Grassland Fuels Management
This content is part of a series on grassland fuels management prepared in collaboration by Colorado State Forest Service with support from Southern Rockies Fire Science Network (SRFSN) and Great Plains Fire Science Exchange (GPFSE).
Wildfire Fuels Management Practices in Grass-Dominated Landscapes
Contents
Fuels Management
The following contains insights derived from discussions with Subject Matter Experts, facilitated by Chad Julian at the Colorado State Forest Service (CSFS).
Designing fuels management treatments in grass-dominated landscapes can present challenges, in part because few of these landscapes are purely grasslands. Many include other fuel types and the potential conversion of native grasses by non-native species, which can include forbs, shrubs and trees, under certain conditions can increase fire behavior and alter fire regimes. Additionally, factors such as drought, climate change and impacts from land use, including the expansion of the WUI, further complicate these management efforts (Leis, 2013).
Fuels treatments can have varying effects, therefore understanding how to interpret their outcomes can support management decisions when evaluating trade-offs between reducing wildfire risks and addressing other management goals. Currently, managing vegetative fuels is the most effective strategy for managing wildfire risk given that weather, climate and topography are factors that can’t be easily altered (Frangieh et al., 2009). Effective fuels treatments can mitigate some of the adverse effects of wildfire while also offering various benefits such as increased biodiversity, improved forest health and watershed protection.
Effective fuels treatments alter the structure and amount of fuels, reduce fire behavior and create access points for fire management resources (Trauernicht et al., 2021). Reducing fuel loads and changing continuity of fuels (both horizontal and vertical) can help to protect lives and assets while also reducing the chances of extreme fire behavior. Therefore, understand local conditions and inherent site characteristics, such as;
- resilience to disturbance
- proximity to structures
- seasonal weather cycles
- susceptibility to non-native species
- land use
- prior or surrounding treatments
- topography
- values at risk
- or other hazards and unique features
These can help guide location, type and method of treatment. This information can also help provide further guidance on when and how frequently treatments will need to be repeated after the initial treatment begins to lose effectiveness. WUI and other private lands adjacent to public lands can facilitate wildfire spreading across a landscape to other areas, including urban sites. When possible, knowing where on the landscape a potential ignition may occur under severe weather events, especially for any that have potential pathways to spread into communities, can enhance treatment effectiveness and reduce risk. It is important that transmission pathways be identified as well as areas where treatments on public or private lands can be implemented to reduce risk. A key first step in determining strategically located fuel treatments is to identify where on the broader landscape a high amount of human exposure is present (Hass et al, 2014).
When considering wildfire risk to the WUI two critical factors should be considered; fire ignition location and potential for fire spread. While the source of ignition can vary from human-caused to natural causes, recreation and roads can contribute to closer and more frequent sources of ignition to populated areas. Although ignitions are the source of the hazard, it is the spread of the ignition that ultimately causes the greatest effects. Under severe weather events high winds and dry fuel conditions can significantly affect fire spread and make suppression of wildland fires difficult and dangerous.
Further information from Rocky Mountain Research Station, “A Wildfire Risk Assessment Framework for Land and Resource Management”, states that wildfire risk analysis involves addressing several critical questions:
- How large are fires likely to grow?
- Which highly valued resources and assets (HVRAs) have the greatest exposure to wildfire hazard?
- What are the likely effects to HVRAs of fire at different intensity levels?
- Where might fires cause harm/damage and where might they lead to benefits?
- How is wildfire risk distributed across the landscape?
- Which HVRAs are most likely to experience loss, how much loss and to which ones specifically?
- Where can wildfire risk be best mitigated?
- What treatments and management activities are feasible?
- Where can different treatments be implemented and to what extent?
- How will treatments affect various risk factors (likelihood and intensity)?
- How will treatments affect potential impacts to HVRAs?
- What combinations of activities can most cost-effectively mitigate wildfire risk?
