FIRE-RESISTANT LANDSCAPING: a GENERAL APPROACH and CENTRAL COAST PERSPECTIVE by Suzanne Schettler
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FIRE-RESISTANT LANDSCAPING: A GENERAL APPROACH AND CENTRAL COAST PERSPECTIVE by Suzanne Schettler hen planning a land- determined that the temperatures within. As the Basin Complex Fire scape in a rural-resi- had reached 3,000 degrees Fahren- in Big Sur approached a home in dential area, it is a natu- heit. I was shown a blob of melted 2008, the owners moved their im- ral impulse to consult aluminum, small enough to hold in portant possessions into a metal stor- Wa list of fire-resistant plants and se- the palm of a hand, that was all that age container on the property before lect only those that are deemed fire- remained of an extension ladder. they evacuated. When they returned, safe. Consulting more than one list Some comparative temperatures the house was untouched but the can uncover a problem, however: may be of interest. Paper ignites at storage container held only ashes. lists contradict each other. For ex- about 451 degrees F, depending on Such incidents of internal ignition ample, our native yarrow (Achillea its composition. Lead melts at 400 to happen more often than we think. millefolium) shows up on some lists 600 degrees F, depending on its pu- as fire-resistant and on others as rity. Aluminum melts at about 1,600 WHAT WE CAN DO flammable. Other species show up degrees F, a blast furnace for making in contradictory fashion as well. steel runs at approximately 2,300 In spite of these formidable con- Most lists are a compilation of opin- degrees F, and a kiln for cement pro- ditions, we are not helpless. Second ions, guesses, informed guesses, and duction is fired at 3,000 degrees F. graders are taught the Fire Triangle other people’s guesses. There are no plants and no struc- (see page 18). The three sides of the tures that can withstand such heat. triangle are heat, oxygen, and fuel— At these temperatures, spontaneous take away any one of them and fire THE WILDFIRE combustion ignites a structure from cannot burn. There is little that can ENVIRONMENT Low-growing native plants minimize potential fire hazard immediately adjacent to this Some factors in the fire environ- home. Native irises (Iris spp. and hybrids), sea thrift (Armeria maritima), Phlox douglasii, ment are beyond human control, and low-growing manzanitas (Arctostaphylos spp.) are featured. Horizontal separation between plants is illustrated by the walkway at the left and center, and vertical separation and are almost beyond comprehen- is illustrated at the right. However, as the vines growing up the house increase in size they sion. First, wildfires are extremely could become a fire hazard. The homeowners might want to replace them with low- unpredictable, burning some veg- growing plantings in large ornamental pots. Photograph by landscape architect R. Lutsko, etation while adjacent sites are left reprinted with permission of Pacific Horticulture. untouched. Second, the rapidly ris- ing column of hot air from a wildfire sucks in oxygen to feed the base of the fire, creating winds of tornado speeds. These high winds carry burn- ing embers (firebrands) a mile or more, igniting spot fires in a leap- frog fashion and spreading the fire farther and faster than the actual flame front. Firebrands are not nec- essarily single burning coals; they can be a shower of small embers, resembling Fourth of July fireworks but not organized into pretty pat- terns. And third, because wildfires often occur during especially hot, dry, and windy weather conditions, the temperatures can be extremely high. After the Trabing Fire near Watsonville in 2008, investigators 46 FREMONTIA VOLUME 38:2/38:3, APRIL 2010/JULY 2010 be done to reduce the industrial tem- peratures described above. And oxy- gen cannot be cut off unless a fire is small and can be smothered. The one element we can control is fuel, and vegetation is fuel. We can con- trol the vegetative fuel when we de- sign a new landscape or retrofit an existing one. HOW FLAMMABLE ARE NATIVE PLANTS? Bert Wilson is a native plant Here interest is focused away from the house, which is casting a late-afternoon shadow nursery owner and former fire- across the patio and lawn. The perennial border creates a focal point viewed from the fighter. In September of 2005, near house, rather than framing the view toward the house. Bulky vegetation is farther back, the end of the dry season, he com- blurring the property line. All photographs by S. Schettler unless otherwise noted. pared the flammability of various plants. He placed a one-centimeter fore are more quickly heated through rounding a home can function as a flame of a propane torch in contact to an ignition temperature than are firebreak, comprising low vegetation with foliage of a clipped branch and larger leaves. Conversely, plants