Designing for Fire Resilience

Primary Author: Deb Lebow Aal

This is a topic every homeowner should pay attention to. As you all know, our fire season in the west is getting longer and hotter. And, while it’s of much greater concern if you live close to the rural/forest intersection, it is now something even urban and suburban dwellers need to know about. Your landscape can actually protect your home from fire, as we have seen in multiple Colorado fires. Some houses are spared while others burn to the ground. We at WOFR are not experts on this, and you should consult true experts when deciding how to landscape for fire mitigation, but here’s are some things we know to be true:

  • Clearing your land is not a good idea (other than in Zone 0, see below). Bare earth allows hot air to travel fast, capturing embers, with nothing to slow it down. There are many examples where people felt that clearing their land would protect their homes only to find that it exacerbates the problem.
  • Native species are generally the best plant materials for landscaping in defensible space. Research in California on native species vs exotic species found a distinct advantage to native plants for fire mitigation (see below for why).
  • Juniper trees: While all vegetation is ultimately flammable, plants in the Juniperus genus seem to be highly flammable and some towns are considering banning Junipers around homes. Conifers in general tend to be highly flammable due to their oil and pitch content. That’s too bad as it is a plant that is important to wildlife, but there are many other alternatives not as flammable.

CSU Extension Service has a wealth of information on this topic. See Fire Resistant Landscaping, which will direct you to many other CSU publications. But here are our thoughts about Fire resistant landscaping. In truth, how you plant and where you plant is more important than what you plant! So, we won’t include an exhaustive list of plants to include in your landscape. The bottom line is to plant in zones.

Zone 1 (The immediate zone): Defensible space around your home should be 3-5 feet, preferably 5, of non-flammable material. In this space it is advisable to clear your landscape of vegetation. Nothing under the eaves of your home, and particularly, no wood! There are lots of fire-safe choices – a cement apron; a flagstone walkway, pea gravel or any kind of rock gravel. This I believe to be hearsay, but firefighters have been known to bypass houses without obvious defensible space, as the chances of saving that house are not good. Be sure to make sure that flammable debris does not collect near or on the home or other structures, and keep your gutters clean!

Zone 2 (the intermediate zone): In the next zone, 5 to 30 feet from your house, plant low succulents and ground covers that are lightly irrigated, and widely spaced apart. No wood piles or wooden fences – metal and stucco are preferable. Do not plant in large masses. Rather, plant in small, irregular clusters or islands. Use rock gravel in pathways, which will slow a fire’s progression. Even if small fires do start in this zone, they are unlikely to spread to the home if surface fuels are not continuous.

Zone 3 (the extended zone): Everything else, i.e., beyond 30 feet, remove non-native grasses and keep up on maintenance, meaning clear weeds and brush, and prune dead branches. Prune up large shrubs and trees so that if the grass catches fire, it will have a hard time spreading to the shrubs and trees. That means, taking branches off to about 3 times the understory height. Don’t clear vegetation – thin it and manage it. Run lots of paths and make some rock sitting areas. Landscaping this way can be really pretty. And irrigate. It’s unfortunate, but a bone-dry landscape, which some of us have attempted, is not going to help in a fire situation. Obviously.

Houses burn from an accumulation of embers. So, get rid of areas where embers can accumulate and ignite. This would include wood decks, wood piles, wood fences, even wooden or wicker furniture.

Beyond that, the more native plants the better, that is if the native plants are grown well, stay succulent, and don’t accumulate debris.  And the best design is planting in islands, surrounded by gravel or other non-flammable paths. The best model I’ve seen of this is Kelly Grummons’ space in Arvada shown in the picture.

And why are native plants so much better than exotics? Well, it has to do with the moisture content of plants. Native plants maintain a much higher “live fuel moisture content” (yes, that’s a scientific term) than traditional plants (although, and I hate to say this, it’s hard to beat a well-watered Kentucky Bluegrass lawn for fire resistance, but we don’t want that!). Lower growing natives exhibit even better fire behavior. And, don’t quote me on this, but because the roots of native plants typically go so deep, I would venture to say that native plants will survive a fire much better than exotics. In other words, even though your landscape may look pretty much dead and burned after a fire, the natives will come back due to their deep roots.

So, hydration takes precedence over plant lists. I am not going to give a list of natives that do best in fire. For that, there is the internet and CSU extension  (e.g., FireWise Plant Material)….but a note about ice plants (Delosperma). They are cheap, and mostly from South Africa. People love them. You see them everywhere, and thanks to Panayoti Kelaidis, horticulturist extraordinaire at Denver Botanic Gardens, for bringing them over, but resist them and go for native plants! Really all succulent plants are good fire-resistant plants if well maintained.

A note about mulch. Wood mulch obviously is flammable, but if it is kept hydrated and not too deep, it can be a good mulch in a fire-resistant landscape. Overhead irrigation is preferred to drip as it gets all of the mulch wet. Much better of course is pea gravel. Native plants tend to like being planted in pea gravel. You have to keep it weeded (and, if it’s 4 inches deep it resists weeds pretty well), but other than that, it’s pretty low maintenance and doesn’t need to be replenished as often as wood mulch.

A note about shrubs. We love shrubs – native shrubs are important for the ecosystem (see article in July 2022 newsletter). The primary concern for shrubs in a fire-resistant landscape is that they are “ladder fuel.” In other words, they guide the fire up. So, don’t plant shrubs near windows, vents or tree crowns; keep the grass low around shrubs; and prune dead branches and sometimes, lower branches.

One more note. We talk about oak trees having a very high ecosystem value. They support a myriad of caterpillar and moth species. Oaks are also good fire-resistant trees. There is no such thing as a tree that doesn’t burn, ultimately, but deciduous trees like oaks and aspens, apples and plums, can be resistant. In the right place, they can protect a house. Oak leaf litter is also good to keep under an oak tree. It will keep the tree much healthier and therefore able to resist fire. Just keep it lightly irrigated, and never within 5 feet of a home or other structure.

And one more note- this from my permaculture class. If you’re actually building a new house from scratch, or have the luxury of siting a house on a large property, fire almost always moves uphill. So, position a house down below, preferably near a pond. If only…. Again, we are not fire experts. Consult the experts when re-doing a landscape to be fire resistant. Hopefully some of these tips can get you started on saving your home in the horrific event of a fire near you.