Fire Wise Plant Spacing Suggestions

Proper placement or replacement of plants is important for any defensible space plan. Through proper plant selection, placement and maintenance, we can diminish the possibility of ignition, lower fire intensity, and reduce how quickly a fire spreads, increasing a home’s survivability.

Recommend separation distances for shrubs:

For areas with dense brush, the recommended separation distance is dependent upon shrub height and steepness of slope.

Note: Separation distances are measured between canopies (outermost branches) and not between trunks.

For example: if your home is located on a 10% slope and the brush is four feet tall, the separation distance would be two times the shrub height or eight feet. The recommended separation distance can be accomplished by removing plants or through pruning that reduces the diameter or height of shrubs (shorter height means less separation is needed.)

Recommended separation distance between tree canopies:

For forested areas, the recommended amount of separation between tree canopies is determined by steepness of slope. If your house is situated on a 30% slope, the separation of tree canopies within your defensible space should be 20 feet. Creating separation between tree canopies can be accomplished through tree removal. Another option is to have a continuous canopy of overstory trees. This approach requires having no understory and adequate pruning of trees.

Fire Wise Plant Choices

All plants – all organic matter – can burn, but the ease of ignition, rate of consumption, and generation of heat is vastly different between species of vegetation. Fuel is variable. 

Far more important than the actual species and its flammability is to be able to recognize and dispose of dead wood, which ignites and burns with much greater ease and intensity than living green vegetative material.

What follows is a list of plants used in landscaping around your home that are labeled either good or bad from the standpoint of fire (only). The lists are not all-inclusive, and many species – both bad and good choices – have been added to our landscapes from worldwide sources. Most of the common native plants are mentioned and more fully described.

Pyrophytic Species are “fire-loving” vegetation which is adapted to or which contributes to rapid burning, high heat output, and ember creation. 

Many brush species – such as 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.

Chaparral species are those brush species which grow in dense, pure or mixed stands and which create impenetrable fields that burn with intense heat and are very difficult to suppress or control.

They include manzanita, coyote brush, buck brush, broom, chamise, mountain mahogany, as well as scrub oaks and other flammable species. Chaparral species are among the hardest to manage and to keep fire safe.

Nevertheless, these can be made fire safe with work. If near structures, all pyrophytic species should be removed, and all dead vegetation removed from the remaining species. What is left should be small trees, and healthy, well-separated selected plants which have had ground fuels removed.  

Resprouting may occur and need to be dealt with annually. Proper attention to timing of repeated removal of sprouts may encourage the roots to gradually lose vigor and die.

Conifers are evergreen trees that have needles for leaves, and most of these have resins and pitch in addition to a tendency toward twiggy growth. Coniferous species should be considered pyrophytic near homes. All can be made less flammable by pruning and removal of dead portions, but conifers tend to grow large and overtop other vegetation, making for potentially long flame lengths, if ignited.

At another extreme are the low conifers – juniper in particular – which contribute to rapid fire spread largely by becoming dense with un-maintained dead and dying vegetative materials that lie in wait for an ember to ignite. In addition to the listed and described species, obvious poor choices near homes include cypress, cedar, all pines and firs.

The one native conifer exception is redwood, which tends to exist in more humid environments and does not have abundant pitch. Redwood can burn, however, as it can quickly accumulate large amounts of debris. Even with low flammability, maintenance and removal of dead vegetative material is key in all vegetation management to prevent wildfire.

Hardwood trees, particularly deciduous species, are less flammable than conifers, with the notable exceptions of non-native eucalyptus, native bay laurel, and a few others. Most local oaks are low in flammability except for the live oaks (which appear evergreen).

The live oaks should be kept in the landscape but pruned away from the ground, with dead material removed to avoid ladder fuels. If surrounding brush species are removed, the fire ladders are also removed and fire safety greatly improves.

This is generally the best course to take in the oak woodland portions of the Wildland Urban Interface (WUI). Excellent trees to have nearby that burn poorly (as long as you keep out the dead materials) are alder, ash, buckeye, cottonwood, maple, deciduous oaks, madrone, and willow.

These should all be maintained free of dead materials and pruned high above the ground.

The following plants are further shown with pictures and descriptions due to their ubiquitous nature. For the most difficult plants, recommendations are made for effective removal techniques.

Replacement Plants

For an existing home, the best way to achieve safety in event of wildfire is to rethink your landscape and to do away with plants in fire-prone locations.  However, many times the removal of plants opens gaps in our landscaping that we may not want or did not anticipate. In such cases, replacement plants may be demanded, for erosion control, visual screens, or landscape enhancement.  In such cases, it is best to use native materials that are drought and deer resistant, in addition to being fire resistant.  This is a tough combination to fulfill, but several choices. which may include some of those attributes, are detailed below. 

