In the world of prescribed burning, three types of fires are used; a “backfire,” “head fire,” and “flank fire,” and each type provides differing intensities for varying results.
Backfire
The slowest and easiest to control is a “backfire,” which burns into the wind. Flame length is sometimes so low you can step over the fire. However, that all depends on the fuel load on the ground. Fuel load is typically the amount of duff, leaves and dead grass on the ground – stuff that will burn. If you are burning slopes, this fire would be started at the top of the hill and slowly creep down slope. In most cases you can out-walk a backfire. Backfires are considered “cool” fires in that little damage is done to the larger trees.
Ironically, because a backfire is slow, the heat remains on site longer. This longer duration is perfect for killing smaller woody stems. It only takes 146° F to break the cambium on small trees, which is sufficient to kill them. If you are managing a native grass stand with woody encroachment, a backfire may be the best way to knock it back. These cooler fires are normally ignited when air temperatures are less than 68°and relative humidity is more than 50%. [Read more…]