“Wildfire season” is still a relatively new phenomenon to those in the pacific northwestern (PNW) region of the U.S. The PNW has always had fires (both wild and cultivated), but due to climate change the environment has become more vulnerable to intense fires that burn longer, wilder, and hotter than before. Drier air and soils, human activity and development, hotter weather, and certain plants all contribute to more intense fires. 

While all plants burn, the plants we’re pointing to are PNW weeds that pose a particular risk of catching and spreading fire. This concept is not exclusive to a particular region; the role of invasive plant species was highlighted most recently by the devastating fires in Maui. As fires are becoming more frequent, many of us are revisiting the topic of how we can protect our homes. Invasive plants are an essential part of the conversation.

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How do invasive species contribute to fire hazards?

“Monitoring and controlling invasive plant species that increase wildfire risk” was 1 of the 12 actions laid out in King County’s 2022 Wildfire Risk Reduction Strategy publication. 

First, let’s be clear that all plant species, native or introduced, have the potential to burn in a fire. But not all plants burn or spread fire in the same way. There are two concepts to consider when thinking about how a plant species may contribute to a fire’s growth: the plant’s role as a potential accelerant or as a ladder fuel. Accelerants catch fire easily and help it spread across a landscape. Ladder fuels help a fire spread upward and into the forest canopy.  

Use the slider below to see what a fire event could look like before and after when your space is fire wise, versus not. Move the slider from the center of the image using the white circle with arrows to slide left and right.

Following a disturbance (e.g. construction, landslide, fire), weeds are often of the first species to establish. By nature, weeds tend to grow faster and can survive harsher conditions than many of our native plants. The risk of ladder fuels and accelerants grows as the diversity of other plant species is reduced. For instance, dense stands of any one species (monocultures) mean that there is no “fire break” to stop or slow a fire in its tracks. If the species is an accelerant or ladder fuel, what may have been a short-lived fire could become a much greater problem. By controlling your weeds, you can reduce this risk.

Often the connection between invasive plants and fire creates a positive feedback loop. This happens when Problem A (weeds) feeds into Problem B (fire), which in turn feeds back into Problem A (weeds) and the cycle continues until it is interrupted. Interrupting this feedback loop is crucial to reducing fire risk and restoring fire resistant habitats. 

What we mean by a “positive feedback loop” between weeds and fire:
1. A weedy site catches fire – fire spreads further & with more intensity than it would otherwise
2. Burned site is now less hospitable, weeds re-establish 1st (they thrive in disturbed areas)
3. Weeds establish before native plants can, become dominant plants in landscape – the cycle begins!

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Hazard weeds

There are many weedy species that can increase fire risk, but most of them fall into one or more of the following categories: 

Dry Grasses

In drier climates, grasses such as cheatgrass (Bromus tectorum) or ventenata (Ventenata dubia) dry out and become a rapid accelerant for fires.  

Climbing species 

 Climbing species, such as old man’s beard (Clematis vitalba), English ivy (Hedera spp.), and field bindweed (Convolvulus arvensis), can help fires reach upper levels of a forest. This increases the potential for a damaging crown fire.  

Woody plants with high oil content 

In climates west of the Cascades, Scotch broom (Cytisus scoparius) and Gorse (Ulex europaeus) are both highly flammable species that create dense brush that can spread fire quickly. Common holly (Ilex aquifolium), is both highly flammable and creates a dense understory that can function as ladder fuel. 

Swipe below to see all 8 hazard species in the order they were referenced.

Both living and dried brush can compromise fire-fighting efforts by adding to a wildfire’s fuel load. Scotch broom (below) shows up often along roadways and through powerline corridors and can sustain and spread fires in those areas.

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What you can do

Note: this list is regarding weed control and fire risk in your living space, this is only one step in being fire wise ready. Visit Are YOU Wildfire Ready? (wildlandfirersg.org) for further guidance.

• Control the weeds on your property, prioritizing those mentioned above. Learn more about ID and control methods for the listed weeds by following the links attached to each plant’s common name. Don’t see your weed? Check here.
• When controlling hazard weeds, clean up and dispose of your yard waste away from your living space.
• Make a buffer zone around your living space, remove all high-risk plants (weeds, dry vegetation, etc.) within 100 ft of the home.
• Plant non-invasive fire resistant plants for your area.  Choose plants that accumulate less dead vegetation throughout dry season and have an open branching pattern.

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Weed control motivated by fire

One success story from King County is the control of gorse, a regulated Class B Noxious Weed. Gorse, Ulex europaeus, has oily leaves and stems, and highly flammable seeds that makes it an extreme fire hazard. Fire is important to gorse’s reproduction because it helps open its hard seed pods, thus it is one of the first plants to appear after a fire. It is native to western Europe and common in rural parts of Ireland. In its native range, gorse is valued as an effective hedgerow shrub. Historically, it was used in the Irish countryside for a variety of needs including cooking fuel for heating ovens and limekilns.

Gorse was introduced to Oregon in the late 1800s and has since spread across the state and into Washington and California. Gorse has fueled a number of large fires in Oregon; one such fire destroyed the entire town of Bandon, Oregon in 1936. Due to the high oil content in the plant the fire was difficult to contain and control.

Because of its impacts, gorse was added to the Washington state noxious weed list in 2000. Its role in wildfire was one of the key reasons for its listing. Thankfully, weed control efforts throughout Washington and in King County have reduced Gorse populations since its initial listing. As of 2023, there are only 11 known gorse sites in King County; all infestations are monitored and controlled annually. Learn how to identify gorse here. Please email us if you find a gorse plant in King County!

Reducing fire hazards is yet another reason to manage the weeds where you live. Visit our website to learn more about how to control noxious weeds in your area: kingcounty.gov/weeds.  

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Local & national resources to learn more about fire risk and fire safety

Follow the links below to learn more:

• King County Wildfire Risk Reduction Strategy 
• King County Firewise Brochure 
• Washington State Department of Natural Resources Community Wildfire Resilience and Preparedness | WA – DNR 
• National Fire Protection Association – Preparing homes for wildfire