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Wildfire Safety
Creating Defensible Spaces Around Homes

by Greg Rubin

Wildfire Safety

Figure 1: After the Witch fire of 2007, the black smudge to the right are all that remains of a rosmarinus prostratus, in contrast to the Eriogonum fasciculatum standing right next to it.

The great San Diego wildfires of 2003 and 2007 taught us a number of surprising lessons. Contrary to popular belief (and hysteria), none of our installation clients lost their homes. This was despite being surrounded by native plants, despite being in the middle of these firestorms, and unfortunately, despite neighbors with non-native landscapes burning to the ground.

Safety in Natives
So what is it about native landscapes that can lead to such fire resistance? For one thing, the plants are hydrated with overhead irrigation throughout the warm months, from early June to mid-October. The amount of moisture delivered is slight; however, it is approximately a quarter inch of equivalent precipitation per watering. That would equate to about 40 minutes on a Hunter MP-Rotator type system. The watering interval on an established landscape is once every 10-14 days, depending on location and exposure. Steep inland slopes may be watered as frequently as every 7-10 days, again depending on exposure. Each watering equates to a summer thunderstorm or fog drip-well within the tolerance range of most natives. The goal is to "dust off" the leaves (dust can actually become a problem on such drought tolerant landscapes) and wet the mulch, but not saturate hot soil. This helps avoid pathogen problems.

This light watering helps with transpiration stress and cooling. Being so drought tolerant, it appears that they hang on to internal moisture even in the face of flames. Other plants surely exhibit these properties; however, it typically requires much less water to hydrate a native plant than an exotic. Figure 1 shows what used to be a planted prostrate rosemary next to a volunteer Flat-top buckwheat. Both were receiving water once every 2 weeks. The rosemary is a black smudge, and the buckwheat, which we tend to think of as a firebomb, is actually covered in green leaves (see Figure 2). Clearly, the rosemary was not receiving enough moisture in contrast to the buckwheat. Ironically, it is often the plants that we think of as firebombs that end up benefitting the most from this supplemental watering.

In fact, it seems that fire resistance has much less to do with plant species than hydration, and that some of the plants considered to be "firebombs" benefit the most from this "thunderstorm level" of irrigation. A study was conducted by the late Bert Wilson of Las Pilitas Nursery where he examined the relative ignition times of various native and non-native plants when exposed to a propane torch. He also notes whether they were hydrated or not. Although admittedly not scientific, his study is fairly unique and useful as a relative measure. Some plants that would ignite in 15 seconds took over a minute once hydrated. Many of the ignition times for natives were far in excess of those for non-natives. Note that Bert was a volunteer firefighter for 14 years.

Wildfire Safety

Figure 2: The Eriogonum fasciculatum is still covered in green leaves, whereas the groundcover rosemary is incinerated. This shows the differing hydration levels in natives vs. non-natives, given the same meager irrigation schedule.

Wildfire Safety

Figure 3: Completely clearing all vegetation (healthy chaparral) for hundreds of feet did nothing to save this home. In fact, it may have made the situation worse. Notice the palms are still standing and alive.

Clearing vs. Thinning
In the panic that followed the firestorm of 2003, many agencies and insurance carriers required that surrounding property be cleared 100, 200, and even 300 feet or more. This meant environmental devastation for huge swaths of land, horrible erosion problems and the establishment of non-native grasses and weeds which become flashy fuels by August. Worse still, many homes that had cleared to bare mineral soil for hundreds of feet still burned to the ground, sometimes surrounded by green lawn and palm trees (see Figure 3). This certainly ran counter to the conventional wisdom that wholesale removal of vegetation should prevent this kind of thing from happening.

Then a thought occurred that awoke the former aerospace engineer in this author: clearing all vegetation around a home actually sets up the perfect conditions for laminar flow. That is, there is nothing to create turbulence and disturb the 80 mph bone-dry winds laden with cinders as thick as the fire-falls of Yosemite. Nothing, except your house. As chaparral-ecologist Richard Halsey explains it, "you have created the perfect bowling alley for embers." On the other hand, low growing, hydrated groundcovers and shrubs can disturb and cool the otherwise uninterrupted flow, while also trapping embers. Leaving some vegetation while planting other irrigated areas may in fact help prevent structures from igniting.

