By almost any definition, whichever plants grow in a garden define the space and give the landscape its character — what's a rose garden without roses, after all? But to create a fire-safe garden, it's actually best not to think so much about the plants.
"The first thing is always not what's planted, but where it's planted," said Eugenia Rendler, education coordinator for the Santa Clara County FireSafe Council, a nonprofit that aims to protect local communities at risk of wildfire.
"People want you to give them a plant list (of fire resilient plants) and then just want to fill up their yard with plants, but every plant will eventually burn," Rendler said.
Instead, structuring the garden to establish what's known as "defensible space" is the most important principle in fire-safe landscaping, Rendler said. Defensible space keeps the immediate area around a house free of anything flammable, and designates zones around the home where carefully spaced plantings can help stop the spread of fire.
Rendler said the use of defensible space is crucial, and required by county ordinance, in areas where neighborhoods touch wooded areas, which are known as wildland-urban interface (WUI). Chapter 49 of the Santa Clara County Fire Code requires defensible space around all buildings in WUIs. According to a map on the City of Palo Alto's website, WUI areas include neighborhoods in the Palo Alto Hills and Stanford Hills, and along parts of both Arastradero and Page Mill roads — about 130 residences.
Though defensible space is most needed in WUIs, Rendler said it’s still smart to apply some of the principles to homes in more densely populated urban areas because fires can happen — and spread — anywhere. In 2002, a fire at the Santana Row shopping center, then under construction in San Jose, spread to homes half a mile away via embers carried on the wind.
Rendler described the three concentric zones used in defensible space. The first zone establishes an area 0 to 5 feet from the house, where it's best to avoid planting any vegetation and instead rely on hardscape like gravel, cobblestones or river rock for protection and decoration.
The next zone, 5 to 30 feet from the house, makes up the main garden area, with small groupings of low plants and shrubs spaced far enough apart to make it difficult for fire to spread between them. "The 'lean, clean, green zone' is a nickname for this zone because that's where things are manicured and usually irrigated. It's where you want to keep out dry leaves, pine needles and any kind of debris (that's the clean part) and the lean part refers to that reduced fuel load. So hardscapes like stairs, gravel or rocks should reach into it to help keep down the amount of fuel," Rendler said.
Finally, the 30-to-100-foot zone includes some carefully spaced and trimmed trees, with small, low plants beneath them to both maintain visual appeal and keep down weeds. Rendler said that avoiding too many plants graduated in height in any zone prevents a "fire ladder," in which fire can hop from small plants to adjacent shrubs and then to trees.
Master Gardener Barbara Hunt also emphasized defensible space in a fire-safe landscaping workshop she recently presented in Cupertino for the Master Gardeners. She described how to set up the three defensible space zones, but also discussed what's best to plant in the two outer zones. She focused on native plants, highlighting fire-resistant specimens that are also less likely to be eaten by deer, a common problem in areas also prone to wildfires.
In terms of plant choice, Hunt said, "The most fire-resistant would be succulents and then things that are green all year (not pine trees because they're oily) but other evergreen plants. And then there's a long list of annuals and perennials that are small, either up to a foot or two high, and shrubs. A few trees — the California Live Oak, for example — doesn't burn very easily."
Both Rendler and Hunt named some plants to avoid. Shrubs and trees higher in natural oils — cypress trees especially — are dangerous choices. Also junipers and eucalyptus, the latter both high in oil and with excessive droppings like leaves and seed pods that will help spread fire. Rosemary, a favorite drought-tolerant plant in many local gardens, is also high in oil, but Rendler said it can be OK to plant sparingly in an outer zone.
Hunt noted that some of the usual suspects that thrive in the hills around Silicon Valley, like sagebrush and manzanita trees, are unsuitable because these plants have adapted to survive wildfire by burning quickly.
She also pointed out that a garden mainstay, wood mulch — so important for helping plants retain moisture — can pose a danger if used too closely to the house, or spread too thickly. Rendler said that a maximum of three inches of wood mulch should be used, and only in the outer zones.
Both Rendler and Hunt noted some additional advantages in using defensible space.
Rendler said that keeping that area closest to the house free of vegetation also makes it unattractive to pests like rodents, with the extra fire safety bonus of keeping pests' flammable nesting materials away from the foundation.
Additionally, Hunt said that defensible space can offer important access for firefighters if a fire is directly threatening a home.
"The goal of fire-safe or fire-resilient landscaping is to keep it low, keep it cool and slow. 'Low and slow' is what the firefighters say — that's how they want the fires so they have the chance to get in there and start putting them out. You've slowed it down by spreading out the fuel, you've kept it cooler because there's not that much fuel, and then they can get in there and fight it," Rendler said.
— Find further resources on wildfire preparedness from CalFire and a map of wildland-urban interface areas on the City of Palo Alto's website.
— If you're living in an area designated as wildland-urban interface, you and your neighbors can pursue a "Firewise community" designation to get help in preparing your properties to protect against wildfire. Visit firewise.org.
— Learn about defensible space, plant lists or request a fire-safe presentation at the Santa Clara County FireSafe Council website: sccfiresafe.org.
• The Palo Alto foothills is at risk of a 'catastrophic wildfire.' Here's how the city hopes to reduce the threat.