(Scott et al. 2013)
Assessing risk may also include climate and weather patterns, historical fire occurrence, and information-gathering on current fuel conditions. It is recommended to identify and quantify any risks associated with wildfire and to use this knowledge to develop mitigation strategies that reduce impacts to ecological, social and economic systems while remaining as cost-effective as possible. This requires knowledge on locations where fires have a greater potential to occur, range of fire intensity levels for the region and possible impacts to HVRAs (Scott et al., 2013). Management of vegetative fuels is only effective when vegetation is regularly maintained, which may require monitoring and integrating management costs into a long-term management plan. To maintain effectiveness, management may require multiple treatments per year with consideration for vegetative growth cycles, whereas sites in wetter areas or during wetter years may require more treatments, with conditions potentially changing seasonally and year-to-year (Trauernicht at al., 2021).
In grass-dominated landscapes, fuels management often requires multiple applications per year and must be responsive to “green-up” events that follow rainy periods. Effective implementation hinges on treatment frequency and timing, and includes factors caused by seasonal variability, climate influences and other potentially dynamic conditions. Optimal efficacy of fuel treatments is typically achieved when conducted prior to periods of increased fire risk, potentially reducing the need for more frequent maintenance (C. Julian, personal communication, May 30, 2024).
The timing of grass fuel treatments depends on several factors, including the type of grass, climate and treatment objectives. Depending on species, and whether it’s predominantly cool-season or warm season grass, treatments should be considered when the majority of fuels are cured. In Colorado’s grass-dominated landscape regions and areas along the eastern slope of the Rockies, the general risk window is from September through April. In these landscapes, an ideal time to conduct a fuels application in grass fuels will likely occur in early Fall (around September), when many of the grasses are cured. This timing aligns with adequate fuel conditions (i.e. curing) and the wind season, whereas mowing during growing season will require additional treatments due to the growth of grasses, adding time, cost and effort, with a greater potential to increase ecological damage (C. Julian, personal communication, May 30, 2024).
For specific guidelines and best practices, it is advisable to consult with local land management agencies and conservation districts. Additionally, for region-specific mowing recommendations, contact your local CSU Extension office, county land manager, or Colorado State Forest Service Field Office.
For more information on wildfire hazard and risk management and analysis, see: https://www.fs.usda.gov/rm/pubs/rmrs_gtr315.pdf.
For current grass productivity levels, see the USDA’s Grassland Productivity Forecast: https://www.climatehubs.usda.gov/hubs/northern-plains/tools/grass-cast-grassland-productivity-forecast
To access CSFS’s Wildfire Risk Reduction Planner for web-mapping designed to support Colorado’s wildfire protection and mitigation planning needs, visit our Colorado Forest Atlas page: https://coloradoforestatlas.org/
Management Approaches
Mechanical alteration to the landscape can include mowing, weed-whacking, haying, raking and plowing. Mechanical methods, especially in grasses, can often be more effective at smaller scales and may not be very applicable to alter fire behavior in spatial scales needed to reduce landscape-scale fire. For smaller scale treatments, such as those in residential areas adjacent to grass fuels, it is recommended to maintain grasses in Zone 2 (5-30 ft from a structure) at a height of 4 inches or less, especially before and during periods of increased fire risk. For Zone 1 (0-5 ft from a structure), it is recommended to not have any grasses (see Low Flammability Landscape Plants) (Live Wildfire Ready, 2024). In Zone 3 (30-100 ft from a structure), grass should be considered ladder fuels and treated as such. Equipment such as weed whackers or mowers can be used in green vegetation but should be used with caution in dry conditions where fine fuels like grasses can easily ignite. When used in dry conditions equipment should be free from faults and mechanical defects that could cause a spark and ignite a fire (Country Fire Authority, 2023).
Mowing, raking and other methods of fuel break construction that overturn soil can create erosion and facilitate the establishment of non-native and invasive species, which can alter fire behavior (Country Fire Authority, 2023). Additionally, excessive mowing in grass-dominated landscapes is not recommended. Over-mowing can alter vegetation in the short term but can result in detrimental impacts to ecological integrity for the treatment site and can exacerbate future fire behavior by facilitating woody encroachment and the introduction of other non-native plants, potentially leading to increased fire intensity and behavior (C. Julian, personal communication, May 30, 2024).
