Flammability of Natural Vegetation and Home Landscapes

Wildfires are an important and necessary ecological process in most natural areas of the southern United States. However, with increasing numbers of homes being constructed in close proximity to natural vegetation, risk of wildfire damage to homes in the wildland-urban interface (WUI) is increasing. To reduce WUI fire hazards most effectively, homeowners need to participate in wildfire risk mitigation. To do so they need information on managing their homes and landscapes to reduce flammability.

Two key components of managing fire risk on private property are to understand both the flammability of natural areas surrounding the home landscape and how landscaping design, materials, and plant species selections influence flammability and fire behavior (see fact sheet 'Understanding Fire Behavior' available at http://edis.ifas.ufl.edu/FR138). Several studies conducted through a partnership between the Centers for Urban and Interface Forestry and the University of Florida are addressing these critical issues.

Research Projects
Three research studies were designed to address the issues of wildfire hazard in the wildland-urban interface and investigate different facets of natural vegetation and home landscape flammability. These studies provide information to help guide firewise planning and to improve fire behavior modeling for urban settings.

1) Flammability of native understory species in pine flatwood and hardwood hammock ecosystems

Regionally specific information on shrub flammability will improve fire behavior models for wildland-urban interface applications and allow fire managers to plan fuel mitigation and firefighting activities more effectively. This study determined the flammability characteristics of six abundant understory shrub species in pine flatwood and hardwood hammock ecosystems, which are dominant vegetation types throughout the southern Coastal Plain. These ecosystems were selected because they were historically affected by fire at different return intervals. Fire-prone pine flatwoods typically experienced fires every 1 to 8 years while hardwood hammocks typically had 30 to 50 year fire return intervals. Although these ecosystems contain several of the same understory species, it was expected that the overall flammability of the understory shrubs would differ since the fire return intervals were historically very different.

Pine flatwoods shrubs were found to be more flammable than oak hammock shrubs, which underscores the importance of fuel management around and within communities surrounded by pine flatwoods. Saw palmetto (Serenoa repens), gallberry (Ilex glabra), rusty lyonia (Lyonia ferruginea) and American holly (Ilex opaca) were the most flammable species studied. In addition, flammability characteristics for saw palmetto and wax myrtle, the two species that were common to both ecosystems, were similar in the two ecosystems. However, higher densities of the two species in the flatwoods may lead to higher flammability levels in that environment, which illustrates that site and environmental conditions, such as available light or moisture, may create variations in flammability within the same species. Therefore, it is important to remember that in extremely hot and dry conditions, even plants considered to have lower flammability may become more flammable.

For more information see the research thesis from this study (Flammability of native understory species in pine flatwood and hardwood hammock ecosystems) at: http://etd.fcla.edu/UF/UFE0001167/behm_a.pdf. In addition, the fact sheet 'Considering Fire in Florida's Ecosystems' provides information on fire hazard ratings for other Florida ecosystems and is available at: http://edis.ifas.ufl.edu/pdffiles/FR/FR13700.pdf

2) Quantifying and ranking the flammability of ornamental shrubs in the southern United States

The shrub species used in home landscaping directly affect fire behavior and the risk of wildfire damage to the home. Long flame lengths and high energy release levels from densely planted or poorly maintained shrubs can easily spread fires into overstory pines or nearby structures. In order to provide better information for firewise plant lists for southern WUI landscapes and to improve WUI fire behavior modeling, this study investigated the flammability of thirty-four landscape shrubs commonly used in the South.

Twenty-two shrub species were found to have low flammability characteristics. These shrubs can be safely planted within the home landscape and are appropriate for addition to firewise plant lists. The eight species found to be moderately flammable can also be planted in home landscapes; however, they should not be planted adjacent to structures and should be isolated vertically and horizontally from other plantings to reduce the potential for fire spread. Gallberry (Ilex glabra), yaupon holly (Ilex vomitoria), Chinese juniper (Juniperus chinensis), and mountain laurel (Kalmia latifolia) were found to be highly flammable and were ranked as inappropriate for use within 30 feet of the home. Because species within the same genus were found to vary in flammability, it is important for homeowners and landscapers not to substitute plants on firewise plant lists with other species in the same genus without verifying flammability characteristics of the substitute species. All shrubs within the home landscape should be routinely maintained by removing dead or diseased branches and foliage.

Flammability Plant List [PDF]

3) Fire spread and structural ignitions from horticultural plantings in the wildland-urban interface

A new study, currently in progress, measures the rate of spread and heat release for fires moving through four common southern mulches under natural conditions. The mulches are also combined with gallberry shrubs planted at several densities to determine how mulches contribute to shrub (and ultimately structure) ignitions. For each of the mulches and shrub/mulch combinations, several drying schedules will provide insight into the value of irrigation for reducing fire risk close to structures. Mulch and shrub flammability will be repeated under controlled conditions in a national fire lab and the results of both studies will provide important information for new models that will be used for predicting fire spread and structural ignitions in a variety of landscape conditions.

Upon completion of this study, the knowledge gained about flammability characteristics of landscape plantings will help answer important questions such as: what are the flammability characteristics of plantings with different compositions and heights, how do drought and irrigation affect flammability characteristics, and what are the effects of planting designs on structural ignitions?

Collectively, these three important flammability studies are providing critical data needed for modeling WUI fire behavior as well as information needed by homeowners, community planners, state forestry agencies, landscape architects, nursery owners, and extension agents to create effective firewise landscapes for communities in the wildland-urban interface and fuel mitigation plans for adjacent natural areas.

Links to Additional Information about Fire in the Wildland-Urban Interface: