It has been a dry summer and flower pots are being blamed for several fires. The news headings are everywhere!
- “Fire Chief Gord Weir stands outside of a home damaged by a fire …. He suspects the fire may have been caused by peat moss left in an old flower pot” (ref 1)
- “Flower pots became a major Montreal fire hazard” (ref 2)
- “Des Moines fire officials said a flower pot spontaneously combusted Wednesday, causing more than $80,000 in damage to a home” (ref 3)
The facts seem simple. Flower pot soil is made mostly of peat moss, something that burns easily. When it gets too hot it ignites and starts to burn. If the pot is near a home, the house also catches on fire.
What is the real story behind all this? Can peat moss spontaneously combust? Lets have a look at the facts.
Fires Started in Flower Pots
There are lots of cases reported in newspapers to support the idea that fires do start in flower pots. Many fire departments also subscribe to the idea. The three quotes above are just a small sampling of cases.
The important question that needs to be asked is, how and why did these fires start?
In some cases it is quite clear that careless smoking, or more specifically, careless extinguishing of cigarette butts are the cause. Dry peat moss in planters will burn if you ignite it – that is clear. Careless people who drop their butts into flower pots should not be surprised that they catch fire. Once you have a fire, the fire can spread to the home – no surprise there.
I find it very interesting that the majority of reports for this kind of fire do not report on the smoking habits of the owners or guests. Many of the fire chiefs reporting on the incident also do not make a point of discussing cigarettes. In fact, many of them think that the flower pots ignited spontaneously.
But ….. can the peat moss spontaneously ignite on its own?
What is Spontaneous Combustion?
Spontaneous combustion happens when material starts to burn without being ignited from an external source such as a match or cigarette butt. In summary, spontaneous combustion happens when the material heats itself up to the ‘point of ignition’. The point of ignition is the temperature at which the material in question will start burning all on its own.
Can Peat Moss Undergo Spontaneous Combustion?
The point of ignition for dry peat moss is 260 °C or 500 °F (ref 5). This means that for spontaneous combustion to happen the peat moss in the pot would need to heat itself up to 260 °C. Note that this would be higher for wet peat moss. To put this into perspective, water boils at 100 °C.
Have you ever touched your planter or soil in the heat of summer and been burned by it? A temperature of 80 °C (175 °F) can cause severe burns in less than a second. The pot needs to become 3 times hotter than this before the peat moss would ignite on its own.
As reported by the Toronto Sun Newspaper, Dr. Murray Thomson, a professor of mechanical and industrial engineering at the University of Toronto, says “Typically, if peat moss gets wet, it doesn’t burn well. It’s hard to believe a plant, which is watered, could ignite from sunshine. The sun would not be strong enough to ignite it. It would take a ignition source of 700 °C (1300 °F) to start peat moss smouldering” (ref 1). Dr. Thomson is in charge of the University of Toronto, Combustion Research Laboratory.
So wet peat moss will not ignite on its own, but what about a flower pot that has been neglected and is now completely dry? Can dry peat moss reach a temperature of 260 °C?
How Does Peat Moss Heat Itself?
There are two heat sources to consider; the sun and the heat from decomposition.
Can the sun heat the peat moss to 260 °C?
The ignition point for paper is 218–246 °C (424–475 °F) (ref 6). This is a bit lower than for dry peat moss. Does a newspaper sitting in the hot sun spontaneously combust? No. If it did, there would be a lot less paper garbage laying around. If the sun can’t heat paper to ignite it, it will also not be able to ignite peat moss.
What about decomposition?
Bacteria start decomposing the peat moss and in the process produce heat. That is why a compost bin heats up. Can this process get hot enough?
Dry peat moss does not decompose. The bacteria need some moisture in order to do their job. Think of the compost pile that needs to be kept moist in order to work. But if water is added to peat moss the ignition point goes up making it even harder to start combustion.
What about a combination of sun and decomposition? The problem with this idea is that bacteria start to die off long before they reach the ignition point. Once that happens they stop adding heat to the process, and you are left with only the heat from the sun.
Decomposition can lead to spontaneous combustion, but only in special cases which require very large piles of material that can trap in the heat to allow it to build up. A flower pot will not do that.
How Hot Does a Flower Pot Get?
Sounds like an easy experiment. I took two 5 gal, black pots and filled them with peat moss. I moistened one and kept the other dry. They sat in full sun for two weeks during the hottest, driest summer we have ever had in Ontario. The temperature was at or above 30 °C for several days in a row, and even reached 32 °C one day – an all time record.
The peat moss in the pots never got above 39 °C. The one with moistened peat moss was always a couple of degrees cooler than the dry one. A temperature of 39 °C is a long way from the 260 °C needed to spontaneously combust.
Peat Moss can not become hot enough on its own to spontaneously ignite.
Why are Flower Pots Burning?
It is clear they are not spontaneously combusting. Something is igniting the peat moss and the most likely culprit is a cigarette butt. Smokers are notorious for butting out in flower pots.