with or hardscape. This also becomes a recorded the time to ignition, re- larger, thicker leaves are slower to work area if firefighters are defend- peating several times per plant. Most ignite. Plants with resinous sap are ing a home and are lugging heavy of the native plants he tested were chemically more volatile than plants hoses to protect the structure. growing in the ground and were with watery sap. Conifers (except unwatered. He performed the ex- for redwoods) are generally fairly DESIGN FACTORS periment in a closed barn so breezes flammable. Species that accumulate would not influence the results. dry litter are a hazard. But even these Foundation plantings evolved What he found was not what he guidelines are relatively minor fac- when it was considered unsanitary expected. The results were not con- tors in reducing flammability. for the first floor of a house to sit sistent, either within genus or spe- close to the ground and plants were cies, or with the literature. The more SEPARATION IS THE KEY needed to camouflage a tall founda- drought-tolerant the plant, the longer tion. Thomas Church and other land- it took to ignite. And unwatered na- The question is not “Is a particu- scape architects began moving away tives fared better than watered non- lar species fire-resistant?” What re- from this approach in the mid-1900s. natives; a watered apple and a wa- ally makes a landscape fire-resistant Rather than using plants to frame tered common lilac burned faster is not the species that are planted the view of the house from the street, than many manzanitas. Manzanitas but the three-dimensional geometry they placed the plantings where they as a group were quite variable, and of their placement and the kind of can be enjoyed looking outward from watered samples were not necessar- care they receive. The spaces between the house and patio. This removes ily slower to ignite than unwatered plants, and the space between veg- the bulk of the vegetation (fuel) from ones. Ceanothus species were rela- etation and a structure, are of great near the house, literally turning the tively hard to burn; a light dust-off importance. There should be hori- design inside-out and creating fire by overhead watering every two zontal and vertical gaps in the veg- resistance at the same time. For fire weeks made them really hard to burn. etation. The individual plants should resistance, it is important to gradu- have elbow room, not just when they ate the vegetation, with least volume SOME GENERALIZATIONS are initially planted but when they near the structure. are full-grown. It may be useful to In a fire-resistant landscape, a In spite of the fallibility of lists of consult a knowledgeable horticul- traditional perennial border is not “fire-resistant” or “fire-prone” plants, turist in advance to identify the ma- located at the foundation of the there are some general guidelines ture size of plants in a given soil type house, but is set at a little distance that do relate to species selection. and climate. And bulky vegetation where it invites exploration. This Plants with fine foliage have a high should be positioned away from concept translates readily to our surface-to-volume ratio and there- structures. The area immediately sur- Mediterranean climate. VOLUME 38:2/38:3, APRIL 2010/JULY 2010 FREMONTIA 47 Islands are a good way to create separation in the vegetative fuel. They THERE IS NO SUCH can either form the framework for THING AS A FIRE-PROOF the layout of a new landscape or can PLANT… be retrofitted by cutting broad trails through existing vegetation such as Iceplant (Carpobrotus edulis) burned chaparral, breaking it up into dis- in the Trabing Fire in 2008. Photograph by R. Casale, NRCS. continuous patches. Human-scale is- lands of mounded vegetation visu- …OR A FIRE-PROOF STRUCTURE. ally remind us of natural topography in the larger landscape, so that one has the experience of being immersed in the landscape rather than merely walking over it. Since fire climbs upward, it is important to eliminate fire ladders. A fire ladder exists when there is continuous or nearly continuous veg- etation extending from the ground up to the tree canopy. Care in the placement of shrubs, and maintain- ing substantial vertical gaps between shrubs and trees, can prevent the development of fire ladders. Steep slopes are particularly vulnerable to fire, as fire low on the slope preheats vegetation higher up. Terracing a steep slope makes the site more useable, allows rainfall to (Before): Mike Evans, co-owner of Tree of Life native plant nursery in Southern California, built this cabin over the course of three years. The cabin was carefully soak in rather than run off, and re- designed for fire safety, there was 100' clearance to mineral soil in all directions, duces the intensity of a potential and the forest understory was cleared for hundreds of yards all around.