By Nikki Hanson, Nursery Manager

As we head into spring with fire season soon to follow, now is a good time to take stock of our yards and properties surrounding our homes and do any tree trimming and brush removal before bird nesting season gets any farther underway.

Driving around the Santa Cruz Mountains lately, I have noticed a trend towards denuding the understory, thinning trees, limbing up remaining trees, and chipping everything that can fit through an industrial sized chipper. As a native plant and wildlife lover, it is hard to see such a change in the landscape. But, with a little information, there are some ways to create fire-smart landscapes that retain some of the natural understory, protect property from fire and invasive weeds, and provide for wildlife.

Healthy oak woodlands, if properly maintained, are a low fire risk and provide essential food and habitat for wildlife. Species such as toyon, coffeeberry, scrub oak, and elderberry produce abundant food (berries or nuts) and their foliage is necessary food for dozens of species of insects, which in turn provide meals for mammals, birds and reptiles. These valuable woodlands should be cared for and not thinned excessively, as this can cause unnecessary problems in the future with regard to invasive species and fire risk (1). By considering just six elements of home and yard maintenance – fire-wise gardening, tree thinning, brush removal, removing invasive plants, pathogen prevention, and home hardening – you can play an important role in fostering healthy ecosystems while protecting your home from fire.

Fire-Wise Gardening

In garden landscapes, there are a number of things you can do to reduce the risk of fire. The main factors that contribute to fire resistance though are proper irrigation and maintenance.

Overhead irrigation decreases fire risk and many native plants prefer it (2). Overhead watering mimics rainfall and encourages mycorrhizal networks to establish throughout the garden soil when it is evenly damp (rather than isolated inundation that drip creates). Some overhead options, such as MP-rotator sprayers, are quite water efficient too, allowing plants to maintain good foliar hydration with just 2 good irrigations per month (in our more moderate climate of northern California).
In the immediate 5-foot buffer around the home, use non-flammable substrate (3, 4). You can do this by creating a walkway with pavement or gravel, or create a path with decomposed granite.
Keep mulch in the garden at least 5 feet away from buildings (2, 3, 4). Arbor mulch should ideally be mostly chips (think less twigs and leaves and fluffy material), so that it lies flatter and is less ignitable. You can water the mulch down as you apply to help flatten it (5).
In the 5-to-30-foot zone around your home, shrubs should be spaced at least twice their height from each other (3). Further away from structures, you can create small clusters of shrubs and/or trees with ~ 10 feet of space between clusters (2).
Breaking up the continuity of vegetation with clusters (rather than just single shrubs or trees) creates fuel breaks while retaining more natural habitat for wildlife (not to mention being more attractive too) (2, 4).
Large, mature trees in your garden should have the lowest tree branches removed up to at least 6 feet to allow space between low-growing perennials and the lowest branches (3, 4). If shrubs are growing adjacent to trees, then tree branches should be removed even higher to allow plenty of space. Small or young trees can be pruned, but do not remove branches from more than the first third of the total tree height (4).
Pruning (not removing) goes a long way to reduce fire risk. Keep plants tidy by pruning them back after flowering and fruiting and removing any dead material. Then compost or remove the trimmings to reduce fuel load.
- Some plants such as yarrow, goldenrod, creeping aster, California fuchsia, bunch grasses, sedges, rushes, hummingbird sage and many other perennials should be cut back or even mowed to 1 to 3 inches tall each year after flowering and fruiting/seeding.
- Salvia species should be cut back heavily to keep their growth less woody (and thus less flammable) (6). When pruning Salvias, I look at the lowest place on the stems where I can still see new growth, and then cut back to just above the new growth. Some Salvias such as creeping sage and Santa Rosa Island sage are much tidier than others and so require much less maintenance (5, 6).
For easier maintenance, choose native plants that are slower growing, so that they produce less leaf litter, and drought hardy, to maintain higher internal moisture such as coffeeberry and toyon (5, 6, 7). See attached plant list for more ideas.