It is essential to develop strategies that are adapted to local growing conditions, as factors such as timing, frequency and other conditions may vary across landscapes. For example, regions characterized by tall-grasses may find it beneficial to remove cut or mowed grasses after treatment to mitigate vegetative fuel accumulation on treatment sites, whereas regions with mixed- or short-grass may not have the same necessity. As a result, a thorough understanding of local site conditions is critical for devising effective strategies.
Herbicides can be used to complement vegetative fuel treatments in wildfire-prone areas. Herbicide treatment can target specific vegetation types, like non-native species, which contribute to vegetative fuel loads while ideally minimizing impact on non-target species, which can help to effectively reduce fuel continuity and decrease the intensity and spread of wildfires.
When combined with mechanical treatments or prescribed burns, herbicide use can assist in achieving more effective fuel reduction treatments. They can be used initially to reduce vegetation density and are then followed by mechanical treatments to further modify fuel characteristics and structure. Herbicide can be an option in areas where prescribed burning or mechanical treatments are difficult or unlikely, and can offer an alternative approach to managing vegetation fuels that address both immediate and long-term wildfire risk.
For further insights into effective application methods, potential environmental impacts, and strategies for integrating herbicide use into comprehensive fuels treatments, consider consulting with local land management agencies and conservation districts to provide specific guidelines and best practices.
Grass-dominated landscapes have historically been influenced by grazing and have played a key role in their evolutionary history (Blair et al., 2014). However, human activities in North America have greatly increased livestock herbivore dominance and nearly eliminated the role of fire. These disruptions include the decline of wild herbivores, livestock expansion and altered fire regimes. These human-driven changes have resulted in major ecological impacts, threatening biodiversity in these landscapes as both fire and herbivores have historically shaped ecological niches (Wendt et al., 2023). These factors, along with a changing climate and alterations in land use, further complicate landscape responses to large herbivores that have historically grazed these landscapes (Engle et al., 2001).
In rangelands, fire behavior can vary based on factors such as grazing history, weather patterns, fire frequency, topography, vegetative composition, and soil conditions (Engle et al., 2001). Wildfire can pose risks to livestock and also disrupt management practices by consuming forage and damaging structures (Middlemis-Brown, 2015). Conversely, grazing can decrease the overall flame length through reducing fuel loads and height through consuming and trampling vegetation. Wild or feral grazers can contribute to the reduction of grass fuel loads; however, effective mitigation requires intentional management of grazing areas at adequate stocking rates to strategically reduce fuels (Trauernicht et al, 2021).
Treatments that involve grazing can effectively reduce fuel loads and continuity while altering the structure of vegetative fuels (Blair et al., 2014). However, trade-offs can exist between fire risk reduction and long-term forage quality and production. Grazing can effectively reduce fire risk, but if not managed properly, overgrazing may lead to increased erosion and contribute to the conversion of woody vegetation over the long-term. This can result not only in increased fire behavior but can also prohibit future grazing due to unpalatable vegetation. Similar outcomes can occur with overuse of mechanical methods (Trauernicht et al, 2021).
While herbivore grazing can help support grass-dominated landscapes conducive to low-intensity fires and reduce fire frequency (even light grazing and browsing can mitigate fire intensity) intensive grazing may have the opposite effect by diminishing grassy vegetation cover and promoting the growth of highly combustible woody vegetation. In some instances, grazing management practices have been linked to an increase in fire ignitions and frequency. However, there are also circumstances in which grazing only reduces fire frequency under certain conditions, such as specific times of the year, under specific management practices or specific vegetation types. In other instances, grazing leads to more frequent but lower-intensity fires, thereby decreasing the likelihood of extreme wildfires. Ultimately, the effectiveness of herbivores in reducing fire frequency is influenced by factors such as the season, grazing intensity and landscape type (Rouet-Leduc et al., 2021).