The Structure Ignition Zone should have only low irrigated or succulent plants. Such non-natives as ice plants and stonecrops (Sedum spp.), and Crassula edulis actually absorb heat and lower the flame temperature as it nears. Good native perennials to use in this space include Yarrow (Achillea tomentosa) which has pretty yellow flowers, Lupines (Lupinus) with multicolored flowers, monkey flower (Mimulus), penstemon (Penstemon) and some sages (Atriplex sonomensis, A. columbariae).

Ground covers are often used in the inner zones, and there are many good nursery choices that might be desirable.  There are native species which are part of the chaparral vegetation type, which means they are flammable – but due to the low form may be used sparingly.  All need maintenance to remove dead thatch.  These include Bearberry (Arctostaphylos uva-ursi) with white flowers and red berries and dwarf coyote bush (Bacharrus pililaris Var. protata) or ‘Twin Peaks’ which can be grown on near-vertical slopes. Since this is the same species which can burn so hot and fast, only a sparse use of the dwarf form should be done. Squaw carpet (Ceanothus cuneatus) is a native low grower which can tolerate drought once established.

Vines can be used to cover a multitude of sins, but be sure to clear out the flammables before planting Jasmine (Jasminium spp.), potato vine (Solanum jasminoides), cape honeysuckle (Tacomeria capensis). Beware the scenic grasses, which have lovely heads and wave in the breeze – but which become both an ember trap and source once they dry.

In the Fuel Reduction Zone, upright plants may be desired. Native species that may already be existing in the landscape that are less flammable than the pyrophytes include toyon (Heteromeles arbutifolia), redbud (Cercis occidentalis), silk tassel (Garrya sp), buckthorn (Rhamnus sp.), and mountain mahogany (Cercocarpus beuloides). Bush anemone (Carpentaria californica) is an evergreen shrub with brilliant white flowers which grows to 4’-6’ tall and wide. It is native California chaparral species which is a good replacement for juniper, coyote brush, manzanita and other pyrophytes. Still flammable, if kept thrifty this attractive evergreen shrub will add to your landscape while replacing known bad actors.

Wild lilac (Ceanothus griseus horizontalis) has beautiful flowers is drought–resistant, but is favored by deer. Well-spaced low-growing saltbush (either Atriplex glauca, or A. semibaccata), Gazania (Gazania spp.) would help to hold a slope in place. Rockrose (Cistus salviolfolius, C.crispus) can be used sparingly in places where erosion control is needed, if cut back after its large white flowers show. Several plants which are drought tolerant and spreading include winter creeper (Euonymos fortunei, E. radicans), and honeysuckle (Lonicera sp.).

For the outer zones, there are many native tree choices when removing pines, firs, cypresses, bays and eucalyptus. Most deciduous species are lower in fire danger, and good choices are maple, walnut, ash, alder, buckeye, and include species in the Prunus genus. Madrone is a native that is having a difficult time with numerous diseases, but periodically cutting out the dying parts should allow you to keep this lovely native in your landscape.  While live oak is more flammable than the deciduous oaks (valley, blue, white, black) these are best kept in the environment if limbed up and dead parts are removed. Some non-natives that are low in flammability and somewhat unusual include carob (Ceratonia siliqua), pepper (Schinus Spp.), and strawberry tree (Arbutus unedo). Edible choices would include fruit and citrus trees, all needing irrigation and thus low in flammability.

When planting new vegetation, always keep in mind the ultimate size of the plants and be aware of the maintenance needs to avoid fuel build up.  You can grow a fire wise landscape.

General Landscaping Tips

We have discussed the why and the where of defensible space activities, but the what and the when and the how need greater exposition. The homeowner and family may do this work, or work can be contracted.

Planning the work:

Defensible space work needs to be planned with recognition of the labor and dangers involved. While a first pass to pick up sticks and dead materials can be done by anyone, this initial step will undoubtedly need to be followed with tools and personnel.

Most people are not used to this type of activity, although it is often an extension of gardening activity, with many of the same tools.

It is best to plan the work in stages, with each step gaining more open conditions. One can mark the vegetation to be cut or to be left, either in the mind or physically with some flagging. Often this stage allows one to step back to see the next steps more clearly before you commit to action.

