Fire is a fickle foe. It can ravage hundreds of thousands of acres, or it can burn quickly and snuff itself out. It can occur on grasslands, in forests, in remote mountainous areas and in suburban tracts. It can start as the result of lightning, a discarded cigarette or insufficiently doused coals. The smallest spark can ignite a conflagration.
Knowing the basics of how fire develops and moves can help you plan a landscape that’s less likely to burn, no matter where you practice. Knowing how to develop a zoned design can help prevent fire from destroying gardens — and structures. And knowing what kinds of plants to spec can prevent those landscapes from becoming fuel.
How fire behaves
We all know that where there’s smoke, there’s fire. But do we know how fire “works”?
The fire triangle: In order for fire to develop, it needs three elements: fuel, heat and oxygen. If one of these elements is absent, or greatly reduced, the fire will diminish and die.
The fire behavior triangle: Understanding how fire behaves is critical to its prevention and suppression. Easier said than done, right? But we can try. Fire behavior is the manner in which fuel (dry vegetation) ignites, flame develops and fire spreads, as determined by the interaction of fuel, weather and topography.
Put a bit more simply, where fuel is plentiful and hot, dry, windy conditions exist, fire risk is increased. Add the element of topography and you can predict, somewhat, how quickly flames will move. Flat terrain? Fire may creep. Hilly areas? Fire loves to run uphill.
If you paid attention in junior high science class, you’ll recall that heat rises. And what that means is that fire will burn faster uphill. Conversely, fire will decrease in speed and intensity if it’s headed downhill; this, however, is a rare event.
Consider these facts:
- FACT – For every 10 feet of slope, wildfire will double its speed
- FACT – When fire increases its speed, it also increases its intensity — it becomes hotter
- FACT – Fire moving uphill will preheat vegetation ahead of itself, providing — you guessed it — more fuel to burn
- FACT – Wind can influence not only the direction of fire, but the intensity, by providing more oxygen
As “predictable” as all this sounds, it’s only a guide by which we can begin to understand fire. Because fire is rarely reliably predictable. Hot spots can flare; winds can shift.
Read more: Firescaping: Creating a Buffer Zone
“Firescaping” design entails thoughtful placement of plants around a home or other structure in such a way that the potential for fire is diminished — by design. Basically, vegetation that can serve as fuel should be kept as far from the house as possible. As we look at the distances recommended, you’ll see that this is not an exact science, and that these measurements don’t fit every site. Suburban property isn’t always large enough to allow for 30-70-100 feet, but the concept of incrementally defensive zones remains sound.
The concept holds no matter where you design; mitigating the risk can help homeowners and firefighters alike breathe a sigh of relief.
So the practice of planting in zones encourages smart plant placement while preserving, as much as possible, the aesthetics. In firescaping, there are three basic zones.
Zone 1: Clear a 30-foot perimeter around the house. Within this area, low-growing perennials and groundcover plants as well as small, deciduous trees and shrubs may be planted. Evergreens, especially conifers, contain highly flammable oils that serve as natural fuel; even if there’s no standing dry brush near the home, needled trees and shrubs can act as torches. If large deciduous trees already exist within this zone, trim branches that overhang any part of the structure. If established evergreens are in place, especially those that serve as foundation plantings, they’ll need to be sacrificed.
The area in Zone 1 is normally the place where we site patios, pools and other hardscape elements; these can serve as fire breaks. Wooden decks, pergolas, gazebos and other structures should be avoided, however. And although it may seem to be common sense, it bears mentioning that firewood storage and fuel sources, such as propane tanks, should not be included in this zone.
Zone 2: Called the mid-zone, this area extends another 70 feet beyond Zone 1. This garden zone is where mowable lawn and mixed beds can be placed. Existing trees should be pruned to remove lower branches, giving a clearance of about 15 feet from the ground.
If the site is hilly, avoid mulch and plant succulent groundcover plants that will help to secure the slope as well as provide scant fuel for fires racing uphill (see “How fire behaves,” above). Perennials and shrubs planted in this zone should be well maintained, as in any garden, but here it’s critical that deadheading and trimming be performed regularly. Dried stems serve as fuel.
Zone 3: This farthest zone should be no closer than 100 feet from the house. This area is likely the site of larger trees which, despite their distance from the house, require appropriate maintenance to prevent wildland fire from breaching the property. Crowns should be separated by a good 10 feet to prevent fire from engulfing adjacent plants, should one tree become involved. Branches should be pruned to a height of 10 feet from the ground, and brush should be kept cleared from the base of trees. Fire can quickly move from ground to crown (called “laddering”) if the fuel is available.
There’s really no such thing as a fireproof plant; like deer, fire will consume what’s available. In general, however, those plants that tend to retain moisture top the list for those considered fire “resistant.” That means succulents, of course, but the palette is not limited to cacti. Delosperma, that superstar of the West, provides color, ground-hugging protection against erosion and water-retaining qualities.
We’re listing a few perennials, shrubs and trees here just to give an indication of the broad range of plants that may be considered relatively resistant. For specific recommendations, state departments of agriculture or state fire protection agencies are an excellent source.
Other recommended perennials and groundcovers include:
- Agapanthus (lily of the Nile)
- Armeria maritima (sea pink)
- Heuchera (coral bells)
- Iris (iris)
- Kniphofia (red hot poker)
- Lavandula (lavender)
- Penstemon (beard tongue)
- Salvia (sage)
- Sisyrinchium spp. (blue-eyed and golden-eyed grass)
- Ceanothus (California lilac)
- Cistus (rockrose)
- Elaeagnus punge (silverberry)
- Lantana (lantana)
- Pittosporum (mock orange)
- Rosa (rose)
- Raphiolepis indica (India hawthorn)
- Syringa (lilac)
- Alnus rhombifolia (white alder)
- Cercis occidentalis (western redbud)
- Ginkgo biloba (maidenhair tree)
- Gleditsia triacanthos (honey locust)
- Liquidambar styraciflua (sweet gum)
- Liriodendron tulipifera (tulip tree)
- Pistacia chinensis (Chinese pistache)
- Quercus suber (cork oak)
The list of plants that can be used in firescaping zones is longer than one would think; proper siting and maintenance also contribute to the resistance factor. Depending on the location of the site — region, hardiness zone, rural vs. suburban — your choice of plants will vary.
Wildfire is a fact of life, and designing to make the most of a site, while protecting the home from fire threat, need not be an insurmountable challenge. With good planning, proper plant selection and careful maintenance, a “firescape” can fit neatly into your portfolio.
Read more: Defending Against Fire