In a 2021 review of 74 studies on the impacts of grazing on wildfire, 21 studies specifically examined the effects of herbivores on fire frequency. Thirteen of these studies found that grazing reduces wildfire frequency. The remaining studies indicated that grazing only reduces fire frequency under certain conditions, such as specific times of the year or different vegetation types. In some instances, the presence of herbivores leads to more frequent but lower-intensity fires, thereby decreasing the likelihood of extreme wildfires (Rouet-Leduc et al. 2021). The effectiveness of grazing in reducing fire frequency is influenced by factors such as the season, grazing intensity and landscape type. For effective land management, it is crucial to understand the conditions in which grazing supports the preservation of grass-dominated landscapes that facilitate low-intensity fires and reduce fire frequency. Conversely, recognizing situations where intensive grazing reduces grassy vegetation cover and promotes the proliferation of woody vegetation is equally important. Achieving this understanding may require local knowledge and experimental approaches (Rouet-Leduc et al., 2021). Reduced grazing and fire suppression in rangelands adjacent to urban and WUI areas may have extended fire regime intervals in these regions, leading to an increase of less palatable woody vegetation, which is considered one of the largest threats to rangelands, and can result in increased fire behavior (Fogarty et al, 2023; Scasta, 2019). These changes, particularly in low to mid elevations, can result in more frequent and larger wildfires (Texas A&M, accessed 2023). In these circumstances, emerging technologies, such as virtual fencing, can offer additional solutions for grazing fences and enhance the effectiveness of certain treatments in areas where prescribed fire may not be an option, with the goal of improving the overall resilience to disturbances of rangelands, especially in areas where impacts from altered fire regimes and land conversion may increase the amount and continuity of fine fuels like grasses (Middlemiss-Brown, 2015; Schachtschneider, 2016; Trauernicht et al., 2021).
Prescribed fire can be a relatively low-cost and effective management method used to reduce fuels at scale and wildfire risk, while potentially supporting other management goals, including the restoration of keystone ecological processes, where frequent low intensity prescribed fires may be a key piece to restoring and maintaining certain landscapes (Donovan et al., 2023; Addington et al., 2020; Vogl, 1979). Fire plays a critical role in grass-dominated landscapes and while management practices, including mechanical, grazing and herbicides, can yield comparable outcomes, they do not substitute for the unique ecological functions of fire (Ellensworth et al., 2022; Vermeire et al., 2020). However, despite the many benefits, prescribed fire is often challenging to implement due to a host of factors, including high-risk fuel loads, proximity to HVRAs (such as homes, utilities and other assets), susceptibility to weather conditions, smoke management and air quality regulations, agency resource capacity and public perception (Ellensworth et al., 2022).
Prescribed fire, also referred to as burning, can be a beneficial tool for rangeland management, offering ecological and economic advantages that can lead to more effective outcomes. Burning has been shown to improve livestock performance, forage quality and palatability. Studies show that burning also provides benefits to wildlife habitat without adversely impacting livestock (Weir et al., 2009). Additionally, because grazers (both domestic and wild) are drawn to the fresh growth that emerges after a fire, this technique can help to move grazers from one area to another, which can reduce the need for fencing (The Nature Conservancy, accessed 2023).
Fire-adapted landscapes rely on fire to maintain their functionality and biodiversity. Fire is essential for the health and sustainability of grass-dominated landscapes, which host native plants and animals that depend on periodic fires for survival. Since European settlement, fire suppression in North American grasslands has led to ecological and economic degradation (The Nature Conservancy, 2022). Many landscapes across North America, particularly in the western continental U.S., have historically been influenced by natural fires, including those ignited by lightning, and human-introduced fires used for purposes like land clearing and fuels reduction.
