Fire-Resistant Landscaping

Creating defensible or survivable space around your home is a primary determinant in helping it to survive a wildfire. Survivable space is the modification of landscaping design, fuels and other building materials that would make a home ignition caused by wildfire unlikely, without direct firefighter intervention. Defensible space is an area around a structure where fuels and vegetation are treated, cleared or reduced to slow the spread of wildfire towards the structure. It also reduces the chance of a structure fire moving from the building to the surrounding forest. Defensible space provides room for firefighters to do their jobs. Your house is more likely to withstand a wildfire if grasses, brush, trees and other common forest fuels are managed to reduce a fire’s intensity. If you have not created defensible space around your home, firefighters may bypass your house, choosing to make their stand at a home where their safety is more assured and the chance to successfully protect the structure is greater. For more information, read Creating Wildfire-Defensible Spaces for Your Home and Property.

People often resist creating defensible space because they believe that it will be unattractive, unnatural and sterile- looking. It doesn’t have to be! Wise landowners carefully plan landscaping within the defensible space. This effort yields a many-fold return of beauty, enjoyment and added property value.

Arizona has great diversity in climate, geology and vegetation. Home and cabin sites can be found from the foothills through 8,000-foot elevations. Such extremes present a challenge in recommending plants. While native plant materials generally are best, a wide range of species can be grown successfully in Arizona. For more information on plant selection refer to Firewise Plant Materials for 3000 ft. and Higher Elevations.

Many plant species are suitable for landscaping in defensible space. Use restraint and common sense, and pay attention to plant arrangement and maintenance. It has often been said that how and where you plant are more important than what you plant. While this is indeed true, given a choice among plants, choose those that are more resistant to wildfire

Consider the following factors when planning, designing and planting the Firewise landscape within your home’s defensible space:

• Landscape according to the recommended defensible- space zones. That is, the plants near your home should be more widely spaced and lower growing than those farther away.
• Do not plant in large masses. Instead, plant in small, irregular clusters or islands.
• Use decorative rock, gravel and stepping stone pathways to break up the continuity of the vegetation and fuels. This can modify fire behavior and slow the spread of fire across your property.
• Incorporate a diversity of plant types and species in your landscape. Not only will this be visually satisfying, but it should help keep pests and diseases from causing problems within the whole landscape.
• In the event of drought and water rationing, prioritize plants to be saved. Provide available supplemental water to plants closest to your house when possible.
• Use mulches to conserve moisture and reduce weed growth. Mulch can be organic or inorganic. Do not use thick layers of pine needles or other mulches that readily carry fire close to structures and other flammable materials.
• Be creative! Further vary your landscape by including bulbs, garden art and containers for added color.

During much of the year, grasses ignite easily and burn rapidly. Tall grass will quickly carry fire to your house. Mow grasses low in the inner zones of the defensible space. Keep them short closest to the house and gradually increase height outward from the house, to a maximum of 8 inches. This is particularly important before and after green-up, when grasses are dry, dormant and in a “cured” fuel condition. In Arizona wildfires can occur any time of the year. Maintenance of the grassy areas around your home is critical. Mow grasses low around the garage, outbuildings, decks, firewood piles, propane tanks, shrubs, and specimen trees with low-growing branches

Replace bare, weedy or unsightly patches near your home with ground covers, rock gardens, vegetable gardens and mulches. Ground cover plants are a good alternative to grass for parts of your defensible space. They break up the monotony of grass and enhance the beauty of the landscape. They provide a variety of textures and color and help reduce soil erosion. Consider ground cover plants for areas where access for mowing or other maintenance is difficult, on steep slopes and on hot, dry exposures.

Ground cover plants are usually low growing. They are succulent or have other Firewise characteristics that make them useful, functional and attractive. When planted in beds surrounded by walkways and paths, in raised beds or as part of a rock garden, they become an effective barrier to fire spread. The ideal groundcover plant is one that will spread, forming a dense mat of roots and foliage that reduces soil erosion and excludes weeds.

Mulch helps control erosion, conserve moisture and reduce weed growth. It can be organic (compost, leaf mold, hardwood bark chips, shredded leaves) or it can be inorganic (gravel, rock, decomposing granite).

When using organic mulches, use just enough to reduce weed and grass growth, usually 4–6 inches. Avoid thick layers. When exposed to fire, they tend to smolder and are difficult to extinguish. Your property might yield an abundance of needles from your native pines or other conifers. Arrange pathways and barriers to use conifer needles as a mulch to prevent fire spread. It is also possible to compost conifer needles to be used in garden areas. For more information on using mulches in Firewise landscapes read  Comparing the Ignitability of Mulch Materials for a Firewise Landscape.

Wildflowers bring variety to a landscape and provide color from May until frost. Wildflower beds give a softer, more natural appearance to the otherwise manicured look often resulting from defensible space development.

