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Knowing how to create a defensible space is key

  • James Campbell
  • In the wake of the most destructive fire season in California history, you’re probably wondering the same thing that I am: What can we do to protect our homes from the risk of another catastrophic wildfire? It turns out that much of what we can do is common sense. If you understand the concepts behind creating a defensible space you will have the key to making the right decisions, whether you’re creating a new garden or maintaining an existing landscape.

    Defensible space is the buffer you create between your buildings and the grass, trees, shrubs and wildland areas that surround it. This space is essential to slow or stop an oncoming wildfire and it will help protect your home from catching fire from radiant heat or direct contact with flames.

    Your goal is to ensure that a fire moving toward your home cannot follow a continuous path through vegetation. To do this, keep your plants small and broken into clusters. The closer to the home, the smaller the plants and clusters should be.

    The first step is to determine the slope of your lot since it will inform how you prepare your defensible space. I found an easy way to calculate slope on the internet just using string and two stakes. (It doesn’t need to be as accurate as a surveyor.) Flat lots and slopes less than 20 percent grade are treated the same way. A moderate slope is land that has falls between 20 and 40 percent grade. Steep lots are hillsides with grades above forty percent.

    I divided my defensible space into two zones: Zone one extends 30 feet for flat or gentle slopes. Moderate slopes, like mine, should be 60 feet and even farther on steep slopes. The distance is measured from all buildings, decks and structures.

    In zone one:
    • Remove all dry leaves, pine needles, dead plants, grasses and weeds
    • Prune trees to keep branches a minimum of 10 feet from other trees and remove branches that overhang your roof, chimney or powerlines.
    • Remove highly flammable trees and shrubs.
    • Remove or prune shrubs near windows (windows can only resist fire for one to three minutes)
    • Be sure to remove shrubs on the inside corners of buildings (these areas ignite more easily than the side of the house)

    Zone two extends out another 70 feet or more from zone one. In Zone two:
    • Mow annual grasses to a maximum height of 4 inches
    • Remove fallen leaves, needles, twigs, cones and branches when a pile exceeds 3 inches.
    • Create vertical spacing between shrubs trees and grasses
    • Create horizontal spacing between trees and shrubs The horizontal clearance between shrubs should follow a basic formula according to slope. If you are on flat ground, make certain there is an area twice the width of the shrubs between them. On a moderate slope, make a space four times the width of the plant between the shrubs. If the lot is steep, the area between shrubs should be six times the width of the plant.

    Trees matter

    The role that a tree plays in the spread of wildfire is often misunderstood.

    People worry a fire in the crown can spread to a structure, but this rarely occurs. In the right position, trees can serve as a buffer diverting wind flow, radiant heat and embers away from a house.

    Keep tree canopies separated by 10 feet on flat or gently sloping lots, 20 feet on moderate slopes and 30 feet on a steep slope.

    There should also be 6 feet between the trees’ lowest branches and the ground. If there is vegetation below a tree, prune it and make certain the lowest branch is three times higher than the underlying shrub. These precautions will help prevent a fire ladder.

    For more information on wildfire protection, go to ucanr.edu/sites/Wildfire.

    The UC Marin Master Gardener column is written by UC Marin Master Gardeners, who are sponsored by the University of California Cooperative Extension.  For questions about gardening, plant pests or diseases, call 415-473-4204 from 9 a.m. to noon, and 1 to 4 p.m. on weekdays, bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato, or email helpdesk@marinmg.org.