Increasing human development has resulted in the need for fire suppression in many of these regions. Estimates suggest that in the conterminous US, burning in the late 20th-century was 7-12 times less prevalent than during pre-industrial times. The consequence of these suppressed and altered fire regimes have led to a reduction or loss of ecosystem services and drastically altered fuels and potential future fire behavior. Without the disturbance of periodic fire, woody plant density increases and landscape structure becomes more homogeneous (Ryan et al., 2013). In almost all regions of North America, the application of fire falls short of its historical prevalence and has not kept pace with historical levels. The combined effects of fire suppression and reduced prescribed burning have significantly altered fire regimes across much of North America. This shift led to a marked decline in fire occurrence in the western U.S., where the total area burned decreased sharply, often due to fire suppression efforts. Consequently, the area being burned today across much of North America is significantly less than what burned historically (USDA, 2023). To preserve these landscapes, the US Forest Service and other agencies and organizations have developed prescribed fire programs. As part of the Wildfire Crisis Strategy, the US Forest Service has identified landscapes that urgently need work to mitigate wildfire risks, in which a major component of this strategy is the reduction of hazardous fuels, with prescribed fire playing a key role in this effort. These programs introduce fire under controlled conditions to achieve specific management objectives (USDA, 2023).
Agricultural land encompasses croplands, rangelands, pastures and other lands used for crop or livestock production. Historically, fire has been an essential tool in agricultural management, a practice that continues in modern agriculture where burning remains a cost-effective method for managing crop residue. Agricultural burning can include practices such as rangeland and crop residue burning, land clearance and ditch lines (USDA, 2023; AAQTF,1999).
Common reasons for burning include reducing pre- and post-harvest vegetation that interferes with harvest, tillage and seedbed preparation, or for pest and weed control, which subsequently reduces the need for herbicides and pesticides. Additionally, certain crops require burning to stimulate growth and increase yield, burning supports nutrient cycling and soil health. Periodic burning is also utilized to reduce fire hazards and for clearing irrigation canals for maintenance of water conveyance structures related to agricultural operations and can be a cost-effective method for managing agricultural lands (USDA, 2023; AAQTF,1999).
For more information on agricultural burning in Colorado, see the Code of Colorado Regulations https://www.sos.state.co.us/CCR/GenerateRulePdf.do?ruleVersionId=1085
Irrigation ditches can contribute to fire spread due to the abundance of vegetative fuels. Ditches that are dry or inadequately maintained can act as pathways for fire spread, especially when there is an accumulation of woody fuels. Under certain wind and topographical conditions, these features can facilitate the movement of fire along corridors, potentially extending into nearby communities. Risks can be compounded by ember production from woody fuels in the ditch and the connectivity of available fuels. Conversely, ditches that are well-maintained or adequately hydrated can act as effective barriers, preventing the spread of fire (Scasta et al., 2019).
Ditch management can include mowing, dredging, herbicide use and burning (Scasta et al., 2019). Treatments that employ mowing or herbicide only reduce risk if accumulated vegetation is removed, and dredging physically impacts the soil which can increase the risk of erosion, a concern in ditch management. The practice of burning ditches presents both advantages and disadvantages. It effectively reduces fuel loads and can improve water management efficiency by removing vegetation that accumulates along ditches, drains and fence lines. While burning can be more efficient and cost-effective for vegetation removal compared to alternative methods, it can also negatively impact water quality, biodiversity, and soil health (US Dept. of Interior, 2017). Therefore, identifying risks and hazards, potential outcomes, goals and objectives, and other considerations in advance can help to guide the most effective treatment.
In Colorado, ditches are generally owned and maintain by private companies. Contact a local company regarding vegetation management in irrigation ditches.
For more information on agricultural burning in Colorado, see the Code of Colorado Regulations: https://extension.colostate.edu/topic-areas/natural-resources/irrigation-ditches-and-their-operation-6-701/
Heterogeneous Landscapes
Heterogeneous refers to something that is composed of diverse or different elements, therefore, heterogeneous landscapes contain mixtures of different varying patches of vegetation. This landscape characteristic can influence fire behavior, especially where there is a high presence of connective fuels, such as grasses.
