composting grass clippings

[Prashun Patel]

My mowed grass piles get incredibly hot - on the border of too-hot-to-touch. Is this a flammability concern when mixing in with dry, dead leaves in the compost bin?

I've never given it much thought until today. There does not appear to be any online flammability warnings against piled grass. Any help appreciated.:)

[Joe Pelonio]

I have heard of large grass clipping piles spontaneously combusting, smoking and in fact burning down a house. pretty rare but it does happen, the deeper the pile the hotter it gets inside.

Piles should be spread out flatter, so the combustible gases and heat can dissipate. I have not heard of a mulch bin catching fire though, because you are supposed to mix the green grass with brown leaves and such, so there's more air circulation and the decay of the grass is slowed down. It should be warm but not hot in it.

[Stephen Tashiro]

How do sawmills deal with the hazard of piles of sawdust catching fire? I assume such a hazard exists since I recall, as a kid, going to local sawmills and seeing charred piles of sawdust. The fires were blamed on spontaneous combustion. Of course, this was in the days when everyone smoked, so perhaps that wasn't always the truth.

[Zach England]

I'd be more worried about the smell. Piles of greens can smell terrible. Mix it with carbon (conveniently sawdust is fine). Compost should be wet anyway.

[Prashun Patel]

I don't believe in sawdust in the compost pile. I will mix in more leaves.

[Zach England]

I don't believe in sawdust in the compost pile. I will mix in more leaves.


Mine composts fine. It takes longer than leaves, but it definitely composts.

[Chris Damm]

Sawdust and grass clippings were made for each other in compost piles. It takes nitrogen to break down the sawdust and grass clippings are full of N.

[jackie gates]

A compost pile with a high value of N can catch on fire. I have not had one actually burst into flame, but had them simmer, smoke and create white ash, like you can see in a slow burning fire. Be safe and place the piles in a secure place away from other combusables.

[Jim Becker]

It's important to turn the pile regularly and include a good mix of browns, greens and other things...like vegetable based food scraps (never meat and bones), shredder paper, and even sawdust/shavings. (not walnut) Moisture is also important. While a compost pile "can" get hot enough to combust, taking care of it will result in really good soil and limited danger from getting too hot.

[John Toigo]

In addition - mixing brown (leaves) with green (grass clippings) allows the pile to aerate better & compost better.

[Mark Patoka]

I have one of those Compost Tumblers and mix my grass clippings with leaves and/or sawdust, about a 3:1 green:brown ratio. Once everything is fully activating, the internal temps will reach 130-140 degrees for a few days while everything starts breaking down.

I haven't heard of grass piles catching fire but had a friend tell me how their very large grass pile was steaming in the winter time. I'd just be careful to make sure it's not next to something else that could catch fire, just in case.

Farmers get the occassional barn or silo fire when very large amounts of fresh cut hay are stored so it can happen, just doesn't seem to be very common.

[Prashun Patel]

Thanks everyone. I spread out the grass a little and vented it. Seems to have reduced the temp considerably until I can get a lot of leaves in there (they haven't started falling in earnest yet).

[Jim Becker]

Prashun, you actually want the heat (within reason) as that is an indication that the composting work is in progress...but waiting a little to get the right mixture isn't a bad thing, either.