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.

Clearing vs. Thinning

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.

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.

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.

Conclusion
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.