Useful Definitions

Aspect: The direction of the slope. Generally, a south or southwesterly aspect presents the greatest hazards from wildfire. The aspect determines the intensity of solar heating and rate at which wildland fire fuels might be expected to dry out during the fire day. Aspect also affects what grows naturally on a slope. 

Canopy: Woody upper layer of a forest formed by mature tree crowns.

Conifers: Woody plants. Typical conifers include cedars, Douglas-fir, cypresses, junipers, redwood and pines. There are over 600 living species within the conifer family.

Crown Fuels: Crown fuels are what are above the stem of the plant – the branches, twigs, needles, and leaves, often called the overstory. Separating crowns from each other gives each plant its own piece of the sky. Cleaning the crown of dead and dying vegetation is important in keeping potential flame height and ignitability low.

Deciduous: Shedding or losing leaves annually – the opposite of evergreen.

Dominant Vegetation: Vegetation type determines fire behavior – the flame length, intensity, and rate of spread – and can be predicted through measurement of fuel type.

Ground Fuels: The low ground fuels are grasses, dead branches on the ground, shrubs and brush. This is often referred to as the understory. Removing ground fuels cools a fire and disallows its spread.

Hardscaping is the use of rock and other non-combustible hard surfaces such as concrete sidewalks, brick patios and asphalt driveways.

Home Ignition: There are three forms of ignition:

  1. Direct flame impingement (convection), in which flames overwhelm a structure. This results from having no defensible space.
  2. Radiant heat, when objects nearby are so hot that they provide sufficient heat for the home to burst into flames. This possibility is greatly reduced as space between fuels is increased.
  3. Embers, which can come as a blizzard and ignite any available light fuels such as needles or dry vegetation.

Ignition-Resistant Construction (fire hardening):  These construction methods or components increase exterior building ignition resistance to a wildfire. Non-combustible roofing materials, double window glazing, vents with minimum openings, fire-resistant siding, and other materials will help to reduce structural flammability. Building or remodeling with these materials adds to overall survivability when done in combination with vegetation management measures for defensible space.

Ladder Fuels: Fuels that provide vertical continuity between layers of vegetation, thereby allowing fire to travel from surface fuels into the crowns of shrubs and thence into trees. Ladder fuels initiate and assure the continuation of crowning (crown fire) during wildland fires.

Overstory: The upper crowns or canopy of a forest.

Prevailing Wind: Many wildland fires are wind-driven. Knowing the direction of prevailing winds and how they might behave is crucial for wildland fire protection planning. Typically, in the late summer and fall, hot, dry northeasterly winds occur. Northeast winds have been a major factor in virtually every California firestorm.

Pyrophytic Species: Literally, “fire loving” vegetation which is adapted to or which contributes to rapid burning, high heat output, and ember creation. Many brush species – such broom, manzanita, coyote bush, and juniper – are highly flammable and burn with an oily heat. Anything that smells when crushed has oils which volatize and burn readily, including bay, fir and eucalyptus trees.

Shrubs: Woody plants. Distinguished from a tree by its multiple stems or lower height, usually less than 15-20 feet tall. 

Slope: The steeper the slope, the greater the rate of fire spread. The critical slopes are those downhill of the structure because these pose the greatest threat. Slope is measured as rise over run. Thus a 20 foot vertical drop in 100 feet of horizontal distance is a 20% slope.

Understory: The area of a forest that grows in the shade of the forest canopy.

Wildlife Urban Interface (WUI): The wildland/urban interface is any location where a fire can spread from vegetation (wildland fuels) to buildings (urban fuels), resulting in multiple house fires that overwhelm fire protection efforts.

Information Resources

Acknowledgements and Appreciation

Some of the content for this electronic brochure was adapted from a 2005 publication, “Living With Fire – A Guide for Homeowners,” written by Ed Smith at the University of Nevada Cooperative Extension, with the assistance of Sonya Sistare, Lucy Walker and Kirah Van Sickle.

