33. Landscape Design for Fire Safety
We’re in the middle of California’s wildfire season and it’s a very dry year, so there’s no better time to talk about the role of landscape design in fire prevention. The memory of the 1991 Oakland Hills Fire is still strong here in the Bay Area - a terrible wildfire that quickly burned through a large residential neighborhood, destroying many houses and threatening much more of the city. The good news is that it helped put in place strong regulations and create a lasting awareness of the danger. (NOTE: this article was written in 2014, before the tragic North Bay fires of 2017 and Camp Fire in 2018.)
Take a minute to think about the potential fire hazard in your neighborhood. The first thing to consider is the terrain, a huge factor in the Oakland fire. Houses that are near natural open space areas, on steep slopes, ridge tops, or narrow canyons are especially at risk. Steep slopes facing into the wind are especially bad, fire can spread fast in those conditions, with hot air from the fire drying out the vegetation ahead of it as it goes. Your local fire department will be able to tell you more about the fire risk of your neighborhood, and in places such as the Oakland hills, there are specific laws and required inspections of home landscapes for fire safety. If you are in a fire-prone area, then the proper design and maintenance of your landscape can go a long way towards reducing the fire risks.
Defensible Space
When looking at the design of a garden in terms of fire safety, the overall concept is to provide a “defensible space” around the house. Depending on the size of the yard there may be a need for several different zones that transition out to natural areas. The primary defensible space zone extends about 30 feet around the house; this is where you want to be most careful about the landscape design to minimize the ability of fire to spread quickly. That 30 foot zone is surrounded by a transition zone where you want to make sure things are not too dense or overgrown, with only a few trees, keeping the amount of fuel for a fire low. The outer zone, extending to about 100 feet from the house, can be more natural and have more trees, but still needs to be well maintained to keep dead wood and other flammable material to a minimum.
Obviously many houses sit less than 30 feet from their property edges, and when you are in a densely populated area away from steep slopes the wildfire risk is usually less. But as the Oakland hills fire showed, fire can occur in a relatively typical suburban neighborhood. Even if the property line is only 20 feet from the house, if you are in a fire-prone area, the entire 20 feet should be designed as defensible space.
Plant Arrangement
Within these zones, the landscape design strategy is to arrange plants to avoid what are called “fire ladders,” where plants of different sizes are grouped so closely that fire can easily spread between them. You don’t want to have a patch of tall grasses right next to a grouping of larger shrubs, sitting right under a densely branched tree. You can imagine how a small ember or spark could ignite the grasses and quickly spread to the shrubs and then the trees. A second part of the design strategy, especially in the 30 foot defensible space zone, is to include many non-combustible landscape elements such as patios, walkways, walls, lawns, swimming pools and other water features. Pools and pathways can even be used to create fire breaks when oriented correctly. A good landscape design also lays out the plants according to their mature size, avoiding the need for excessive pruning in the years ahead.
Maintenance
Landscape maintenance is naturally a key factor for a fire safe landscape. Once the planting is installed, keeping vegetation healthy, well pruned, and thinned of dead branches reduces the amount of fuel and the fire ladder potential. Regular mowing of grass areas and composting of the trimmings and other plant debris or removal off the property is also important. Keeping the irrigation system operating properly is another critical maintenance task for fire safety, it is good for plants to stay moist and less flammable. However, over watering can also create problems. Some plants, especially California natives, prefer not to have too much water and may die back if over watered. Other plants may take advantage of the extra water to grow a lot bigger than you planned and provide a path for the fire to spread.
Plant Selection - Pyrophytes
There are certain plants which are considered especially flammable, and some that can be more fire resistant. Historically California and many other parts of the west had regular wildfires, and the native vegetation adapted to that, some plants even relying on fire to activate seeds for growth. For this reason we have quite a few native plants that aren’t great in the defensible space zone around your house. Plants called Pyrophytes are considered highly flammable because of their oils and resins. Eucalyptus, Juniper, Coyote Brush, and Monterey Pine are common Pyrophytes that should be avoided. Eucalyptus trees were a primary cause of the intensity of the Oakland Hills fire, they grow fast and choke out other vegetation, leaving a large amount of dry leaf and bark litter on the ground that can easily burn.
Fire Resistant Plants
There is no such thing as a completely fire-proof plant, any plant can dry out and become combustible, but some plants are often better at resisting fire. These types of plants tend to generate fewer dead leaves and branches, have less volatile sap, and generally have moister, more succulent leaves, plants like Aloe, Agave, Ice Plant, or Camellia. Unfortunately there is no authoritative list of fire resistant plants that I know of. As mentioned above, maintenance is a key factor in making any plant fire resistant; if any plant is well trimmed and properly irrigated then it will naturally have less flammable material.
A fire safe landscape doesn't have to be a bleak stretch of concrete, but it can’t be an overgrown mess either, a good landscape architect can help find the right balance. The house design is also important, especially its location, and the materials used for the roof and decks. As long as you have thought through the potential hazards and minimized them, a fire safe yard should be just as enjoyable as any other. If you feel the urge for a really big fire, go out to the ultimate fire safe landscape, the giant dried lake bed of the Black Rock Desert, where some truly talented pyromaniacs are doing it properly this weekend!
Important Note: This writing is in no way intended to provide expert professional advice specific to a certain landscape and in no way guarantees fire resistance of a landscape. Please consult with a professional and your local fire department directly for specific advice on the best fire-safe practices for your yard.
References:
Bay Friendly Gardening Guide and Landscaping Guidelines
Fire Prevention Matters: A Homeowner’s Guide to Fire Prevention in the Oakland Hills, by Bob Sieben, June 2013
Firescape: Landscaping to Reduce Fire Hazard, East Bay Municipal Utilities District, Oakland, October 2004
Pyrophitic vs. Fire Resistant Plants, by Ray Moritz and Pavel Svihra, University of California Cooperative Extension, October 1998.