A concern with wildflowers is the tall, dense areas of available fuel they can form, especially in dormancy. To reduce fire hazard, plant wildflowers in separated beds within the defensible space. Do not plant them next to structures unless the beds are frequently watered and weeded and vegetation is promptly removed after they die or go dormant. Use gravel walkways, rock retaining walls or irrigated grass areas mowed to a low height to isolate wildflower beds from each other and from other fuels.

Shrubs lend color and variety to the landscape and provide cover and food for wildlife. However, shrubs can add significantly to total fuel loading around a home. The primary concern with shrubs is they can serve as “ladder fuel” and carry a relatively easy-to-control ground or grass fire into tree crowns. Once a fire reaches into the tops of trees (the crowns) it is difficult, and sometimes impossible, to control.

To reduce the fire-spreading potential of shrubs, plant only low-growing, nonresinous varieties close to structures. Do not plant them directly beneath windows or vents or where they might spread under wooden decks. Do not plant shrubs under tree crowns or use them to screen propane tanks, firewood piles or other flammable materials. Plant shrubs individually or in small clumps apart from each other and away from any trees within the defensible space.

Mow grasses low around shrubs. Prune dead stems from shrubs annually. Remove the lower branches and sucker sprouts from trees to raise the canopy away from the ground.

Trees provide a large amount of available fuel for a fire and can be a significant source of fire brands — embers from stems and branches, carried in the smoke column ahead of the main fire spreading the fire in a phenomenon known as “spotting”. Radiant heat from burning trees can also ignite adjacent shrubs, trees and structures.

Arizona’s elevation and temperature extremes limit tree selection. The best species to plant generally are those already growing on or near the site. Others may be planted with careful selection and common sense.

If your site receives enough moisture, plant deciduous trees. These species, even when planted in dense clumps, generally do not burn well, if at all. The greatest problem with these trees is the accumulation of dead leaves in the fall. Remove accumulations close to structures as soon as possible after leaf drop. A list of deciduous trees is available in Firewise Plant Materials for 3000 ft. and Higher Elevations.

When site or available moisture limits recommended species to evergreens, carefully plan their placement. Do not plant trees near structures. Leave plenty of room between trees to allow for their growth. Spacing within the defensible space should be at least 10 feet between the edges of tree crowns. On steep ground, allow even more space between crowns. Plant smaller trees initially on a 20- to 25- foot spacing to allow for tree growth. At some point, you will have to thin your trees to retain proper spacing.

As the trees grow, prune branches to a height of 10 feet above the ground (though height may be lower on full-crowned trees such as pinyon and juniper). Do not overprune the crowns. A good rule of thumb is to remove no more than one-third of the live crown of the tree when pruning. Prune existing trees as well as ones you planted.

Some trees tend to keep a full crown. Other trees grown in the open may also exhibit a full growth habit. Limit the number of trees of this type within the defensible space. Prune others as described above and mow grasses around such specimen trees.

When building a deck or patio, use concrete, flagstone or rock instead of wood. These materials do not burn and do not collect flammable debris like the space between planks in wooden decking

Where appropriate on steeper ground, use retaining walls to reduce the steepness of the slope. This, in turn, reduces the rate of fire spread. Retaining walls also act as physical barriers to fire spread and help deflect heat from the fire upwards and away from structures.

Rock or masonry walls are best, but even wooden tie walls constructed of heavy timbers will work. Put out any fires burning on tie walls after the main fire front passes

On steep slopes, consider building steps and walkways around structures. This makes access easier for home maintenance and enjoyment. It also serves as a physical barrier to fire spread and increases firefighters’ speed and safety as they work to defend your home.

A landscape is a dynamic system that constantly grows and changes. Plants considered fire resistant and which have low fuel volumes can lose these characteristics over time. Your landscape, and the plants in it, must be maintained to retain their Firewise properties.

• Always keep a watchful eye towards reducing the fuel volumes available to fire. Be aware of the growth habits of the plants within your landscape and of the changes that occur throughout the seasons.
• Remove annuals and perennials after they have gone to seed or when the stems become overly dry.
• Rake up leaves and other litter as it builds up through the season.
• Mow or trim grasses to a low height within your defensible space. This is particularly important as grasses cure.
• Remove plant parts damaged by snow, wind, frost or other agents.
• Timely pruning is critical. Evaluate trees and shrubs annually to determine if and where pruning is needed, and prune only as necessary. Pruning not only reduces fuel volumes but also maintains healthier plants by
  producing more vigorous, succulent growth.

Landscape maintenance is a critical part of your home’s defense system. Even the best defensible space can be compromised through lack of maintenance. The old adage “An ounce of prevention is worth a pound of cure” applies here.

This educational material is based on and draws heavily from a publication titled “Fire-Resistant Landscaping” written by F.C. Dennis and produced by the Colorado State Forest Service. Firewise is a multi-agency program that encourages the development of defensible space and the prevention of catastrophic wildfire.