Special Thanks to:

Butte County Fire Safe Council 
CAL FIRE/Napa County Fire Department 
Darren Drake, Fire Marshal, City of Napa Fire Department 
Don Gasser, FireSmart Defensible Space 
Monty Dolack Gallery, Missoula, Montana 
Orange County Fire Authority
Peggy Delaney, Orange County Fire Authority
Pete Martin, Fire Safe Sonoma 
Roger Archey, Roger Archey & Associates 
The Napa Communities Firewise Foundation 
USDA Forest Service 
360 Fire Protection

And a host of fire service personnel who continue to add to what we know about wildland fire.

This website is intended to be a dynamic document. We encourage you to share your defensible space photographs, success stories and best practices. Contact us at

Plant Glossary

ACACIA is a non-native tree or shrub that comes in many forms. Species commonly planted in Napa are often highly flammable, seed profusely, and also sprout. They can be beautiful yellow in springtime, and many have large pods in the autumn. It must have full sun to live, and will grow quickly, producing many offspring.

Acacia can be removed by cutting and then removing sprouts and seedlings in subsequent years, but roots can be very deep (acacia came from dry climates). Many acacias can be managed as single stem trees, and kept with a crown well off the ground, which would lower flammability as long as sprouts are controlled.

BAY also known as Laurel, Pepperwood, or Oregon myrtle, is a native tree that grows in generally moist places on the west coast. Its leaves are aromatic and used in cooking. Because of the high oil content, the leaves are very flammable, particularly if dried out. The tree has high protein content and is a preferred deer feed in some locations, which helps to keep the plant under control. The leaves are allelopathic (want to be left alone) and will inhibit the growth of other plant species.

Older tall stands of bay trees often have little understory, and also low flammability if there are no dead ground fuels underneath the canopy. Once cut, the stumps sprout profusely and the low branches and leaves are very flammable, often torching if ignited. Management of sprout clumps requires either periodic removal of

sprouting clumps or cutting and management to a single stem while also removing surrounding ground fuels. In a few years with both natural and human pruning, the bay can grow into a tall shade tree with relatively lower flammability. Seeding of large trees is prolific, but repeated pulling of seedlings will deplete the viable supply quickly if the overstory trees have been removed.

BROOM plants – Scotch, French, Spanish, etc. – came to America from Europe and are invasive perennials that spread easily and grow six to ten feet tall. They are extremely flammable due to oil content. Often beautiful in the springtime with golden flowers, they are abundant seed producers and should be removed prior to seeding. Broom readily colonizes open grasslands, coastal plains, mountain slopes, and disturbed areas such road cuts and managed land. Seeds mature June through July.

One approach for removal is to annually pull the plants prior to seeding or flowering. Do not allow these plants to seed as they have a persistent seed bank that will require ongoing removal. The entire plant with root system must be pulled since it readily resprouts. This is a plant that will need continual revisiting until the seed bank is exhausted, but will get easier with time as the roots decrease in ability to support the plant.

CHAMISE is a native shrub that grows on shallow soils and forms extensive fields, often mixed with other brush species. It both sprouts and seeds prolifically and is aggressive in establishment. It needs full sun in order to grow. Young plants are palatable to deer and goats, but older stems tend to get woody and have low value as food, and these older plants create a very hot fire when ignited.

Chamise removal is best done by cutting off the above-ground portions. The plant has a deep root and will re-sprout, necessitating a re-cutting the following spring. If done at the right time, the roots will be starved of nutrients and will die. It may take two such cuttings to be successful in complete removal of the plant.

CHAPARRAL is a native plant community well-suited to our Mediterranean climate. It consists of many species of low shrubby plants (manzanita, coyote brush, and numerous brush species) that can grow very dense. It houses numerous bird species but can become impenetrable to mammals. Chaparral is perhaps the most fire-prone vegetation to be found, and burns with a fearsome intensity. Chaparral can return very quickly after fire, through both prodigious seeds and sprouting. It should be kept far from any buildings, since it can generate tremendous heat very quickly if ignited.

COYOTE BRUSH, also called greasewood, is a dense multi-stemmed native shrub that is aggressive in colonizing open sites. Its white flowers are welcome in January, but by July the plant is dry, oily, and ready to burn. Older plants tend to develop dead material within the crown of the plant. This plant both seeds and sprouts readily, and growing in the open can easily become a ten foot tall shrub within ten years.