Remove flammable invasive species such as junipers, Italian cypress, iceplant (which can deceivingly create a dead underlayer), bamboo, pampas grass, jubata grass, and Mexican feather grass (6, 8).

Selective Tree Thinning

An open grassy area that has had trees limbed up and surrounding area mowed and weed-whacked.

You can create a shaded fuel break and reduce invasive weeds by selectively removing overcrowded, small-diameter trees and keeping existing large-diameter trees and a smattering of small trees (7, 9). Keeping the tree canopy intact provides shade and keeps more moisture in the understory, which is essential in our drought-strained conditions. Follow these guidelines as you approach tree thinning:

When deciding which trees to remove, keep in mind that some native trees are less flammable than others (see attached plant list for some ideas). For example, on my family’s 40 acres in the rural wooded Santa Cruz Mountains, if there is a choice between keeping small-diameter Douglas fir, bay laurel, and (healthy) coast live oak, I choose the oak, as it is less resinous and burns more slowly than both the fir and the bay.
Create a minimum of 6 feet of vertical space between lowest tree limbs and the ground. If shrubs are growing below trees, increase this vertical space so that ideally there is 3 times the height of the shrub and the lowest tree limb (10).
Large-diameter redwoods are fire resistant if lower limbs are removed (10).
Oaks make excellent fire-resistant canopy trees when limbed up (10).
Some native deciduous trees, such as big leaf maple and California buckeye, can be limbed up and are great low-flammability choices (7, 9).
Another excellent small understory tree is hazelnut, which goes dormant for about half the year but has beautiful structure and edible nuts that local critters love. Hazel grows quite slowly and tends to be naturally spaced in the understory, making it a reasonably low fire risk.
Firs are considered flammable, but large diameter native trees can have lower branches removed from the first 12 or more feet to make them more fire resistant. Keep firs away from the home or other structures (10).

The Art of Brush Removal

Douglas firs with intact understory of sword ferns, polypody ferns and huckleberry.

For larger properties with crowded thickets of coyote brush, poison oak, chamise, and other twiggy material, it is important to assess the big picture and strategize how best to ensure fire safety while maintaining habitat health (1, 2, 11). These two things are not mutually exclusive! Over clearing land also leads to erosion (2, 11).

Identify the top priorities for brush removal. For example, first remove shrubs in the closest vicinity to structures, and prioritize removal of invasive species (such as broom) and the most flammable species (such as juniper, chamise, and coyote brush).
On a large property, the first 30 feet should have mostly low-growing species and well-pruned trees. The next 70 feet beyond this first zone should ideally have “islands” of a few shrubs and trees spaced at least 10 feet apart (this spacing should increase with steeper slopes). If a home is surrounded by dense chaparral, thinning the existing shrubland to approximately 50%, leaving a tidy array of shrubs at varying heights, is a good goal (2).
Select the least-flammable native species to anchor your “islands.”
- Coffeeberry and toyon are fantastic native shrubs that are drought tolerant, fire resistant, and are wonderful food and habitat structure for wildlife. Ceanothus, if kept tidy or limbed up, is another fantastic native species for wildlife and can be quite fire resistant if healthy (5, 6, 9, 12).
Some native shrubs are more flammable, such as chamise and coyote brush. However, coyote brush is an amazing plant, providing food for 100+ native insect species and birds, and protection from voracious deer browsing of young trees (12). You can keep coyote brush in the garden by coppicing down to the stump every other year or try using the “Pigeon Point” selection, which stays low (2).
When eliminating dead material from woodland understory, be careful not to confuse dead with dormant. Many native species go dormant during fall and winter but are alive and even still fire resistant.
- Some examples are hazelnut, ocean spray, currant, gooseberry, thimbleberry, elderberry, snowberry, and native roses, which all lose their leaves during the fall and winter months. Don’t be fooled - they are still alive!
If removing large amounts of shrubs and understory is necessary, it is important to prepare for early mowing and removal of the subsequent influx of weeds that thrive in disturbed soils and sunny habitats (1, 7, 11).
- Thistles, stinkwort, exotic annual grasses (such as those that turn our California hills golden brown in the summer), poison hemlock, and non-native mustards all produce masses of growth each year and then die, leaving an easy tinder for fire.
- For some erosion control and weed suppression, maintain or plant native perennial species that tolerate annual or even biannual mowing, such as yarrow, creeping aster, western vervain, and foothill needlegrass (in sun), and mugwort, beeplant, strawberry, California blackberry, and California fescue (in shade).