It is a shame that fire departments don’t understand this topic better.
References:
- Burning Controversy Over Burning Flower Pot Fires; http://www.torontosun.com/2011/06/05/burning-controversy-over-flower-pot-fires
- How Flower Pots Became a Major Montreal Fire Hazard; http://www.cbc.ca/news/canada/montreal/flower-pots-fire-milano-building-little-italy-1.3654128
- Can a Flower Pot Start a Fire?; http://www.kcci.com/news/central-iowa/Can-a-flower-pot-start-a-fire-Yes-it-can/15603834
- The Subtle Signature of Spontaneous Combustion;
- Peat Moss Material Safety Sheet; http://www.avpeat.com/cansorb-safety-sheet.html
- Auto-ignition Temperature; https://en.wikipedia.org/wiki/Autoignition_temperature
- Photo source; Craig Myran Photography
We need to think about infrared energy. The curved concave inside surface of the flower pot could be concentrating the infrared rays in sunlight and, at specific angles, could concentrate enough energy on a tiny point to act as an ignition source. We tend to think of glass and mirrors as light reflectors, and this is true for visible light rays, but invisible infrared (heat) rays are long wave and are reflected and concentrated very well by other surfaces that one would not normally consider “reflective”. I have not done the experiment but, having built solar thermal water heaters, and my experience as an extra-class amateur radio operator (ham radio uses longwave radiation to communicate around the world and can easily melt chocolate) and retired electronics / laser printer engineer (laser printers use heat – not lasers, but other heat sources – to melt / fuse the plastic toner particles to the paper), my experience and background in thermodynamics suggest that the interior surface of a flower pot might be able to concentrate some wavelenghts of sunlight, possibly heating a small point to ignition temperatures. Once ignited, a microscopic point source can provide enough heat for the flames to grow sufficiently large to consume a house under the right fuel / air / temperature conditions. Your experiments were, no doubt, done well, but this does not prove that under ideal circumstances, peat moss in a flower pot will never be ignited by sunlight. I have witnessed enough pyrolysis (look it up) in laser printer fusers – and done enough research to improve fusing efficiency in laser printers – without setting to page on fire – to believe that sunlight in a flower pot could ignite peat moss under ideal conditions (but not “general” conditions). Sunlight is about 1100 watts per square meter. Concentrate that on to a pinhead and ignition is possible. – Tony Barrett, N7MTZ
I guess that is possible, but most containers are not very reflective on the inside.
Weirdest thing happened this morning our pot plant thats on a second floor balcony, caught on fire by itself how is even possible how did this happen, was it the compost its winter here so ye have no damn idea wasn’t arsen as it is on the balcony were no one can get to it very odd but ye thought we would share it! We took a video and posted it on you tube link
https://www.youtube.com/watch?v=Y4Ux15DEUpE
It was not spontaneous combustion.
At least twice you used “but” instead of “butt”.
You’d think the fire people would think to look for butts though – they don’t tend to burn away in a mild fire…
Thanks for the spelling correction.
London had a building that was “melting” cars a few years ago. Headlines exaggerated things but someone managed to cook an egg. Cheap plastics were obviously melting but the recorded temp was only around 100 Deg C.
https://en.wikipedia.org/wiki/20_Fenchurch_Street ( see solar glare section)
I dare say some objects could focus the sun enough to act like a magnifying glass but it could only be an odd freak occurrence otherwise it’d happen all over the place and be widely known about.
Your guess about cigarettes has to be the most probably cause or even deliberate acts for insurance purposes.
Thank-you for this wise account of the stupidity of smokers and the gullibility of fire departments! I really admire your work. I love the experimental methods you devise and the simple way you explain everything – so clear, and not in the least patronising.
Perhaps blaming the flower pot for starting the fire can be considered an “act of God”. This claim may make the insurance companies pay for the damages rather than smoking homeowner negligence. This is speculation on my part.
Nice analysis Robert
psst you have missed n out of burning in your last title
Thanks
Hello Robert,
I admire your clear thinking and expression. As we say in French “ce qui se conçoit bien s’énonce clairement” i.e. “what is properly thought-out will be clearly expressed” (Boileau, 17th and 18th centuries) and may I add, wil be clearly understood.
Concerning self-ignition of peat moss, what of pots standing behind a glass-pane ? Can the glass act as a heat concentrator ? As naughty kids, we’ve all played with a magnifying glass and burned the back of our hands, haven’t we ?
Cheers from France
Marie
A glass pane does not concentrate the light energy like a magnifying glass. Inside a greenhouse can get hotter, but nearly hot enough.
Something I’ve never thought about. But you’ve done the analysis
step by step to show spontaneous combustion of peat moss is impossible under normal conditions. I’d like to think that most people should be bright enough to realize it’s an impossible feat for peat moss to ignite on its own. What you have demonstrated is gullibility and naivete where it shouldn’t exist, namely with fire departments.
Very topical, interesting information. Thank you!