Plastic Pots and Danger

Well, it's been awhile since I've been able to post.  First, the hard drive on my computer decided to crash and it took some time to get that up and working again.  That happened last week, which brings me to the next event that kept me too busy to write and led to the title of this post.

Last Monday, my husband and I arrived home from work at around the same time.  It was a beautiful spring evening, and we smelled wood smoke as we stood outside and visited for a few minutes.  I commented that the neighbors must be burning their brush piles and that maybe we should do the same, since it was the first period of calm winds that we'd experienced in what seemed like months.  He went off to take care of chores in the barn and I went into the house to change clothes before starting on my chores.

When I got in the house, I thought "Hmm.  We must have left the windows open - it's really smoky in here."  About that time, my husband threw open the front door and yelled "Get Out!  The south wall of the house is on fire."  He raced to get water hoses out of the garden shed while I grabbed the phone and called 911.  Fortunately, my husband used to volunteer on the rural fire department, so he knew just how to rip off the siding and get the water in between the walls where it could do the most good.  We had most of the fire out by the time the fire department arrived and had limited the damage to the exterior of the south wall, and inside, just some buckled and scorched sheetrock in one of the bedrooms.   Oh, and an awful smoke, charred wood, and burnt plastic smell throughout the house.  That left the fire department with just the job of trying to figure out what had started the fire.

It could have been so much worse!

The final determination was that the fire had started in a plastic flowerpot that sat beside the house.  After investigating, they are pretty sure that someone wandered into the yard while we were gone and used the flowerpot for an ashtray, igniting the organic matter and the dead foliage that I leave in the pots over the winter to discourage cats from using them as a litter box.  We live pretty close to one of the busiest highways in this state, both for vehicle traffic and hitchhikers, and have had strangers wander into the yard for a little rest before (one even helped himself to a jar of spaghetti sauce that I'd just finished canning), so that is a plausible explanation.

The scorched foundation plate and 2 x 6's

The other explanation is not very likely, but there have been documented cases of spontaneous combustion of the organic material in the plastic pots.  That's where the danger comes in.  According the assistant fire chief, the "perfect combination" of chemicals from the plastic, organic matter in the potting soil, moisture, and heat can cause a reaction that causes the organic matter to heat up, ignite, and smolder.  The organic matter will not burst into flame, but will smolder for hours.  In our case, the process of smoldering, whether from spontaneous combustion or a lit cigarette, caused the wood foundation plate, the 2 x 6 wall studs, the OSB plywood sheeting, insulation, and the Tyvek wrap to begin smoldering behind the steel siding.

Most potting soil mixes contain peat moss or wood products, both of which will burn when exposed to flames or heat. Some potting mixes also contain fertilizer, which acts as an oxidizer that can cause a fire to spread more quickly.  Spontaneous combustion occurs when chemical changes in the material creates heat. Organic material with moisture content between 26 to 46 percent moisture and a temperature that exceeds 200 degrees F may ignite on its own, according to the University of Minnesota Extension Service. 

Although temperatures this high generally only occur in very large piles with limited air flow, such as hay or silage piles, cases of spontaneous combustion in flower pots have been documented.  Thankfully, this is very rare.  It is a little more likely to occur in humid climates.

There are some things you can do to prevent this from happening, other than simply giving up container growing of plants.  While it is not impossible for potting mixes to spontaneously combust in clay, ceramic, or concrete containers, it is more likely in plastic.  If spontaneous combustion does occur in a non-flammable pot, it likely will smolder slowly without causing much damage.  In plastic or resin pots, the heat causes the container to melt, which spills the smoldering contents out into areas that are much more likely to catch fire.  So, I will be replacing all my outdoor plastic pots with some type of hard, non-flammable material.

Here is what is left of two of my plastic containers.  A third one just completely disappeared.

Another way to prevent fires is to keep the containers well-watered (remember, spontaneous combustion occurs when moisture content is 26 to 46 percent) and keep all dead foliage cleaned out of them.  I guess this Fall, I will clean all the dead foliage out and maybe even empty the potting mix into the garden before I put things to bed for the winter.  After all, it will be cheaper to replace potting soil every year than to replace the house and all its contents.

I'll move those containers a little further away from the house, rather than tucking them right up against the wall.  I'm also reconsidering the wisdom of covering my cedar deck with containers, although I switched to all ceramic containers there purely for aesthetic purposes a couple years ago.  Maybe a drip watering system so those containers don't get the opportunity to dry out quite so much on those hot, windy days?

Other than that, I am considering building a barricade or an alligator filled moat around our yard to keep out careless visitors.