Discourage and Remove All Invasive Species (if Possible)!

Example of influx of invasive French broom after extensive clearing of understory along rural road.

One of the biggest factors to consider before taking out all understory plants is that this allows invasive plants to get a foothold in a previously occupied location. These, coincidentally, are also among the most flammable plants as they grow quickly and produce lots of dead material. Additionally, if you remove too many trees, this opens up the canopy to allow more sun and creates a drier landscape that is more susceptible to fire and more appealing to weedy species that thrive in disturbed, sunny habitats.

Along Highway 35 between Page Mill Road and Woodside Road, you can see a good example of this phenomenon. Portuguese broom and French broom have usurped a large stretch of roadside so quickly that what was once mostly native shrubs and a ground cover of woodland strawberry, creeping snowberry, and yerba buena is now a massive thicket of invasive broom. Broom grows very quickly and can form a uniform mass of small-diameter branches reaching up towards the light, creating a fuel ladder that links the understory to low-hanging branches of trees. Additionally, invasive broom species are devoid of much use to wildlife other than their profuse (but brief) flowers in the spring. Unfortunately, broom species have extremely long-lived seeds that can stay viable for 30+ years in the soil, so it is a costly problem to fix if left unchecked (13). The following are some key invasive species to target:

Invasive flammable trees to remove are eucalyptus, acacia, cypress and many exotic conifers (5, 6, 8, 14).
Troublesome shrubs to remove are junipers (these are an absolute fire hazard), all species of exotic broom, and cotoneaster, which degrade habitat value for wildlife and produce dead twiggy material (5, 6, 8, 14).
Exotic grasses such as bamboo (yes, this is a grass too), pampas or jubata grass, and Mexican feather grass are highly flammable (5, 6, 8, 14).
Finally, ivy, non-native thistles, stinkwort, vinca, iceplant, and other weeds should be removed as they are either flammable, cause habitat degradation or both (5, 6, 8, 11).

Plant Pathogens Increase Fire Risk

Remember that dead plants are the most flammable, so keeping a healthy forest or yard is important. Sadly, I often see well-intentioned roadside pruning result in a wide-scale spread of plant pathogens. Follow these best practices for preventing the spread of plant pathogens:

Clean chainsaws by brushing off debris and then cutting through some clean wood. You can do this by making a few cuts in some healthy wood on a branch that will be removed, before making the final cut that is closest to the trunk.
Clean pruning shears and handsaws by brushing off and then spraying with 70% isopropyl alcohol.
Brush off soil and debris from tires and shoes between locations and spray tools and shoes with 70% alcohol if possible.
To further reduce the risk of Sudden Oak Death (SOD) and subsequent die-off of oaks, remove bay laurel trees near oaks when possible. Contrary to what is often thought, oaks cannot spread SOD oak to oak, but rather oak to bay to oak, so removing bay trees among oaks will greatly reduce risk of SOD.
Finally, if you live adjacent to wild lands with intact native habitat, I highly encourage you to source plants for your landscape that are as pathogen-free as possible. Some excellent small-scale nurseries grow native plants in accordance with best management practices to reduce the risk of introducing potentially devastating plant pathogens such as one of the many Phytophthora species, which unfortunately are abundant in many nursery stock (15, 16).

Home Hardening

Finally, it is important to remember to implement some key home hardening principles in addition to maintaining your home landscape. Keep in mind that 60-90% of all homes that burn are ignited by embers floating in from 100+ yards away, NOT because of flames reaching the home via burning shrubs and trees (19). Embers may land on the roof or get sucked into an open vent or land on a flammable doormat or recycling pile. Below are some first easy steps for home hardening (17, 18, 20).

