Great Brush Pile Burning

Author with leaf blower brush pile burning in the background.
There is more to burning a brush pile than dousing it with gasoline and throwing a match at it.

Mid winter is a great time for brush piles burning in Wisconsin. My wife Anette and I unpack the Forrester: first the sled; then the leaf blower, pump sprayer and torch fuel. Fill the chainsaw with gas and bar oil. No need to bring along extra as the saw will see limited work today. Some snacks, tea and juice will keep us happy through the upcoming afternoon. Finally, I tuck some dry kindling and a roll of old construction prints what will be our starter.

A mid January thaw during last week put the burn day in doubt. Forecast snow on Sunday night meant plenty of snow cover for the Martin Luther King holiday and our brush burning. A moderate snow, light winds and weather in the twenties meant we will be comfortable all afternoon.

The sled is half full and that is just fine with me. Anette grabs the rake closes up the car are we leave the county highway behind. The local snowmobile club came through this fall after harvest and dragged a beautify trail across the corn stubble. Access to the Vermont creek will be quick and smooth. I throw the rope across my shoulder and make fast work of getting to the easement.

Our first brush pile was made during two days work in early and late fall. The pile is made up almost entirely of honeysuckle and buckthorn with a couple small boxelders for good measure. The first task is blowing the snow off the pile and clearing a small ring around the pile.

It takes one calorie to raise the temperature of one pound of liquid water one degree Fahrenheit. To melt that same pound of ice to liquid water requires 36,150 calories of heat. To take that pound of water from 32 degrees to its boiling point requires a mere 180 calories of heat. However, to boil off that heat and drive it out from soaking wet wood require a colossal 241,765 calories. This is called the latent heat of melting and the latent heat of evaporation.

That frozen wet wood on my burn pile needs to thaw out heat up and drive off its water, then continue up to the 700 degrees needed for wood to Ignite. Now consider that there is a cold wind trying to carry off the precious heat from my fire. No wonder it is so hard to get a brush pile to burn in the dead of winter.

Blowing the snow from the pile and around it suddenly makes a lot of sense. So does bringing along paper and dry kindling. These will make sure I have fuel that can easily light and stay burning long enough to dry our surrounding wood in the pile.

Torch fuel is a two to one mix of diesel fuel and gasoline. Diesel has a low ignition point but a high flash point which makes it much safer to work with than straight gasoline.

A wise friend finally put me straight about using accelerants on brush piles. “You don’t want the torch fuel to burn. You want the torch fuel to make the wood burn. Let it set there for ten minutes and see what difference it makes.” He was right! All my life I would pour fuel on a pile and light it, never understanding that the fuel was sitting on top of the wood and burning itself off without heating up the wood enough to get it to ignite.

Now I pour on a quart where I would have used a gallon. By walking away for ten of fifteen minutes, the diesel has time to soak into the wood. There is no big whoosh of flame, instead the dry kindling and paper take off and heat up the surrounding wood that is ready and raring to burn.

Snow on the ground means embers will not ignite surrounding vegetation, making it safe for a couple of people to burn several piles at once. Many towns are weary of issuing burn permits in the spring and fall when dry grass and leaves make spot fires from brush piles a real danger. Winter burn permits are easy to get and are easy to watch.

Anette keeps vigil using the rake to push remaining branches from the edges into the coals where they are quickly consumed.

I head upstream to a downed tree that needs to be cut and stacked. It takes a half hour to get the pile set, but because this is a black walnut that is not yet finished drying out, our efforts to burn the pile meet frustration. After several failed attempts, it is time to move on to a third pile.

Like the first pile this is one has plenty of honeysuckle, which burns easily, buckthorn that burns okay when dry and preheated, and boxelder that has laid there for several years. As this large tightly packed pile springs to life, Anette breaks out cookies, chocolate hot tea. The snow is flying all around; we relax and enjoy the now fading sun as it forces itself through the snow and cloud cover.

As the sun begins to fade, I once again take out the leaf blower. There is a large bed of coals and large sections of trunk burning in the middle of the fire. There remains plenty of brush around the downwind edge of the pile that I am in a hurry to dispatch. Braving the smoke, I rake the remaining unburned fuel into the hot center where it sits listlessly. The small pile seems to be waiting for an invitation to burn.