Zoning and Defensible Space
It is critical that the firefighters have a zone around the house where they can safely fight the fire. This is what is known as "defensible space." The first 30 feet is probably the most critical, where a passing fire crew quickly assesses whether it is safe to stop and set up a perimeter or move on. They do not particularly want to lose their lives saving your possessions. This first zone is where you want to have a considerable amount of hardscape - flagstone, boulders, pavers, cement, gravel, etc. Plantings should be low in fuel volume and hydrated with once per week watering. Many native perennials and low growing shrubs would fit the bill here. Try to avoid planting directly under the eaves. Many clients benefit from a 3-4-foot apron here. In Figure 4, the plantings immediately outside this zone and for the first 30 feet are hydrated, low growing, and well-spaced. This home survived both the 2003 and 2007 fires.

Zone 2 is the 30 to 100-foot space (this may extend up to 300 feet if your house is located on a ridge or at the end of a north or east facing box canyon). Figure 5 shows another view of the same house where the first 30 feet of well-watered plantings are enclosed by a low rock wall. Outside of that perimeter are planted native groundcovers that are irrigated about every 10 days in the summer. In this particular case, a road was constructed around the house approximately 100 feet away. This "country lane" actually doubles as a fire-break which gives access to the firefighters.

Wildfire Safety

Figure 4: The 8-foot decomposed granite apron around this house has helped it withstand 2 major fire events in 2003 and 2007.

Wildfire Safety

Figure 5: Most of the landscape within 30 or so feet of this home is hardscape and hydrated plantings protected by a wall. The next 30-100 feet are landscaped in hydrated natives, and then a road was constructed around this perimeter that doubles as a firebreak. Note the metal roof, which prevents embers from igniting the structure.

If there is existing chaparral growing in Zone 2, thin it by about 50%, because this actually removes about 70% of the fuel volume (See Figure 6 and 7). Clear cutting or bulldozing only creates more problems. Thinning implies cutting the shrubs to the ground, not removing by the roots. This is to prevent further erosion and soil disturbance that brings up even more weeds. Chamise, Laurel Sumac growing near the house, and maybe some buckwheat and sage are targeted first. Plants like Manzanita, Ceanothus, Prunus, Rhus, and cneoridium are usually preserved, although dead wood is removed. It is advisable to prune up and open their structure when possible. All trimmings are mulched and placed back on the areas that have now been opened up, to help suppress weeds. This is also an opportunity to lace the area with 4-5-foot-wide paths that double as fire-breaks and which further open up the vegetation. One can bring in benches, bird baths, low fuel-volume perennials, signage, and other features to transform once impenetrable chaparral into an inviting, mature native landscape (Figure 8). Clearly, the environment does not have to be destroyed in the name of fire safety.

Maintenance Considerations
Site hygiene is everything when it comes to fire safety. Non-native weeds are typically annuals and perennials that are dead or dormant by August. They tend to be rich in lignin which means their dry, dead carcasses sit on top of the soil, having robbed the system of nutrients and moisture. This is opposed to wildflowers that usually get reabsorbed into the ecology after death to the point that there is little evidence by summer of the previous spring's show. Unlike native chaparral that tolerates intense but infrequent fires, these invaders welcome and promote frequent burning. It is therefore essential that they be controlled and removed. Most native plant communities actually possess a natural weed inhibition if left undisturbed; however, the thinning process is a disturbance that allows for infection by weeds.

If left to their own devices, weeds will severely compromise the ecology of native plant communities. They act as fire ladders into the remaining native shrubs and trees that are now weakened and even more fire prone. This is the worst of all possible situations - an unhealthy plant community depleted of its moisture and full of the driest tinder so flashy that acres can go up simultaneously. This is what leads to desertification and is unfortunately, happening in California.