Fire burns more intensely and more rapidly with homogenous vegetative fuels, particularly in continuous fine fuel beds (Trauernicht et al, 2021). Conversely, fire spread is hindered in environments with fragmented vegetation like those found on heterogeneous landscapes, where fire spread relies on embers, which can travel and ignite other fuel sources farther than several hundred meters (or almost 1000 ft) from the nearest area of wildland vegetation (Caggiano et al., 2020). As a result, efforts that disrupt the continuity and increase the patchiness of fuels can reduce the rate of fire spread and fire intensity (Trauernicht et al, 2021).
Fuels reduction treatments resulting in heterogeneous landscapes often employ more than one method of treatment and can include combinations of methods (Trauernicht et al, 2021). This strategy can lead to smaller and less intense fires, reducing loss of life, lost values at risk, suppression costs, and potentially reduce the size of the treatment area and therefore reduce management costs (Trauernicht et al, 2021). Certain combined treatments, such as prescribed fire and grazing, play crucial roles in various ecological processes, including nutrient dispersal, carbon storage, habitat structuring and biodiversity maintenance, and can produce similar outcomes on the landscape (Wendt et al., 2023). Whereas the interaction between timing, seasonality and frequency of grazing (or mowing) and fire, can have the greatest effect on vegetative biodiversity (Fynn et al., 2004).
Climate, fire and grazing are three critical drivers influencing the composition, structure, and functioning of rangelands (Blair et al., 2014). In these landscapes, prescribed fire promotes heterogeneous vegetation patterns, which can more effectively reduce fire behavior compared to homogeneous treatments, while also enhancing biodiversity (Trauernicht et al, 2021; Fynn et al., 2004). Prescribed fire applied in patches (patch burning), combined with grazing, can result in patchy vegetation patterns and less fuel loading and continuity, ultimately reducing rates of spread and fire intensity (Trauernicht et al, 2021).
Management practices that incorporate fire and grazing employ some of the very ecological processes that shape these regions and can further promote heterogeneous landscapes through trampling, soil compaction, soil tunneling and redistribution of nutrients, and through selective consumption of certain species (Blair et al., 2014). Grazing and fire interact in a dependent manner, with spatial and temporal patterns of grazing often influenced by fire and, conversely, grazing patterns shaping future fire occurrences (Blair et al., 2014; Fuhlendorf et al., 2009). Fire can increase the likelihood of a patch being grazed, since grazers regularly prefer foraging in recently burned areas. This practice results in heterogeneous landscapes from grazers choosing between burned and unburned patches, with fire occurrence dependent on fuel removal by grazing (Fuhlendorf et al., 2009).
Both methods remove vegetation, reduce fuel accumulation and change fire behavior by altering height and fuel loading, however, the combined effects of fire and grazing on landscapes can vary (Middlemis-Brown, 2015). Research shows that low to moderate levels of grazing can significantly reduce fire behavior, however while some rangelands exhibit resilience to relatively intense grazing pressures, others can quickly degrade, leading to woody encroachment and conversion to shrub-dominated landscapes (Schachtschneider 2016; Blair et al., 2014). Determining treatment methods and frequency will depend on management goals, landscape and climate conditions, and fuel load and type, where combining techniques may increase treatment effectiveness (Leis, 2013). To achieve optimal outcomes that result in fuels reduction and ongoing grazing land use, it is important to determine the ideal placement, timing and intensity of grazing treatments (Middlemis-Brown, 2015; Schachtschneider, 2016; Trauernicht et al, 2021).
Defensible Space
The following contains insights derived from discussions with Subject Matter Experts, facilitated by Chad Julian at the Colorado State Forest Service (CSFS).
Building loss from wildfire has mostly occurred in areas with low building densities and high vegetation cover, where most structures that burn ignite due to embers or firebrands starting small fires in fine fuels that can grow to ignite heavier building materials (Zhou, 2019). As a result, WUI and rural intermix environments with wildland vegetation have experienced higher losses due to a greater potential for fire spread and building ignition. Conversely, building loss in more dense environments may be influenced by unique environmental factors related to the buildings themselves, such as construction materials, landscaping and proximity to neighboring buildings, all of which may facilitate building-to-building ignition, or urban conflagration (Caggiano et al., 2020).