Coyote Brush is deep rooted, and repeated cutting of the plants will be necessary. It is palatable to goats but resprouts with vigor. Timing pruning in spring to rob the roots of carbohydrates may make it easier to kill permanently. Those plants that seed in and grow during the fall can be pulled out (with a weed wrench) before they become established, particularly easily done when soil moisture is high very early in the season (i.e., by January).

EUCALYPTUS comes from Australia and is among the worst of the pyrophytes, having shaggy bark, oily leaves and branches, with very rapid growth and development of debris. In addition, the leaf can lift easily into the wind, and cast burning embers for great distances. It both seeds and sprouts prolifically and is very difficult to remove. Its flowers contribute to allergies and its roots are allelopathic, disallowing all other species in the vicinity (except for poison oak).

Eucalyptus produces hot-burning firewood, one of its few beneficial effects. Because of its rapid sprouting and tall growth, it has fueled many wildland fires and made many more difficult to suppress. It should be kept well away from structures as it can toss embers for hundreds or thousands of feet. If it is to be kept, it requires extensive cleaning at least annually to remove excessive peeling bark, dense litter of leaves, twigs and seeding pods. Removals of big trees are expensive and need follow-up for years to keep sprouts under control. Both keeping and cutting this flaming invader require annual diligence (the author is still cutting sprouts from trees initially cut three decades ago).

FIR Douglas fir grows extensively in the higher areas of Napa, particularly on east- and north-facing slopes. It is an ecological climax species, and promotes itself through prolific seeding. It can be seen in each of its sizes in dense stands, and many pole-size stands are heavy with dead and dying trees, since it tends to successfully sprout in much greater density than can be sustained over time. The natural forest sows seedlings of its own death, since these dense forest stands tend to be totally consumed in stand-replacing fires every century or so. Many of the huge fires in Napa County (1890, 1923, 1964) which destroyed thousands of acres and many homes saw embers carried into the fir crowns, spreading rapidly over the landscape through crown fire.

Fir does not resprout and it is easy to get rid of the worst of the fire dangers by cutting the dead and unhealthy trees. Thinning the remaining trees to space them out will reduce the ability of fire to spread, while pruning the branches of the residual trees to an 8 to 10 foot height will markedly decrease the flammability of the forest. Care must be taken to remove the dead branches from within the forest stand.

One of the worst impacts of this species is to overtop and kill the hardwood trees which over time are shaded out by these tall trees. Madrone and oak do not do well in fir shade, and throughout the Mayacamas Mountains fir is overtopping, overtaking, and killing the hardwood forest. Those wishing to retain the less-flammable hardwoods are encouraged to remove fir trees before they become huge flammable problems with a dying forest underneath.

JUNIPER is a coniferous pyrophyte that grows easily and well in most environments. There are many species, some low shrubs and some growing into trees. Juniper is often used as a quick ground cover, since it grows fast, acts as a visual screen, and is easily cared for. Its berries attracts birds and mammals. However, beware this plant, which develops great volumes of dead litter underneath, and whose foliage is highly flammable. No juniper should be planted within 30′ of a structure, and preferably much further.

MADRONE is a hardwood native to the west coast. Its bright orange bark, white flowers and red berries make it attractive to have nearby. It is moderately flammable, and needs management in order to be sustained in the landscape. Susceptible to fire and a host of diseases, many madrones are actively dying and dead parts are contributing significant fuel loads where madrone exists.

Madrone is not tolerant of shade and unless provided with full sunlight can be expected to fade in vigor over time.

MANZANITA comes in a variety of forms. It has a large number of species and is related to madrone, easy to recognize by the bright smooth orange or red bark. It has abundant oils and tends to become twiggy and dry with a lot of dead material in the lower crown. Some species may be only 12 inches high, while some are over 15 feet tall. Some resprout after cutting and some do not. A local nursery should be able to advise about the desirability of each species.

