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Wilfire Protection and Defense


The most effective way to increase the protection or your home from wildfire is by creating defensible spaces. Defensible space involves creating concentric zones around structures, with increasing fire resistance provided in zones closest to structures. Plants in each zone perform a distinct function. The transition area between zones creates a break to slow ad­vancing flames. A minimum distance of 100-150 feet around your home needs this type of comprehensive landscaping. See Figure 1 for a description of each zone. Steep slopes or windswept exposures need greater defense distances (see Figure 2).

Figure 1. Defensible zones

Figure 1 revised from "Fire Risk Ratings for Homes" Wildfire Hits Home, Fire Control Division, Washington State Department of Natural Resources, Lacey, WA.

Zone 1: Moist and trim
In zone l, low-growing plants which resist catching fire and provide little fuel are used. Turf, perennials, groundcover and annuals form a greenbelt that is regularly watered and maintained to eliminate dry plant litter. This zone may contain individual shrubs and trees located at least 10 feet from the house.

Zone 2: Low and sparse
In zone 2, slow growing, drought-tolerant shrubs and groundcovers are used to keep fire near ground level. Native vegetation can be retained here if it is low growing, does not accumulate dry, flammable material, and is irrigated.

Zone 3: High and clean
In zone 3, native trees or shrubs are thinned and dry debris on the ground is removed. This zone requires removing overgrowth and pruning trees every three to five years. Specimen trees can be planted at the edge or this zone, if well cared for.

Zone 4: Natural area
Zone 4 is composed of native plants that are selectively thinned. If possible, highly flammable vegetation is removed and replaced with less fire­prone species.

Before any discussion of reducing fire hazard through the selection of proper plant materials this point should be made clear- all plants will burn if there is enough heat and other conditions are right.  The term "fire resistive" is used when referring to plants that are less flammable than others. The condition of a plant is as (or more) important as the species. Depending on a plants' growth form and access to water, the same species may be fire-resistive in one environment and combustible in another. Summer irrigation may make the difference between an extremely flammable plant and one which will not burn readily. Some species, such as cheat­grass, can actually serve as a fuse and start flash-fires when overheated.

Figure 2.  Sloped Sites

House without defensible space

House with defensible space
  • House at crest of hill at risk.
  • Overhanging wood deck is a big danger.
  • Shrubs below deck and tree through deck add to fire danger.
  • Shake roof burns easily.
  • House set back from crest is sheltered.
  • Stone walls deflect fire.
  • Fire proof roof protects house from embers.
  • Small watered lawn prevents ground fires.
  • Enclosed eaves reduce fire risk.
  • Greatest clearing downhill from house.

Plants that ignite readily and burn intensely, known as pyrophytes or "fire prone" plants, typically share certain characteristics:

  • They are water-stressed
  • Usually accumulate fine, twiggy, dry, or dead material
  • Have leaves and wood containing volatile waxes, fats, terpenes, or oils
  • Are typically aromatic (crushed leaves have strong odors)
  • Have gummy, resinous sap with a strong odor
  • Are usually blade-leaf or needle-leaf evergreens
  • Have stiff, leathery, small, or fine lacy leaves
  • May have pubescent (hair covered) leaves
  • May have loose or papery bark
  • Are plants that flame (not smolder) when preheated and ignited with a match.

"Fire-Resistive" plants share the following characteristics:

  • They have a high moisture content in their leaves
  • Are drought-tolerant
  • Have little or no seasonal accumulation of dead vegetation
  • Have a low volume of total vegetation
  • Have a non-resinous woody material
  • Have an open, loose branching habit
  • Are slow growing.

Landscape Maintenance

In maintaining defensible space you must actively reduce potential fuel accumulation by regular pruning, mowing, raking, and removal. . The less accumulated plant debris, the slower a fire will spread.

To modify existing vegetation to reduce wildfire hazard:

  • Clean the debris from your roof and yard several times a year
  • Remove grassy fuels for 30 feet around all dwellings
  • Remove highly flammable brush from around each home for a distance of not less than 100 feet
  • Apply all known cultural practices (irrigation, fertilization, etc) that improve health and vigor of trees/shrubs around homesites
  • Do not plant shrubs at the base of structures
  • Where planted, they should be kept well watered and pruned
  • Keep tree branches at least 15 feet away from chimneys and stove pipes, which should be covered by screens. Tree branches should also be kept at least 15 feet from utility lines and roofs
  • Store firewood 30 to 100 feet from any structure and create a defensible space around the pile
  • Remove dead shrubs and trees
  • Thin shrubs growing 100 feet and further from each house into individual plants
  • Eliminate "ladder fuel" configurations in vegetation (ladder fuel refers to the growth of a plant community structured like the rungs of a ladder (leaves, grasses, small shrubs, large shrubs and trees). Removing these "rungs" decreases the development of destructive crown fires
  • Reduce the probability of surface fire climbing into tree crowns by pruning the base of the crown 6 to 15 feet from the ground
  • Reduce lateral movement of fire between crowns by cutting branches that span between crowns to 10 feet or more apart
  • Prune all dead branches
  • When possible, keep the surrounding landscape healthy by thinning, controlling insect and disease problems, and reducing fuel accumulations
From University of Idaho Cooperative Extension System, Station Bulletin 67, March 1998, Revised June 2002

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