In grass fuel types, it is just as critical to focus on structure ignitability from a flaming front of a grass fire. Fires in prairie lands can move 400 to 600 ft/min with the potential for just as many losses occurring in this fuel type as the areas with heavier concentrations of fuels (Gray et al., 2007). Structures are only susceptible to fire spread if they meet the necessary fuel and heat requirements sufficient for ignition and maintained combustion (Cohen, 1999). Ensuring there is adequate clearance of vegetation, structure ignition models indicate that large flame fronts like those from crown fires will not ignite wood surfaces greater than 40 m away (120 ft) (Cohen, 1999). For structures 100-850 m (328-2788 ft) from wildland vegetation, buildings may still be at risk of igniting but exposure is likely caused by embers and ignition becomes more influenced by the built environment (Caggiano et al., 2020). However, in most simple interfaces characterized by predominantly grass fuel models, significant ember production is generally not observed due to the rapid ignition and combustion of these flashy fuels. In many North American landscapes, grass serves as the primary carrier of fire rather than trees, even under extreme weather conditions. This is true across various landscapes, including pure grasslands, shrublands and coniferous forests (with the exception of lodgepole pine and spruce), where grass fuels primarily drive fire spread.
Fire spread occurs in three main ways:
- direct flame contact (conduction)
- ember attack (convection)
- transfer of heat (radiant)
The CSFS Home Ignition Zone (HIZ) defensible space and home hardening guide, initially designed for ponderosa pine, Douglas-fir and mixed-conifer forest types within the HIZ, maintains consistent recommended practices for addressing a home ignition zone around structures and assets and can be applied in grass-dominated landscapes. However, if a complex interface is present and structures are also surrounded by woody fuels like shrubs and conifers, significantly greater distances than 30 ft is recommended (Caggiano et al., 2020). This HIZ Guide is also available in Spanish.
This is due to risks of ignition resulting from wind-driven embers (convection), transfer of heat (radiant) and direct flame contact (conduction) in complex cover types. Areas that experience high winds should consider even greater distances; a 2020 Colorado State University study found that 95-100% of all fire-related WUI building losses occurred within areas that were within 100-850 m (300-2550 ft) of wildland vegetation (Caggiano et al., 2020).
Home Ignition Zone
When managing vegetation in built environments, in interfaces like grass-dominated landscapes, conduction and radiation occur on scales of inches and feet, therefore the first 5 ft surrounding a structure is critical. In this area, Zone 1 (0-5 ft from a structure), it is recommended to not have any vegetation present, including grasses. This is because grasses can ignite structures through direct flame contact (conduction) and heat transfer (radiation). Generally, embers are not produced from grass fuels but will come from woody vegetation from irrigation ditches, encroachment or from forest cover types with 1.5 miles (approx. 2400 m) from structures and urban vegetation (C. Julian, personal communication, May 30, 2024). In Zone 2 (5-30 ft from a structure), it is recommended to maintain grasses at a height of 4 inches or less. In Zone 3 (30-100ft), grass should be considered ladder fuels and treated as such. Ideally and wherever possible, a radius of 30 ft minimum clear of vegetation from the base of each structure will provide further reductions in risk.
However, where this guidance doesn’t permit, i.e. in urban areas with less than 100 ft between structures, an issue more common in subdivisions and higher density areas, it can be highly beneficial to consider grouping all structures into a single feature anywhere Zone 3 overlaps with adjacent properties. These conjoined features can create a single large HIZ and can consist of a high number of homes. In these cases, in grass-dominated interfaces, the distance into the built environment from the interface should consider at least the first three rows of homes, or approximately 400 ft, as Zone 3. This is where the most critical areas of mitigation measures exist and actions will need to be made in collaboration with surrounding neighbors and adjacent landowners (C. Julian, personal communication, May 30, 2024).
