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Defend your home against fire with these landscaping techniques

by Timber Press on March 21, 2013

in Design, Gardening, Regional

Well-maintained, irrigated native plantings and ample hardscape create defensible space around this home. Photo: Saxon Holt

Well-maintained, irrigated native plantings and ample hardscape create defensible space around this home. Photo: Saxon Holt

Wildfires are increasingly common in many parts of the United States, especially in California. Authors Greg Rubin and Lucy Warren assert that growing native plants diminish the impact of fire on the landscape and our homes. The following selection is from their book, The California Native Landscape.

Defending your home against fire

Vegetation often has nothing to do with whether or not a structure burns. Some of the most important steps you can take to defend your home against fire involve the architecture of the house itself. Important firedefense areas include roofing, vents, eaves, windows, and secondary elements that immediately contact your house, like shade structures, decks, and fences. Maintaining low-growing, hydrated groundcovers and shrubs disrupts and cools the otherwise uninterrupted flow of fire. Expanses of clear, mineral soil may not be defensible. Allowing thinned and maintained natural vegetation to remain, in addition to irrigated landscape plantings may, in fact, help prevent structures from igniting.

Low-density, fire-resistant landscaping within 30 feet of a home. Photo: Greg Rubin & Lucy Warren

Low-density, fire-resistant landscaping within 30 feet of a home.

Fire Defense Zone 1

It is critical that firefighters have a perimeter or zone around a house where they can safely fight a fire. This area is known as “defensible space.” The first 30 feet is probably the most critical. A passing fire crew quickly assesses whether it is safe to stop and set up a perimeter or to move on. You want to have a considerable amount of hardscape—flagstone, boulders, pavers, cement, gravel, etc.—in the first zone. Avoid planting directly underneath the eaves of the house. It is prudent to create an apron about 4 feet out from the house simply covered in gravel or concrete. Just these aprons alone have been proven to save homes during wildfires.

Plants near the house should be either lower growing or have an open “see-through” structure to limit potential fuel for a fire. They should be hydrated with once-per-week watering. Most conventional landscape plants are appropriate. However, many native perennials and low-growing shrubs would work well in this area.

Thin natural vegetation within 100 feet of a home. Paths and features carve out a mature native landscape.

Thin natural vegetation within 100 feet of a home. Paths and features carve out a mature native landscape.

Fire Defense Zone 2

Zone 2 is the area 30 to 100 feet from your house. Zone 2 may extend up to 300 feet if your house is located on a ridge or at the end of a north- or eastfacing box canyon. If there is existing chaparral growing in Zone 2, thin it by about 50 percent. This one step removes about 70 percent of the fuel volume. Clear cutting or bulldozing only creates more problems. Thinning may involve cutting the shrubs back as much as to the ground, but not removing the roots. Thinning prevents further erosion and soil disturbance that encourages even more weeds in the future.

Open up plants’ structure whenever possible by pruning lower branches. Mulch all trimmings and then, to help suppress weeds, return the mulch to the areas where the plants were thinned out. This is also an opportunity to lace the area with 4- to 5-foot-wide paths that double as firebreaks and further open up the vegetation. Bring in benches, birdbaths, non-woody perennials, signage, and other features to transform once-impenetrable chaparral into an inviting, mature native landscape. It is not necessary to destroy the environment in the name of fire safety. There are many creative, aesthetic landscaping “solutions” that lower the risk of fire danger.

Maintenance Considerations

Good site maintenance is critical for fire safety. Non-native weeds are typically annuals and perennials that are dead or dormant by August. They are rich in lignin. Their dry, dead carcasses sit on top of the soil. Compare this to wildflowers, which are reabsorbed into the soil after they die; by summer there is little evidence of the previous spring’s show. Non-native weeds welcome and promote frequent burning. Responsible maintenance dictates that they be controlled and removed.

Most healthy and undisturbed native plant communities, by virtue of their specialized and finely adapted ecology, do not  support the growth of non-native annual weeds. The litter layer or mulch formed by these natives kills most weeds. Additionally, most of the native plant community’s nutrition is contained in the mycorrhizal fungi and is not available to the weeds (which are usually non-mycorrhizal). Disturbing the plant community opens up the canopy and makes nutrition available to invasives.

Left to their own devices, weeds will severely compromise the ecology of native plant communities by robbing moisture and nutrition from the system. Worse, they act as fire ladders into the remaining native shrubs and trees 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 highly combustible dry tinder. Death of annual weeds by summer’s end leads to desertification. Humidity levels drop in the weedy areas, because no moisture is being held onto in living tissue.

This homeowner completely cleared all vegetation for hundreds of feet, leaving the home exposed to fast-moving embers in a firestorm and erosion. Photo: Richard W. Halsey

This homeowner completely cleared all vegetation for hundreds of feet, leaving the home exposed to fast-moving embers in a firestorm and erosion. Photo: Richard W. Halsey

Controlling annual weeds can be challenging. Using tree trimmings for mulch helps. Hand pulling may be sufficient in small areas. However, with a typical seed bank of 10,000 to 100,000 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 as weed free as possible once it has been opened up.

Keeping the canopy coverage pruned to around 50 percent is important for fire safety. Whenever possible, prune trees 6 feet up from their base. Keep shorter perennials and shrubs pruned to a height of about 18 inches when practical. A good rule of thumb is to provide clearance between tree limbs and groundcover (shrubs, perennials) of at least three times the height of the lower plants. Remove all dead wood. Most plants that have been cut to the ground annually will regenerate from basal burls. Allow the plants to grow for up to one year, then cut them to the ground again once their newer green growth starts to become woody.

If Zone 2 (30 to 100 feet from the house) is cleared and planted in irrigated natives, the maintenance routine is straightforward. These plants should be lower-growing (under 18 inches) and spaced for final size. This prevents overcrowded plants from forming a flammable woody thatch. Mulch with shredded redwood bark (gorilla hair) matted down with water so that it is poorly aerated. Thickly applied mulch is effective at controlling annual weeds, especially when combined with hand weeding, or with pre-emergents in large, hard-to-maintain areas where hand pulling is not practical.

After a 2007 fire, this native slope was scorched but not burned, saving the wooden deck at the back of the house.

Three years after the fire, the native slope has completely recovered. Photo: Greg Rubin & Lucy Warren

Three years after the fire, the native slope has completely recovered.

Fire-wise planning and planting

Zone 1 must be irrigated, ideally with overhead irrigation at least once a week. This ensures that the plants are always hydrated and less likely to burn. Install lots of hardscape (flagstone, interlocking pavers, decomposed granite, gravel, etc.), including an apron of these same materials extending beyond the eave line. Numerous native plants will both tolerate this frequent watering and provide low fuel volume. A good decorative mulch is 6- to 12-inch boulders placed on the plants’ rootballs surrounded by gorilla hair, but the bark must be watered down to compact it immediately after planting.

Zone 2 ideally consists of either thinned chaparral or lightly hydrated native plantings. Plant them in groups of three or less to prevent creating a large fuel mass. Leave about 10 feet between these small groups of larger shrubs.Create small firebreaks by incorporating lots of trails or paths at least 4 feet wide in this area. Fully established Zone 2 plantings must be irrigated with overhead irrigation about once every 8 to 14 days during the warm months to promote adequate hydration. It may be possible to lightly irrigate existing chaparral in Zone 2 as long as the natural soil is not saturated. Test results are pending.

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This thoughtful guide tells you which plants will perform best and how to use them most successfully. —Susi Torre-Bueno, Former president, San Diego Horticultural Society

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