The way a wildfire burns depends most heavily on (1) the weather preceding the fire, (2) the fuels available to burn, and (3) the topography of the physical location. The adjective class rating, promoted by the Wildland Fire Assessment System (WFAS), accounts for weather and fuel. There are measures, described in more detail below, that homeowners can take to minimize the chances of a wildfire loss.

Impact of Drought Conditions

It will come as no surprise that wildfires tend to burn more easily and more intensely when the atmosphere and surrounding area are drier than normal. The Keetch-Byram Drought Index (KBDI) compares, on a scale of 0 to 800 in hundredths of an inch (0 to 200 mm) and assuming the soil can hold eight inches (203.2 mm) of water at full saturation, how much moisture it would take for the ground to be fully saturated with the amount of water actually available, also called the “moisture deficit”; a 0 on the KBDI index means the ground is fully saturated, and an index of 800 (200mm) means there is absolutely no moisture in the soil. A high moisture deficit indicates drier conditions, which could contribute to the intensity of a fire by having more dry fuel available to burn. For example, soil is considered “moist” up to a KBDI of 200 (50 mm), meaning the water is 1/200 of what it should be.

The North American Drought Monitor (NADM) provides ongoing monitoring of drought conditions across North America. They provide a weekly map of dry and drought conditions.

Hotter than Hot

Different types of fires burn at different temperatures. For example, the optimal temperature for roasting marshmallows is between 600 and 900 degrees Fahrenheit (316 to 482 Celsius). Large bonfires can reach 2,000 degrees Fahrenheit (1,100 Celsius). For wildfires, it depends on what part of the fire you’re referencing. Surface fires, which burn along or close to the ground, the temperature is usually in the neighborhood of 1,472 degrees Fahrenheit (800 Celsius). Fire burning in the tops of the trees usually exceeds 1,832 degrees Fahrenheit (1,000 Celsius) or, in the right conditions, even 2,192 degrees Fahrenheit (1,200 Celsius).

What is Defensible Space? 

In the early 2020s, the Insurance Institute for Business & Home Safety (IBHS) created a “Wildfire Prepared Home” designation where eligible homeowners can take specific steps to increase their home’s resilience to wildfires. Read about it here.

In wildfire prone areas, homeowners are encouraged–or required, depending on the location–to create what is known as a “defensible space” around their dwelling. FEMA defines a defensible space as “an area around a building in which vegetation, debris, and other types of combustible fuels have been treated, cleared, or reduced to slow the spread of fire to and from the building.” Fire has to have fuel to burn, so you would want as little fuel close to your home as possible.

There are three main “zones” to a defensible space. There are different guidelines for creating each zone, each with the goal of reducing the chances of a fire as much as possible. For homeowners living in wildfire-prone states, like California, creating a defensible space may be required by law. Per the California Department of Forestry and Fire Protection (Cal Fire), the exact size of each zone may vary based on local government regulations. Individuals living in certain counties–Los Angeles, Orange, Ventura, Marin, and Santa Barbara–should visit the county-specific website to obtain the proper regulations. For the purposes of this article, we will be discussing the general guidelines available from Cal Fire and defensiblespace.org.

The first zone is the ember-resistant zone. This zone, according to Cal Fire, is the “most important” because, of the thousands of homes lost to a wildfire, a majority of them are ignited by a single ember or spark that has been carried on the wind from a distant fire. The ember-resistant zone includes the dwelling itself and the five feet around it. This zone should be free of any vegetation, including removal of any tree branches that come within 10 feet of a chimney or stove outlet. Paths leading to the house should be made of gravel, paving stones, or concrete, and any “mulch” should be made of rock or gravel. Any firewood, lumber, or other combustible items must be moved out of the ember-resistant zone.

The second zone is the home protection zone. This zone spreads out a minimum 30 feet from the house and acts as a buffer to (hopefully) slow down or even halt the spread of a fire. Having a handful of plants is fine, but don’t go overboard. Remove any invasive species and replace those plants with fire-resistant native plants. Though no plant is fireproof, fire-resistant plants have physical qualities that make them less susceptible to catching fire. Avoid plants that die back or dry out in the summer, because dry plants are a major source of fuel for fires. Aim instead for plants with thick leaves that have a high moisture content. Avoid plants with a strong scent, like roses or lilies, and keep trees trimmed and free of dead branches. Any trees planted in this zone should be spaced so the crowns (topmost leaves and branches) are at least 10 feet apart.

The third zone is called the reduced fuel zone.This zone is all about reducing the amount of potential fuel for a wildfire near your house. Place boulders or retaining walls in strategic locations across your yard. Create a dry creek bed with large, flat stones. The lowest branches of a tree should be a minimum of six feet off the ground. If a shrub or bush is under the tree, the minimum distance from the top of the shrub to the bottom-most branches of the tree should be at least three times the height of the shrub. As far as the distance between plants goes, it depends on whether your yard has a slope, and how steep it is if you do. On slopes that have less than 20 percent grade, trees should be at least 10 feet apart, and the space between shrugs should be at least double the height of the shrub. A slope graded between 20 and 40 percent should have a minimum distance of 20 feet between trees, and the distance from bush to bush should be four times as long as the bush is high. A hill any steeper than 40 percent should put 30 feet between trees, and the distance between bushes should be at least six times as long as the bush is high. Check your county’s website to see if there are specific guidelines you must follow regarding what to plant.