This area, referred to as the Home Ignition Zone (HIZ), outlines the space around a structure where the two main factors determining a home’s ability to withstand a wildfire are found. It’s recommended to keep Zones 1 and 2 clear of fuels like trees, shrubs, grasses, mulch, firewood and wooden fencing, as much as possible (see Low-Flammability Landscape Plants).
Learn More About Defensible Space
Research shows that treatments surrounding housing developments can provide the greatest level of protection in reducing transmission potential into a community, compared to treatments on federal lands, where reductions in transmission were very effective for fire management on federal lands but comparatively did little to reduce exposure from fire in housing developments.
Adapted from Thompson et al., 2022
Fuel Breaks
A fuel break is defined as a “natural or human-made change in fuel characteristics which affects fire behavior so that fires burning into them can be more readily controlled” (National Wildfire Coordinating Group, accessed 2023). Fuel breaks encompass an area where the quantity of fuels is strategically altered by reducing or eliminating vegetative fuels. In addition to reducing the overall quantity of vegetative fuels, changing the vegetation structure within and along the edges of the fuel break can reduce fire risk. Fuels can be reduced across entire areas, around specific resources or known areas of frequent ignitions and as linear breaks with strategically located strips of land.
The objectives of fuel breaks are to:
- reduce fire intensity
- increase survivability on roadways during evacuation
- decrease the rate of spread
- alter fuel continuity
- reduce spotting potential
- improve access for firefighters and emergency responders
- create safer and more defensible space
- reduce risks to assets
- allow for possible engagement of suppression activities
- lessen fuel loads
Fuel breaks may be particularly important in grass-dominated landscapes adjacent to urban areas or structures (Leis, 2013). When grass fires move rapidly over large areas, fuel breaks and fuel treatments disrupt the horizontal continuity of vegetation, which can slow fire spread. Fuel breaks, particularly in grass-dominated landscapes, must be monitored and maintained to remain effective. Variations in weather and growing conditions can affect how often maintenance will need to occur. The lack of regular maintenance has been identified as the biggest cause of fuel break failure in nearly all fire manager discussions (Trauernicht et al, 2021). This is especially true in grass-dominated landscapes where vegetation can recover very quickly following fuels reduction treatments.
The most frequently asked question about fuel breaks is “How wide should they be?” The answer from a risk reduction standpoint is “As wide as possible and as wide as necessary.”
When determining fuel break width it’s important to consider:
- fuel
- fire intensity
- weather
- slope
- where the fuel break is in relation to the fire itself
- the value of the resource to be protected
Mowed fuel breaks are common in grass-dominated landscapes and can help to reduce fire intensity, reduce risk of ignition to other nearby fuel sources by reducing the flame height, and altering vegetative fuel characteristics, but it may not stop fire spread. The minimum recommended fuel break width for wildfires is approximately 300 ft for level ground, without taking into account additional factors like topography and local weather patterns within forested fuel models (Dennis, 2005). In landscapes with mixed vegetation, like grasses and shrubs, the presence of woody fuels within 20 m (65 ft) of the fuel break significantly increases the potential of ember loft and should be removed or compensated for by increasing the fuel break width, as this material can increase fire intensity, increase risk of spotting, and act as a bridge for fire across a fuel break (Trauernicht et al, 2021; Country Fire Authority, 2023).
There has been little experimental work on fuel break effectiveness to provide specific dimensions for grass-dominated lands. Additionally, the effectiveness of any type of fuel break will be limited during conditions of extreme fire behavior. Fuel breaks may need to be wider in areas where other factors may alter fire behavior, such as midway on slopes or in areas exposed to frequent high winds. However, fuel breaks should also be balanced with land management objectives and ideally, ecosystem health. Consequently, selection of fuel break type and extent in grass-dominated landscapes depends on fuel load and type, site condition, maintenance demands, landscape and goals (Trauernicht et al, 2021).