All manzanita can be highly flammable, with rapid burning potential. A dense hillside of this is terrifying when flaming. Many sprouting species can be very difficult to permanently remove, since they have deep roots with great vigor in few shoots. In addition, many manzanita species have seeds triggered by fire, so a profusion of seedlings can follow a fire. This plant should not be allowed near structures; further away, single specimens with 20 feet of non-flammable terrain in all directions may be kept, as long as they are maintained and all dead vegetation is removed.

OAKSare Napa’s hallmark species, and Napa County is home to more oak species than any other county in America. The oaks range from towering heritage trees to small and barely significant scrub oaks. The deciduous oaks are less flammable than the live oaks (which are evergreen). Some of the oak species are low-growing and difficult to protect from fire due to their low profile and inability to separate ground fuels from canopy fuels. All parts of all trees are flammable, particularly when dry.

It is desirable from a number of standpoints to keep oaks in the landscape and only thin or prune oaks to prevent fire spread. Oaks are extremely important for wildlife. Oak stands are often different on neighboring properties, since regeneration has been aided by the lack of wildfire, while grazing of oak woodlands has contributed to the loss of oak regeneration.

Many oaks actively resprout following pruning, removal cutting or fire, and continued diligence is needed to create a landscape which is healthy and growing, but kept fire safe. The wood is among the best for burning, and it is best to remove cut or fallen materials from the landscape near homes to avoid feeding a wildfire. Some oaks are susceptible to Sudden Oak Death (live oaks, tanoak, black oak), and rapid tree failure and breakup may follow this disease.

PALMS deserve a special mention because they are planted throughout the Napa area despite being non-native. The reason they are of concern herein is the tendency for dead fronds to develop over time either high in the crowns or low in the skirts. Because they are difficult and even dangerous to work in, they are often left with highly flammable dry parts which can easily catch flying embers and become torched. Annual removal of the dead parts is the only remedy to counteract fire tendencies, unless the entire plant is removed.

PINE trees are native almost worldwide. The leaves consist of bundles of needles which may number 1 to 5 held together by a fascicle. While not deciduous, many pines lose great quantities of needles in the fall, which contributes to the accumulation of light flashy fuels at the worst time of year. In addition, pines are pyrophytic, and many depend upon periodic fire for the survival of the species. Pine tends to be heavy with pitch and have flammable bark, cones, and branches. They do not resprout, but seeding success can be prolific and they can often be found in thickets.

Pines should either be removed with a low stump, or left and pruned high into the crown. Trees need to be separated from each other and should have perhaps 10 feet between crowns of other trees, with no understory fuels. Needles should be raked for at least 30 feet around houses, further if the pine is downslope from the house. All dead branches and twigs (more than 1/2 inch in diameter) should be removed within 100 feet of structures.

POISON OAK is not flammable, but is ubiquitous in the wildlands of Napa County. It is often the reason many people avoid doing work in defensible space. It is most often obvious by remembering “leaves of three – let it be.” Unfortunately, it is most contagious when it is least noticeable, in January and February, when it may appear as only a slightly purple stem with no leaves. The stem tends to extend and crawl just underneath the soil ground surface to arise in new places. It can be killed by a diligent digging out of the roots, but otherwise will sprout evermore, ready to do its itchy worst.

Having fought it for over three decades (unsuccessfully), the author has some advice: Find some unsusceptible person (there are a few) to remove it for you. Otherwise, go after it hard after liberally applying Tecnu (, cutting wherever visible and digging where possible, leaving it on the surface to dry out. Go inside and take a bath with Tecnu, and keep the clothing in a bag to go into the washing machine without more contact. Return to the site with a shovel after several weeks and dig out the roots that have resprouted. Remember that tools and clothes and body will once again be saturated in oils, but that once washed off it will not spread further.

REDWOOD – Non-Pyrophytic Species Redwood is considered a non-pyrophytic species, due to its verdant crown, thick bark, and moist growing environment. It can burn, however, as evidenced by the large fire scars that can be found in native forests. Active in sprouting from the stump, redwood is commonly found in clumps or rings that surround the original tree stump. Redwood does produce large quantities of needles and branches which can dry out, making removal of ground fuels a necessity from time to time and also demanding pruning of the lower branches until a clear, fire-free bole is created.