The goal of this Web-based reference guide is to provide the citizens of Napa County with a comprehensive, yet easy-to-understand source of information on defensible space planning and practices.
The content of this guide is drawn from official fire service documents; national fire prevention associations and councils; and individual knowledge experts within the fire service, vegetation management and related fields.
This reference guide was produced by the Napa Communities Fire Wise Foundation in May of 2009. User comments and suggestions are welcome – please feel free to contact us at firstname.lastname@example.org.
What is defensible space
Defensible space is a law as well as an approach to vegetation management. It includes an understanding of wildfire behavior and contribution of plants to wildfire intensity. It recognizes the risks posed by wildfire to us by our natural surroundings and our vegetation management choices. Another term used in the context of defensible space planning is the “Home Ignition Zone” or HIZ. The HIZ primarily determines a home’s ignition potential based on the proximity to available fuel types within 100 feet or more from a home.
Defensible space is the area around a house/structure and reaching toward an oncoming wildfire. This is an area where the vegetation has been modified to reduce the wildfire threat and to provide an opportunity for firefighters to effectively defend the house/structure. Simply put, defensible space is a zone where vegetation and fire fuels have been modified to reduce or stop fire spread. The goal is to save homes and lives.
The state law governing defensible space can be found on the CAL FIRE Web site at www.fire.ca.gov. Reference Senate Bill 1595, dated September 2008.
The Napa County Fire Hazard Abatement Ordinance can be found on the county Web site at: www.co.napa.ca.us
The City of Napa 2009 Weed Abatement Ordinance and information can be found on the City Web site at: www.cityofnapa.org
Why is defensible space important
When you consider that 75% of Napa County’s 500,000 acres is considered a high hazard fire environment, and that over 250,000 acres of Napa County have burned over the past 60 years of recorded fire history, the connection to fire preparedness and survival becomes more obvious. Many areas have burned more than once in the last century. We live in the type of environment that has and will support large, intense, and uncontrollable wildfires.
The good news is that there is a way to live with fire and survive. It’s all about planning and taking responsibility for our personal safety and the safety of our neighbors.
For our discussion, we are breaking up defensible space into four vegetation management zones. Each zone is established at a particular distance and is designed to affect the behavior of a wildfire entering the outermost zone. From wildland to residence, we want to reduce the heat output and burning characteristics of the fuels.
Planning & Preparation
The four zones of defensible space
A Structure Ignition Zone is an area that must be made non-combustible within approximately 0 – 10 feet from the structure. This area around the perimeter of the structure is crucial for wildland fire survival.
In this area, only low volume, well-irrigated vegetation should be planted, and extreme care should be taken to keep this area free of all materials that could ignite during or after a flame front (advancing wall of flame).
There should be no fire fuels under decks or windows. This is especially important during the high wind conditions of late summer and fall. It is imperative to patrol for and remove all fuels, such as leaves and pine needles, which tend to build up and become traps for burning embers.
A Firebreak Zone is a radius of approximately 10 – 30 feet from the structure and requires reduction in the volume of the fire fuels (plants, shrubs, trees), and elimination of ladder fuels, in order to create an environment that will not sustain a wildland fire. In doing so, the fire is denied fuel and the effect of heat and flame on a structure will be substantially reduced.
Grasses should be kept low during fire season, and dead vegetation accumulations removed. Mowing and thinning are normally needed annually. If mowing during fire season, work before 10 AM when there are high humidity conditions, low temperatures and no wind.
Many non-pyrophytic (low flammability) shrubs can be in the next zone out (Firebreak Zone), with only occasional non-pyrophytic trees in this area. Open walks, lawns, non-flammable ground covers will help to keep this area usable, lovely and free of wildfire.
A Reduced Fuel Zone is a radius of approximately 30 – 100 feet (or more) from the structure. In this zone large non-pyrophytic trees can be maintained and grown with other non-pyrophytic species. Shrubs must not constitute continuous fuel beds. The separation distance charts are helpful in determining wise plant spacing.
Depending on the factors influencing fire intensity, behavior, and flame spread, this distance may be expanded much further from the structure. If your structure is above a 20% slope, this zone should extend 200 feet downhill. If above a 40% slope, you may need 400 feet of managed space.
This zone is an area of overall fuel reduction designed to create an environment that calms a wildfire by separating the available fuels and reducing the heat and flame generated by the fire. If done correctly, fire “offense” can be most effective here.
The required distance usually radiates out in all directions but often expands downslope from the structure, further as the slope increases. Steeper slopes will also require greater protection from erosion if fuel management measures are undertaken.
An Access Zone is 10 feet on either side of roads and driveways. The ingress and egress route(s) from road or highway should not be flammable for at least 10 feet on either side of the traveled way. To allow for emergency access, there should also be at least 14 foot clearance over the road. Addresses should be clearly marked. Remember that emergency vehicles may be entering as others are leaving.
Two access routes at minimum are recommended. As you plan your defensible space, also plan your escape routes.
