Spontaneous Hay Bale Combustion

Many people declare autumn their favorite season. With its colorful leaves, jack-o-lanterns, pumpkin spice, costumes, and crisp weather, what’s not to love?

Lurking in the background of many Halloween landscapes are seemingly innocuous rectangular prisms or cylinders. Sitting in fields along country roads, providing seats for tractor rides at pumpkin patches, outlining mazes at fall festivals, or just providing some harvest charm, bales of hay scream autumn. These collections of dried grass or cornstalks are nothing but positives – feed for animals and rustic decorations – right?

Or are they insidious booby traps of the spirits of Samhain?

Did you have any idea that, while you pacifically ride a tractor to the patch to pick out your yearly carving pumpkin, the bale of hay beneath you could spontaneously combust?

And not in the likely mythical spontaneous-human-combustion fashion, either, but the this-definitely-happens fashion.

Though spontaneous combustion of hay is real, the explanation is boringly not evil denizens of Halloween.

Counterintuitively, water is partially the culprit of self-inflaming hay. When baled and stored, if the grass has too much moisture (more than approximately 20%), a curious chain can lead to fire. Though the grass is cut, it continues to respire; in conjunction with the functions of microbial life – bacteria and fungi – in the grass, which munch on plant sugars, this recipe can produce ethanol. The chemical reactions produce heat in the bales, which are natural insulators. Ethanol has a flash point of just 57 degrees Fahrenheit. If some sort of spark, such as static electricity, catches the ethanol, the entire bale can go off. Further, even without the ethanol combusting, the chemical reactions can produce heat inside the bales, which can grow thanks to insulation, sometimes to points over 400 degrees Fahrenheit. At these temps, the bales easily go boom.

Below the moisture threshold, these reactions do not run out of control, and the hay remains the dry grass we all know and love.

The fiery phenomenon is hardly rare. Farmers let hay dry until the wetness levels are just right. If they judge incorrectly, or rains come to the hay, the bales are at threat of igniting for up to six weeks. Many a barn, silo, or entire crop has fallen prey to hay-bale spontaneous combustion. Hay farmers need to check the temperatures of their stored bales frequently. If things get too toasty, they need to take drastic cooling actions.

The next time you find yourself near hay, think about asking the proprietor about the moisture content in the bale. I’d like to think any bales left near the public would be dry enough to avoid spontaneous combustion, but, especially near Halloween, do you really want to leave that up to chance?

It’s never the costumed, axe murderer that will get you at a haunted house, it’s the hay bale in the corner you never suspected.