The goal of “firewise” landscaping is to reduce the intensity of fire and slow its advance as it nears the house. The basic principles are simple and few. Harden the target by making the house as resistant to fire as possible. Keep the area next to the house free of anything that will burn. Design and maintain planted areas farther out to provide no continuous path for fire to reach the house or move up into the tops of trees. Retain sufficient vegetation to buffer the house from airborne embers.
These principles can be implemented on the ground in many ways. Most communities are subject to state or local ordinances that specify details such as required or recommended spacings between plants and the distance from the house that regulations apply. Within these guidelines is usually a continuum of actions that might meet the specified requirements.
Hardening the target may mean something as simple as screening exterior vents and closing off the space under a raised deck; or it could entail the considerable expense of replacing a shake roof and installing triple-pane windows. Clearing the area next to the house may mean removing foundation plants and moving the woodpile uphill; or it could involve the installation of a continuous band of nonflammable materials — brick or flagstone patios, a concrete apron, or gravel mulch — all around the house. Interrupting pathways for the advance of fire could mean no more than pruning up trees and removing selected shrubs; or it may require a redesign of the entire landscape.
How much you do to protect your home from wildfire will depend on your assessment of risk. If you live in a community surrounded by forest or native chaparral you likely are at some risk for wildfire. If your house is on a steep slope, near the top of a ridge, or in a canyon, your risk may be higher. If access is reasonable, you may want to focus on providing space for firefighters to maneuver and defend your home. If roads are narrow and winding, vegetation is dense and overgrown, water supply is less than ideal, and long driveways provide no safe access for firefighters, you may want to prepare your house to survive a wildfire on its own.
Landscaping that follows the guidelines for reducing the risk of wildfire can add usable outdoor space, simplify maintenance, and make plantings more attractive from different viewpoints. Firewise designs can help to control erosion, improve drainage, open up views, and even make the experience of approaching the house more interesting and inviting.
Dry-stacked rock walls and terraces that interrupt the path of fire also stabilize slopes, slow rainwater runoff, provide flat seating areas, and improve drainage for the many plants that prefer it. Spacing plants farther apart gives them room to show off their natural form and provides them with better air circulation. Separating planted areas with wide gravel paths simplifies maintenance while making it possible for visitors to stroll comfortably throughout the garden. Gravel mulches conserve moisture and make it easier to remove weeds. All of these measures also help to reduce the intensity and slow the path of approaching fire.
Experience gained from recent wildfires has added another principle: Coordinate with neighbors to extend the protected area beyond your property. Some of the most destructive wildfires in recent years have spread explosively from one house to another, often without even reaching the tops of the tallest trees. Hardening an entire neighborhood or community may provide the best protection.
[excerpted from Gardening in Summer-Dry Climates]