2) err on the side of caution.