After assessing the needs and difficulties, starting with a lopping tool allows one to get rid of the small stems that inhibit movement through the wildland. Cutting at the base (but out of the dirt) will allow for the entire plant to be removed at once, and allow for easy future management of the stump, if needed. Of course, a worker should be able to recognize poison oak as well as the pyrophytes you want to remove.

As the ground begins to get more open, it becomes easier to see what is needed next. Once the space is opened up, a weed whacker may allow for easier movement.

At each stage, the preparation and use of each tool should be done to ensure safe and efficient operation. Debris should not be left in the way, but pulled out in readiness for the next step.

Once the undergrowth is either removed or trimmed to remove all dead branches, the trees can become the focus. Trees to be retained may need pruning with a handsaw. Other trees may need to be cut using a chainsaw.

Preparing for chipping means getting the material to a location next to access and “indexing” the ends. This means stacking the brush to be chipped in a parallel manner, with large end toward the access road, and relatively flush for easy grabbing.

Piles can be as large as 20 feet long and 6 feet high. For additional information about chipping, visit the Napa Firewise Web site at www.napafirewise.org.

Pruning:

Pruning will need to be done on trees that have branches reaching below head height (or so). Breaking the fuel ladders is an essential part of defensible space.

Pruning can be done with hand saws, shears, and/or loppers. Most branches can be cut off with one cut, and larger branches may need three cuts (see inset).

Every pruning cut on live wood is a wound. A branch needs to be cut so to not damage the tree and also to leave no stub. This does not mean making a cut flush with the bark, but instead cutting just outside the branch collar. Cutting in this way minimizes the impact, since branches compartmentalize decay and keep rot away from the main stem.

Many large trees can be disfigured or killed by improper pruning, so be sure of yourself or call a certified arborist before work begins.

The branches to remove are those which are either dead or which have parts which reach below eight feet (or so) from the ground surface. Steeper slopes often need higher pruning. For tree health, never prune more than ½ the height of the tree.

Branches which provide a screen or which are particularly useful or attractive may be left, if the area of the lower tree crown is open and has no flammable vegetation for 10 feet around the area.

Overhead pruning work can be and dangerous. Head and arm protection are essential, while face and neck covering will make falling sawdust a great deal easier to endure.

Pruning done in the spring and summer can attract bark beetles to conifers, while pruning oaks in areas of sudden oak death should be done during the dry months. Constraints and hard dirty work make it easy to ignore the need for pruning, but remember that fuel ladders promote crown fires – the worst of wildfires. The good news is that once done, pruning needs only occasional attention, since the removed fuel will not return. back to top …

Stumps:

Stumps should be cut as low as possible for both aesthetic and for sprouting reasons. If sprouts quickly return and grow, they can be cut almost as quickly.

Landscape as habitat:

While managing vegetation to make your situation more fire safe, you are also altering the habitat for what lives there.

Many times, removing debris piles also reduces the rat population. You also may uncover snakes, squirrels, birds of all types, and certainly arthropods and other small life forms. For the most part, whatever is disturbed will quickly find a new home further away from your areas of highest value.

Remember that removing dead vegetation more than 1/2 inch in diameter is a key to preventing the passage of a hot wildfire.

The forest floor has a life of its own, and generally should be left undisturbed. The duff and litter will turn into mulch with vital nutrients to nourish the forest over time. When chipping, casting the chips on the surface to a depth of 4 inches is a good way to retain soil moisture while also inhibiting weed growth. Deeper piles may build heat as vegetation decomposes and should be spread out to avoid potential ignition.

Workers removing trees and shrubs should be alert to what is living there. If active nests are encountered in trees planned for removal, it is best to cut from August to March to avoid impact to young birds.

Another consideration regarding when to cut may be the activity of insects and diseases. With the threat of bark beetles, the disease Sudden Oak Death (SOD), and many other pests, it may be difficult to find the right time to best fulfill your plan by doing the actual cutting and pruning. All host material for SOD is best left on site where it is.

This is not a time to delay! If concerned, call the Master Gardeners and they can usually answer your specific questions over the phone. They are available M-W-F from 9am-12pm at 707-253-4221 or toll free at 1-877-279-3065.

If working in a drainage where water sometimes flows, it is best to tread lightly and remove only pyrophytes and small dead materials, leaving in place any big wood (more than 12 inches in diameter or more than 4 feet long). The canopy should not be reduced in such places, which usually have higher humidity and lower tendency to burn.

Working on steep slopes (more than 60%) should be avoided, and surface litter should be left to protect against erosion.

It is often recommended to leave big wood on these slopes, removing the smaller dead wood only (1/2 inch to 12 inches in diameter)