So I supply just such an invitation. My leaf blower comes out of the sled and springs to life. Its supercharges stream of air turns the bed of coals into a blazing forge. The coals become furious and through intense flames and heat that make short work of the remaining branches that threatened moment earlier to suffocate the those same coals.

A few minutes running around the edge of the fire with the leaf blower moves the mixture of snow, leaves and twigs at the margin out into the snowy barren reaches beyond. A wide patch of charcoal black mineral soil now separates the two worlds and provides a safe barrier for the remaining wood to finish consuming itself.

We pull back across the field and wipe off the snow from the bottom of the sled. It slides into the back of the Subaru as the last rays of sunlight fade.


7 thoughts on “Great Brush Pile Burning

  1. What Is The Greatest Hazard With Diesel Fuel? – Almazrestaurant

    […] Torch fuel is a two to one mix of diesel fuel and gasoline. Diesel has a low ignition point but a high flash point which makes it much safer to work with than straight gasoline. A wise friend finally put me straight about using accelerants on brush piles. via […]

    1. conser36_wp

      Diesel fuel is a petroleum distillate. When you apply it in large quantities, its smoke is a pollutant. Like other petroleum distillates, diesel fuel is a ground and surface water pollutant, so again, you don’t want to pour diesel fuel on the ground.

  2. Sandra Stevens
    Sandra Stevens says:

    What about the wildlife that depends on that brush pile for habitat in the winter? What about insects, beneficial insects, that are there in the form of eggs and larva and such. ?

    1. conser36_wp

      Brush piles can be important refugia for bunny rabbits, and other small mammals. It too can provide a home for insects and place for their eggs to overwinter. I have several brush piles constructed specifically for that purpose on my property. However, many Wisconsin woodland owners have more brush piles than needed for refugia and want to recycle that biomass. Building brush piles as refugia is a great idea for a separate blog post. Those brush piles are constructed differently, based on the wildlife you want to use them. Thanks for pointing that out.

  3. Kiss you ash goodbye - Conservation Digest

    […] it can be used as a super plant food, check out this video. You can also check out my blog post on Great Brush Pile Burning for help making that chore easier and more […]

  4. Is Diesel Less Flammable Than Gasoline? – Bescord

    […] Torch fuel is a two to one mix of diesel fuel and gasoline. Diesel has a low ignition point but a high flash point which makes it much safer to work with than straight gasoline. A wise friend finally put me straight about using accelerants on brush piles. “You don't want the torch fuel to burn. via […]

    1. conser36_wp

      My experience has been that 2:1 mix is not the best torch fuel mix. Gasoline has a real low ignition point. So, that mix will burn really fast, for me; too fast. That might seem like non-sense, but here is the basic science. Wood does not burn. You say, “What are you talking about? That is crazy; of course wood burns.” Actually, no; it does not. Fire is an oxidation reaction where oxygen combines with carbon based molecules producing heat; CO2 and water vapor … as well as other minor combustion gases. In order for fire to ignite, the fuel must be raised to its combustion temperature. That permits the fuel molecules, in this case wood, to begin vaporizing forming a very thin layer of gaseous carbon based molecules coming off the surface of the wood. It is those vaporized molecules that combine with oxygen in what we call fire.

      When you put torch fuel on a brush pile and light it, the flame from the burning torch fuel heats the wood. It is only when the wood reaches its ignition temperature that that vapor is released and the wood fuel itself begins to burn.
      Gasoline burns at a much lower temperature than diesel fuel, which makes it burn faster. The problem is that water molecules in the wood absorb part of that heat. There is more to the discussion, having to do with sensible and latent heat, but that is longer topic.

      The wetter the wood the more heat is absorbed by the water and the less heat available to raise the temperature of the wood to its ignition temperature. The result is that torch fuel burns itself up before the wood starts burning.

      I use a much more diesel heavy 4:1 torch fuel mix. The torch fuel burns slower and the heat it releases has time to bring the wood up to its ignition temperature. The result is similar when using a drip torch to ignite a broadcast (prescribed) burn.

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