Wildfire Safety

Wildfire Safety

Figures 6 and 7: Before and after photos of properly thinned chaparral. The canopy is reduced by about 50%, which also removes about 70% of the fuel volume. The trimmings are chipped and placed right back as a mulch, which helps discourage weeds (ladder fuels) from forming.

Controlling annual weeds can be a challenge. Certainly, the redeposition of the mulched tree trimmings helps. Hand pulling may be enough when the weed loading is low enough to permit it. However, with a typical seed bank of 10 to 100 thousand seeds per cubic foot, post and/or pre-emergent chemical treatment may be required. Whatever method is chosen, it is essential that the site be maintained in a clean condition once it has been opened up.

Another important maintenance step is continuing to keep the site at around 50% coverage. Trees should be pruned up 6 feet wherever possible. Lower perennials and shrubs should be held to 18 inches when practical. A good rule of thumb is to provide clearance between tree limbs and groundcover that is a minimum of 3 times the height of the lower plants. All dead wood needs to be removed. In addition, most of the plants, like Chamise, that have been "stumped" will regenerate from basal burls. Such plants can be allowed to grow for up to 1 year; however, they will have to be stumped back again once their newer green growth starts to become woody.

If Zone 2 is devoid of naturally occurring vegetation and is instead planted in irrigated natives, the maintenance should be fairly straightforward. The plants should mostly be lower growing (under 18 inches) and spaced for final size. This prevents plants growing on top of each other and forming a woody thatch. Well consolidated shredded redwood bark (gorilla hair) is the mulch of choice and is usually quite effective at controlling annual weeds, especially when combined with pre-emergents after planting.

Wildfire Safety

Figure 8: A mature native landscape carved from impenetrable chaparral. Adding bird baths, benches, and other features enhance the experience, and path double as fire breaks.

Firewise Planning and Planting in Zones 1 and 2
Zone 1 must be irrigated, ideally with overhead irrigation once a week. There should be lots of hardscape (flagstone, interlocking pavers, decomposed granite, gravel, etc.), including an apron of some type that extends beyond the eave line. There are a number of native plants that will both tolerate this frequent watering and provide low fuel volume. Some nice evergreen shrubs would include lower growing manzanitas like Arctostaphylos 'Carmel Sur,' 'Radiant,' 'Emerald Carpet,' and 'Pacific Mist,' as well as medium manzanitas like 'Sunset' and 'Howard McMinn'. Lower growing garden tolerant wild lilacs would include Ceanothus thyrsiflorus repens 'Blueblossom,' 'Anchor Bay,' and 'Heart's Desire'. Native perennials that could tolerate these conditions would include 'WR,' Matole river fuchsia, and Goldenrod. Monkeyflower may be shorter lived under these conditions but will certainly put on a show for the 2-5 years it survives (just get a new one if it dies). Rocks and Gorilla Hair can be used for mulch, but the bark must be watered down and consolidated immediately after planting.

Zone 2 ideally consists of either thinned chaparral or lightly hydrated native plantings. Baccharis 'Pigeon Point,' Ceanothus 'Yankee Point,' Arctostaphylos 'John Dourley,' and Iva Hayesiana are all excellent choices if this area is to be planted. A smattering of larger shrubs, like Ceanothus 'Blue Jeans,' 'Concha,' Rhamnus 'Eve Case,' 'Mound San Bruno,' and Heteromeles arbutifolia are all fine as long as they are situated in groups of 3 or less with about 10 feet between groups. It is a good idea to incorporate lots of trails in this area of at least 4 feet in width. Fully established Zone 2 plantings must be irrigated about once every 8-14 days during the warm months with overhead irrigation in order to promote adequate hydration. The possibility of lightly irrigating existing chaparral in Zone 2 (wetting leaves and mulch, not to saturation) is being investigated.

Fire in Southern California is an unfortunate inevitability; however, homeowners can create defensible space around their homes that avoids wholesale environmental destruction. Proper hydration of landscape plants, utilizing a large proportion of hardscape within the first 30 feet of structures, low growing, low fuel volume, and regularly irrigated plantings are recommended to keep homes defensible in the event of a fire.

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July 17, 2019, 8:32 pm PDT

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