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Monitoring thatch prevents fire creep

By Frank Motal
Research Associate 3

Posted Dec. 1, 2014

Prescribed fire is a practice many landowners use in managing their properties to improve forage quality, improve wildlife habitat or control brush encroachment. Conducting safe burns is important, but issues can arise while burning. One such issue is fire creep. For the purpose of this article, fire creep in the context of burning can be defined as unnoticed smoldering of plant material (e.g., thatch) that is thought to be extinguished. The buildup of thatch along the fireline can cause fire to "creep" across the firebreak, which can result in an escaped fire.

Thatch is a mat of undecomposed, accumulated plant material next to the soil. Thatch in firebreaks is caused by mowing; mashing down tall, dense vegetation; and weather conditions that are unfavorable for proper decomposition of plant material. Dry soil aids fire creep because drier soils heat up easier and will remain hotter than moist soils, keeping thatch nearest the soil surface warm and dry.

thatch settledPreviously mowed thatch that has settled next to the soil

When using mowed firebreaks, water is most often applied (creating a wet line) along the fireline to keep fire in its designated area. When thatch is abundant, water may not penetrate thoroughly, leaving a dry layer next to the soil which can remain dry and warm after ignition and initial wet line application has occurred. When conditions become favorable, it begins smoldering and slowly begins "creeping" across the firebreak until it reaches and ignites unburned fuel outside of the burn unit, resulting in an escaped fire. This can happen later in the day when weather conditions change, the burn crew is further along the firebreak, or when the burn unit is thought to be contained and extinguished, and everyone has gone home. This is why it is important to have a backup suppression crew following behind the ignition crew, monitoring the fireline all the way back to the starting point.

thatch not settledRecently mowed thatch that has not completely settled next to the soil

Preventing fire creep can be accomplished with different techniques, including: 1) mowing at least twice prior to burning, with intervals between mowings, to reduce the amount of thatch and give the plant material time to break down; 2) raking thatch away from the burn unit, exposing bare soil for a wet line; 3) thoroughly wetting the thatch down to the soil; 4) using a leaf blower ahead of ignition to blow thatch away from the burn unit; 5) where practical, using a lawnmower to blow thatch away from the burn unit; and 6) where practical, creating a bare soil firebreak by disking. Disked strips work well as long as any grass or thatch is properly turned under. If not, fire can creep through the disked strip where plant material is contiguous, escaping outside of the designated burn unit.

Abundant thatch in firebreaks should be monitored throughout the day of the burn and addressed quickly if it begins smoldering. Checking the amount of thatch prior to a burn and deciding in advance how you will deal with it will assist you in conducting a safe burn.

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