As your landscape matures, it will need annual maintenance, and it will also require removal of some plants as they grow large and start to close in. The separation guidelines are useful for planning; you should also be wary of the development of ladder fuels. Ladder fuels are fuels that allow a fire to move from the ground or lower-levels of vegetation such as grasses and small shrubs into taller shrubs and low trees and into the upper portions or crown of the vegetation.
The three R’s of defensible space landscaping
Part of any defensible space plan is vegetation management or landscaping. This will likely mean altering your existing landscape by removal, thinning, pruning and perhaps replanting. The goal is to grow your landscape into a more desirable level of fire safety and to ultimately reduce the need for annual maintenance.
This doesn’t have to happen all at the same time. The goal is to develop a landscape plan with fire in mind and beauty as a given. Defensible space is the true expression of form and function.
You will be surprised what a little pruning can do to open up vistas you never knew existed. Above are several examples of nicely landscaped defensible space.
The objective of defensible space is to reduce the wildfire threat to a home by changing the characteristics of the adjacent vegetation. Defensible space practices include:
- Increasing the moisture content of vegetation
- Decreasing the amount of flammable vegetation
- Shortening plant height
- Altering the arrangement of plants.
This is accomplished through the “Three R’s of Defensible Space” – REMOVAL, REDUCTION AND REPLACEMENT.
This technique involves the elimination of entire plants, particularly trees and shrubs, from the site. Focus should be on pyrophytic vegetation.
Removal of plant parts, such as branches or leaves, constitutes reduction. Examples of reduction are pruning dead wood from a shrub, removing low tree branches, and mowing dried grass.
This is substituting less flammable plants for more hazardous vegetation. Removal of a dense stand of flammable shrubs and planting an irrigated, well-maintained flowerbed is an example of replacement. For more examples, visit our list of replacement plant choices.
Preparing a defensible space plan
- Begin by establishing the required 100-foot outer zone. With a fire awareness checklist in hand, start from your home and work outward. The assessment should identify fire risks to both your home and property.
In the zone reaching out 100 feet (Reduced Fuel Zone), large tree plantings can be made, using non-pyrophytic or low fire ignition species. Shrubs should not constitute continuous fuel beds. The separation distance charts are helpful in determining wise plant spacing and future maintenance requirements. The separation guidelines are useful, as well, in keeping an eye on the development of ladder fuels.
The 100-foot outer zone is based on your structure being located on level land. Significant slope variations and fuel types may require extending the zone(s). If you are in doubt reference the following charts or contact your local Fire Marshal’s office
- Once you have completed the assessment, you will have a better idea of the scope of your project, and you can prioritize your next steps. Don’t be overly concerned if the size of the project seems daunting. The goal is to take this one step at a time.
- Your priorities should focus on making the most impact with the least amount of effort. Removing dead and dying vegetation (including dry leaves, pine needles, limbs and small branches, brush parts, dead materials on the ground, unused timber, and debris piles) is first in all zones, starting closest to the house.
Dry, dead material that is ½ inch to 3 inches in diameter provides for high heat and is much easier to ignite than green materials.
- You will also want to break up the continuity of fire fuels by removal and spacing.
- Ladder fuels, or fuels that tend to help a ground fire spread from ground level into the upper reaches of trees and brush, need to be removed.
- Mow grasses for at least 50 feet around your structure each year, and maintain at a height of less than 3 inches throughout fire season.
- Maintain an ongoing vegetation management program. Removing a source of fire fuel one year may not keep it from returning with a vengeance the following year. Plants grow and many resprout. Reference our plant list to help with removal techniques and timing.
- In developing your new fire-safe landscape plan, only low-growing, irrigated plants should be planted within 10 feet of structures (Structure Ignition Zone).
Think “Lean, Clean, and Green”
Part of the process of defensible space planning is creating a new consciousness about vegetation management and how the decisions you make now will influence your overall defensible space planning. The following “Lean, Clean and Green” checklist should be revisited periodically as part of your overall vegetation management plan.
Emphasize the use of low-growing herbaceous (non-woody) plants that are kept green during the fire season, through irrigation if necessary. Herbaceous plants include lawn, clover, varieties of ground cover, bedding plants, bulbs, perennial flowers, and conservation grasses.
Emphasize the use of mulches, and “hardscaping” such as rock and non-combustible hard surfaces such as concrete sidewalks, brick patios and asphalt driveways.
Deciduous (plants that shed their foliage) ornamental trees and shrubs are acceptable if they are kept green and free of dead plant material. When it comes to deciduous scrubs, the shorter the better
Minimize the use of ornamental coniferous (evergreen) shrubs such as juniper and manzanita. Also minimize the use of tall exotic grasses such as pampas grass.
If you do retain coniferous shrubs and trees, make sure they are healthy and free from dead wood; they are pruned regularly to reduce the amount of fuel and height; and ladder fuels are removed.
You need to be aware of local regulations about removing substantial amounts of vegetation near streams or creeks and where erosion can become a factor. If in doubt, check with your county agriculture department, state forestry office and Department of Fish and Game. Most landscape contractors and tree services are also knowledgeable about local regulations.