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Courtesy of http://cecalaveras.ucdavis.edu/land.htm

It is costly to live safely in the fire-prone hills of California. Control of chaparral is a major expense, and a hillside, whether landscaped with native shrubs or other ornamentals, requires maintenance that includes water, since few ornamental plants can survive on our normal rainfall. However, you can choose plants with drought-tolerant characteristics and those that require minimum maintenance.

Before any discussion of the reduction of fire hazard with proper plant materials, this point should be made clear: THERE IS NO SUCH THING AS A PLANT THAT WILL NOT BURN. THE TERM "FIRE RESISTANT" HAS BEEN USED AND MAY BE MISLEADING. ALL PLANTS WILL BURN IF THERE IS ENOUGH HEAT AND OTHER CONDITIONS ARE RIGHT.

Plants on California hillsides grow mostly during late winter and spring, when most rainfall occurs and after air temperatures are frequently in the 60ís and 70ís. Growth slows or ceases during the long, rainless summer months, and both dead and living plant materials become increasingly dry and flammable.

Unwatered landscapes increase the fire risk except in places where there are succulent ground covers and groves of certain cleanly maintained trees. However, many native California plants can be used successfully in the unwatered landscape. The important thing to remember is this: The more a plant grows and the larger it becomes, the more potential fuel it produces and, consequently, the greater the fire hazard it poses. When a plant is green and growing vigorously, its potential as a fire hazard is small. As its soil becomes dry, however, the plant is able to take up less water and has a lower moisture content. When moisture in the plant decreases, the plant becomes more and more likely to burn. This holds as true for native plants as for ornamentals purchased at the nursery. Therefore, one or two irrigations in midsummer may make the difference between an extremely flammable plant and one which will not burn readily.

During a major fire, any plant may burn, even though some may be slower to catch and carry a fire. However, recent large fires showed that plants on well-watered and well-maintained landscapes did not burn as readily as dry plantings. Tall trees often formed a barrier that prevented flying burning material from reaching buildings. Irrigated ground covers, like ivy or iceplant, usually do not carry fire. Sprinkler systems that can be operated at critical periods can increase the effectiveness of the plant protection. A wide choice of plants can be used in irrigated landscaping. However, caution must be used in placing coniferous evergreens and some resinous plants.

In recent California fires, well-pruned, cleanly maintained, irrigated landscapes held up remarkably well. Ground covers, especially of succulents, generally did not carry fire. Maintenance had a important influence in this, for iceplant did carry fire where litter had accumulated and dead patches occurred. Plantings of Algerian ivy were often scorched or wilted by the heat, but did not burn or carry fire when they had been kept free of dead material. In the watered landscape, consideration must be given to soil erosion problems to determine how much water can be applied to a particular hillside and consequently what plants to use there. Generally, chaparral plants cannot be mixed effectively with plants that require a great deal of water, since many species will be injured or killed by overwatering.

Many native shrubs of California can be used successfully in the unwatered landscape, but unwatered chaparral plants become highly flammable under drought conditions. If used as part of the landscaping, these should be well spaced so that they will be less likely to carry fire. Dense chaparral is especially dangerous when it is growing in a canyon or other location below a house where a chimney draft situation might exist if a fire should occur there. In any case, fire tends to race uphill. Wherever feasible, chaparral should be replaced by a low or discontinuous ground cover that will produce less fuel.

With careful planning, you can prevent your unwatered landscape from becoming a fire hazard, although it will not equal the safety of the watered landscape. The most important step is to minimize the fire hazard. Separate flammable native shrubs by removing adjacent plants. On the remaining shrubs, prune and remove old growth that would spread fire easily. Ask your fire department for local regulations on how far to keep brush and grass from your buildings.

The following ground covers, shrubs, and trees represent only a few of the many excellent plants available for landscaping irrigated hillsides for fire protection. Your local nurseryman can help you with others for special locations. Always keep in mind the need for reducing fuel volume to a minimum while achieving the desired protective and esthetic effects.

Baccharis pilularis prostratus (dwarf coyote bush): Deep-rooted and good soil binder plant that needs some supplemental irrigation. Grows to 18 inches high. Likes full sun. Flowers are not showy. Hedera canariensis (Algerian ivy): Widely used ground cover with large glossy leaves, widely spaced on the stem. Once established, grows rapidly and will not carry fire easily if kept clean and well irrigated. It prefers heat and sun but must have water, since the leaves burn in hot weather if allowed to dry out. Plant on 18 to 24 inch square. Rooted cuttings obtained from nurseryman or from existing plantings will soon spread to cover intervening spaces. Helianthemum nummularium (sunrose): Grows to 1 foot high and flowers in the spring in several colors. Plants are available in flats and gallon cans.

Osteospermum fruiticosum (African creeping daisy): Good hardy cover for banks and erosion control. It has a vigorous trailing growth habit and attractive light green foliage the year round. Plants grow 12 to 18 inches high and bloom profusely in early spring with daisy flowers about 2 inches in diameter. These are adapted to the seashore as well as inland valleys. Plants are available through local nurseries.

Rosmarinus officinalis prostrata (creeping rosemary): Gives a quick, drought-resistant cover. Root system not as deep as some other species. Produces blue flowers which are very attractive to bees. Readily available at nurseries.

Ceanothus griseus horizontalis (Carmel creeper): Low growing, drought-resistant, evergreen shrub with thick leathery leaves and bright blue flowers in the spring. It is used as individual specimens or in clumps of two or three plants. A single plant will spread to as wide as 10 feet. Most nurseries have it. Cistus vellosus (rockrose): Low, spreading evergreen shrub, drought-resistant, attractive purple flowers.

Heteromeles arbutifolia (toyon): One of the best California natives. This drought-resistant plant bears masses of brilliant red berries that remain for many months. It should be used as a large specimen shrub or tree, and is available from most nurseries.

Nerium oleander (oleander): Sturdy, tough, attractive summer-flowering shrub that is very tolerant of drought. The plant grows to a height of 20 feet and may reach a width of 25 feet.

Phamnus alaternus (Italian buckthorn) Large evergreen shrub or small tree with dark green leaves. It bears deep blue berries in the fall, and is extremely drought-tolerant. A limited quantity is available.

Rhus integrifoia (lemonade berry): Native shrub that is very drought-tolerant once established. A limited quantity is being produced.

Ceratonia siliqua (carob): Readily available tree commonly used in landscapes, parks and streets in southern California. It is pest free and tolerant of alkaline soil and drought.

Eucalyptus spp.: Many species of drought-resistant trees, especially adapted to California climate. May be chosen in all shapes and sizes, flower, color, etc. Produces considerable litter and must be well pruned. Should be kept away from structures.
Schnus molle (California pepper): Very drought-tolerant, must be well pruned and produces some litter. Generally available. Schinus terebinthifolia (Brazilian pepper): Grows 15 to 30 feet high and requires little care except occasional pruning to maintain its shape. It requires irrigation and is not hardy where temperatures drop below 20 degrees F. Generally available.
Umbelludlaria californica (California laurel): California native recommended for erosion control. Not all nurseries carry it.
Washingtonia spp. (fan palms): Upright palms which are drought-resistant and well-adapted in milder areas. Old fronds must be removed to eliminate the fire hazard.