Defensible Space News & Opinion

Defensible Space: Protecting Your Property From Wildfires


How To Landscape For Fire Resistance

When wildfire season comes around, you’ll want your property as fire-resistant as possible. Creating defensible space improves the chances of your house surviving an out-of-control fire by creating a buffer between your house and anything else that might burn through either direct flame contact or radiant heat. Ideally, this buffer of cleared-out space can slow – or even stop – the spread of fire to the really important bits of your property, but it takes some thoughtful consideration of how to go about this type of landscaping.

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Issues to Keep In Mind

Vegetation like grass, brush, and timber can all be highly combustible in the right conditions. They can burn intensely, producing embers that present a hazard in any sort of windy condition.

Combustible plants have particular characteristics that are worth recognizing. They can contain oils and resins that make them more prone to burning (you can usually tell these because they become aromatic when crushed). Narrow leaves and long, thin needles (like pine) will burn really easily. Waxy and fuzzy leaves will generally go up in a blaze of glory. Trees or other plants that have papery bark will light up pretty easily.

Wildfires can move horizontally from tree to tree, but also vertically from the ground up to the treetops. This might seem like common sense, but it’s worth your consideration when planning out landscaping that creates defensible space.

Combustible vegetation around a building will, obviously, increase the risk of setting that building alight in a wildfire. But, it also makes a firefighter’s job that much more difficult because it restricts the space that he can safely work in. It also increases the chance that the wildfire will spread further into adjacent wildlands.

The Three-Zoned Approach to Creating Defensible Space

All of this means that you’ll have to take into account the materials and vegetation that could potentially cause problems by spreading both horizontally and vertically.

Horizontal Spacing

Trees will need to be pruned and/or thinned out to ensure that their tops (crowns) do not intersect. The idea here is that by providing horizontal space between the crowns, you minimize the risk of fire spreading across the tops of the trees.

For trees, the general guidelines look like this: for a 0% to 20% slope, you’ll want to clear at least 10′ worth of space between the tree crowns. On slopes from 20% to 40%, you’ll want to clear 20′ or more. Any slope greater than 40% will require 30′ of clearance between trees.

For shrubs, the formula for creating defensible space is simple. Measure the height of the shrub. On a 0% to 20% slope, you’ll want to leave twice that height between any adjacent shrubs. From 20% to 40%, leave 4x the height. And, for any slopes greater than 40%, leave 6x the height of the shrub.

Vertical Spacing

You’ll need to account for vertical spreading by maintaining a vertical separation between the lowest branches and the tops of shrubs and grasses. This vertical distance can vary depending on tree and shrub/grass species, but the general rule is that the minimum amount of vertical space you should leave between the top of a shrub and the bottom of the tree is 3x the height of the shrub.

The idea here is that, if your shrub catches fire, you’ve cleared enough space that it won’t catch the bottom branches on fire, and will hopefully keep the fire from laddering up the tree.

Zone 1

FEMA suggests that you create three distinct and concentric zones of defensible space, if possible.

This first section is the closest one to your home, and you’ll want to create as much of a firebreak here as possible. For this section of defensible space, you’re aiming for 30 feet worth of noncombustible space in all directions. This section will require the most intensive maintenance of plants and materials, and you’ll want to consider taking these steps:

  • Install hard surfaces in this zone or use noncombustible mulch products, like rock mulch.
  • Keep this area regularly watered to prevent dry vegetation.
  • Remove any dead vegetation from this area.
  • Keep firewood stacks and other combustible material at least 30 ft away from your home. Ideally, you’ll want to place firewood uphill or on the same elevation as your home, and it wouldn’t hurt to keep your firewood on a graveled area free from other vegetation
  • Clear your gutters and roof of any dead branches, leaves, or pine needles. Trim any tree branches that hang over your roof, especially those that might hang over top of a chimney. Speaking of chimneys, keep yours cleaned and screened.
  • Keep grasses mowed to 4″ or less. Bag up and dispose of clippings.
  • Make certain that your driveway is wide enough to accommodate a firetruck. Make sure that overhanging tree branches are trimmed back enough to provide clearance.

Zone 2

The second zone marked out by your defensible space landscaping should take up the next 30 ft (give or take) of your yard. Again, you’ll be looking to keep up with tree and shrub spacing and maintenance; you’ll want to ensure that zone 2 includes only individual and well-spaced trees and shrubs.

This is also the zone where you’ll ideally want to place auxiliary structures like sheds or pergolas. As per the defensible space guidelines, keep these structures at least 50 ft away from the home. Increase this distance if you plan on storing any combustible materials in this structure.

If you plan on adding some plants to your landscape, plant fire-resistant, low-volume vegetation that holds onto moisture well and needs a minimal amount of pruning and upkeep.

Again, hardscapes like paved or gravel driveways and walkways are a good idea as they provide firebreaks.

Zone 3

This zone will stretch out to around the 100′ mark. In this zone, you’ll still thin and prune out trees and shrubs, though you can be a bit more limited in your application than in the first two defensible space zones. The goal here is to improve the health of the surrounding wildlands while providing a way to slow down any approaching wildfires. It also makes for a nice transition between your trimmed-up zone 2 and the untouched surrounding vegetation outside of your defensible space.

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Creating defensible space through landscaping doesn’t have to be rocket science, but it does require some upkeep. Many of the steps listed above will likely only require yearly upkeep, but the weekly chores like mowing and raking will take some care when it comes to disposing of yard litter.

Finally, you’ll want to consult your local fire agency or fire management specialist about your local codes and standards relating to defensible space.

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