First (less expensive) steps to making your home more fire resistant:

Replace all vents (under eaves, in foundation, side of home) with screens with less than 1/8” mesh. “Vulcan,” “Brand Guard,” and “Embers Out” are trusted brands to use.
Move wood piles, loose recycling, leaf piles, and yard clippings away from structures.
Do not store flammable things under or on decks.
Do not use wood or wicker deck furniture; stick with metal.
Put away doormats in the dry season (jute is very flammable).
Use metal gates and/or fences that connect to structures (or at the very least prop open wood gates if you have to evacuate in a fire threat).
Keep roofs and roof gutters clear of leaves and debris.
Plug or caulk any gaps around eaves, windows, and doors, as embers can enter these.

Further (more expensive) steps you can take to make your home fire resistant:

Use roofing material that is metal, asphalt-composition shingles, tile, or made of other Class A materials. (This is a very important piece of making your home fire resistant!)
Replace single-pane windows with dual-pane tempered glass.
Replace plastic gutters with metal (ideally closed to reduce maintenance).
Use siding that is cement fiber shingles, stucco, or other fire-resistant material.
When building or rebuilding decks, apply newest methods such as:
- Wider (¼”) spacing between boards
- Wider (24”) spacing between joists
- Apply foil faced self-adhering tape to the top of each joist

Sources for this article are listed below, as well as a starter list of local native plants that are good for landscaping. Remember though, it is not as much about the species of plants you choose but rather how you maintain them. Local native species have evolved to be most adapted to this climate and have co-evolved with native insects, which so critically form the base of our ecological system (12). These plants also tend to grow slower and produce less fuel (therefore requiring less maintenance) compared to exotic and common landscaping species. Native species (as long as they are planted in the right spot) even require less water on average, which is a very good thing in these drought-prone and fire-prone conditions to which we are all adapting. Happy planting-- the wildlife and our future generations thank you for fostering biological diversity!

FIRE SMART PLANT LIST

Annotated Bibliography:

Brennan, T.J. and J.E. Keeley. 2017. Impacts of mastication fuel treatments on California chaparral vegetation structure and composition. Fire Ecology 13-3: 120-138. doi: 10.4996/fireecology.130312013. http://www.cafiresci.org/research-publications-source/chapmasticatio
→ This has results from an experiment showing the hazards of over clearing brush by chipping everything, click on either the full article or the summary
Rubin, Greg. 2019. Home Landscaping in Fire Country: A Southern California “Case Study”. California Native Plant Society Fire Recovery Guide 50-53. https://www.cnps.org/wp-content/uploads/2019/08/cnps-fire-recovery-guide-2019.pdf
→ Greg Rubin is a landscaper in San Diego with over 20 years of experience landscaping homes in high fire risk areas
Fire Safe Marin. 2019. Defensible Space. https://www.firesafemarin.org/defensible-spac
→ This is a detailed explanation of the different zones of fire risk around a home and what actions should be done to make it fire safe
National Fire Prevention Association. 2019. Public Education: Preparing homes for wildfire. https://www.nfpa.org/Public-Education/Fire-causes-and-risks/Wildfire/Preparing-homes-for-wildfire
→ This is another version of general recommendations for fire prevention within 100 ft of the home
Wilson, Bert. 2014. Leaf Burn Times of California native plants (& Several Non-Native Plants). Las Pilitas Nursery. https://www.laspilitas.com/classes/fire_burn_times.html
→ This has super interesting results from testing how different native plant leaves burn
University of California Cooperative Extension Division of Agriculture and Natural Resources. 2020. Fire Resistant Plants. https://ucanr.edu/sites/fire/Prepare/Landscaping/Plant_choice/
→ This University of California extension site has helpful descriptions of general qualities of fire resistant plants and also lists some plants to watch out for and why
Schettler, Suzanne. 2010. Fire-resistant landscaping: A General Approach and Central Coast Perspective. Fremontia: Journal of the California Native Plant Society 38:2, 38:3:46-50. https://www.cnps.org/wp-content/uploads/2018/03/Fremontia_Vol38-No2-3.pdf
→ Article describes landscaping ideas for fire-prone areas, including some plant lists
Fire Safe San Mateo County. 2019. Fire Prone (Pyrophytic) Plant List. https://firesafesanmateo.org/resources/defensible-space/fire-prone-pyrophytic-plants
→ This is a list of fire prone plants to avoid, suggested by San Mateo Co.
Fire Safe San Mateo County. 2019. Fire Resistant Plant List. https://firesafesanmateo.org/resources/defensible-space/fire-resistant-plant-list
→ This is a list of California native fire-resistant plants, suggested by San Mateo Co. Note that some species are native to southern CA, but it is a great list
California Department of Forestry and Fire Protection. 2019. Prepare for Wildfire: Maintain Defensible Space. https://www.readyforwildfire.org/prepare-for-wildfire/get-ready/defensible-space/
→ This is a general list of recommendations for making your property more fire safe
Keeley, Jon E. 2010. Fire-resistant landscaping: A General Approach and Central Coast Perspective. Fremontia: Journal of the California Native Plant Society 38:2, 38:3:2-6. https://www.cnps.org/wp-content/uploads/2018/03/Fremontia_Vol38-No2-3.pdf
→ Article describes landscaping ideas for fire-prone areas, including some plant lists
Tallamy, Douglas. 2019. Restoring the Little Things that Run the World. California Native Plant Society - Santa Clara Valley Youtube Channel. https://www.youtube.com/watch?v=bF5e-vyKLw0&feature=youtu.be
→ This is a fantastic lecture by Douglas Tallamy on the importance of boosting native insects
Zouhar, Kris. 2005. Cytisus scoparius, C. striatus. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/shrub/cytspp/all.html [2020, April 12].
→ Fact sheet about scotch broom, a common invasive plant
Fire Safe Marin. 2019. Fire-Resistant Plants Common to Marin County, CA. https://www.firesafemarin.org/images/articles/plants/FIRESafe-MARIN_plant-list_2019.pdf
→ This list includes some differences from mine, but still has many useful ideas
Phytosphere Research. 2015. Phytophthora in Nursery Stock and Restoration Plantings. http://phytosphere.com/soilphytophthora/index.html
Swiecki, T.J., Bernhardt, E.A., Garbelotto, M., and Fichtner, E.J. 2009. The exotic plant pathogen Phytophthora cinnamomi: A major threat to rare Arctostaphylos and much more. Proceedings of the CNPS Conservation Conference 17-19 Jan 2009:367-371 http://phytosphere.com/publications/Swiecki_etal_367_371CNPSproc2011.pdf
→ This is a sobering article on how a Phytophthora species can be introduced from adjacent landscaped gardens and cause massive die off of native plant populations
University of California Cooperative Extension Division of Agriculture and Natural Resources. 2020. Wildfire Preparation: Preparing Your Home. https://ucanr.edu/sites/fire/Prepare/Building/
→ This is a more detailed list on home hardening, i.e. making your home fire resistant
National Fire Prevention Association. 2019. Fire Spread on Ember-Ignited Decks. https://www.nfpa.org/-/media/Files/Firewise/Fact-sheets/FirewiseFactSheetsEmberIgnitedDecksCustomizable.ashx
→ This link has details on making decks more fire safe
Lando, Todd. (2020, February). Defensible Space, Vegetative Management, and Home Hardening to Reduce Wildfire Impacts. Presented at the symposium of Home Landscaping in a Changing Climate: Fire Smart, Water Wise and Biodiverse, sponsored by Marin Chapter California Native Plant Society, Bee Audacious, Marin Conservation League, and Marin Art & Garden Center, Ross, CA.
Disastersaftey.org. 2019. Protect Your Home From Wildfire. https://disastersafety.org/wildfire/protect-your-home-from-wildfire/
→ Details how to make your home most resistant to embers, which cause most home fires

From the Field

How to Collaborate with Your Landscape: Creating Fire-Resistant Habitat