Poison hemlock is a highly toxic plant that has been growing in the United States since the 1800s, but it wasn’t listed in Washington state until 1988 and was only selected for control in King County in 2019. Since then, King County’s Noxious Weed Control Program has worked to prioritize control of this abundant species.

Today, the Program is tracking nearly 900 sites (and counting) throughout King County. When poison hemlock was first regulated, control was required only on public lands.  In 2026, after six years of regulation demonstrated a need for additional control, requirements expanded to select priority sites on private lands.

But what makes a site a “priority”?

To help answer that question, the Program developed its first poison hemlock prioritization tool. The tool considers various impact areas of the species, but as we apply the tool, we’ve found limitations and considerations that need to be addressed.

P.S., new to poison hemlock? This blog dives deep on the prioritization strategy, but not the species itself. For general information, visit our 2024 blog: “Poison hemlock: Know it, Dig it, Trash it.”

The Poison Hemlock Prioritization Tool

There are many factors to consider when trying to prioritize hundreds of sites. To make this easier, the program took a more structured approach. Meet: the scoring matrix.

Sites are scored using a rubric that considers 6 factors:

1. Public exposure risk: Proximity to places where people move through and commute frequently
2. Community density: How densely populated an area is
3. Agricultural/pastoral impact: Distance from agricultural cultivation or livestock production site
4. Likelihood of spread: How close major vectors like highways and flowing waterbodies are
5. Proximity to other infestations: Number of infestations that are in community
6. Habitat quality: Quality of native vegetation and level of ecological disturbance

Each factor is scored from 0-5. The scores are added up, and the total score tells us whether this site is a priority or not. Scores range from 0-30, a score of 0-10 appoints low priority, 11-20 is moderate, and 20-30 is high.

Example case study: applying the matrix to different site types

Picture this: it’s been a hot spring, and all your poison hemlock sites are starting to flower at once. This gives you only a few weeks to manage dozens of sites.

Let’s see how this tool can help us narrow down site priority.

Sample sites:

Table 1: Utilizing the Scoring Matrix for Poison Hemlock Prioritization on 3 Sample Sites

Site	Public Exposure Risk	Community Density	Agricultural/ Pastoral Impact	Likelihood of Spread	Proximity to Other Infestations (EDRR)	Habitat Quality	Total Score
1: Rural Roadside Near Agricultural Area	0	0	4	2	4	0	10
2: Industrial Urban Construction Zone	3	1	0	4	0	1	9
3: Suburban Near Restoration Area	4	4	0	4	2	3	17

Interpreting the matrix results

The matrix alone gives a clear priority site: #3. This is a site that near a neighborhood restoration site and trails. It lost priority points in not being near agricultural areas and in being near other infestations (we prioritize controlling infestations that may prevent poison hemlock from establishing in an area).

This tool is a helpful first step for specialists to narrow down site priorities. Beyond comparing different types of sites, it could be used to compare and prioritize similar sites, such as multiple rural sites. Specialists can apply this tool to dozens of sites throughout the season to help decide which sites to attend to first.

Beyond the matrix: applying a public health lens

While the tool is a helpful guide, it cannot consider every factor. Another aspect we wanted to ensure we considered was environmental public health. Environmental public health is the study of how human health, disease, and the environment are related. Poison hemlock can act as a threat to public health to those who are regularly exposed to it without proper protection. After using the rubric in over 60 sites across rural and urban properties, we confirmed there is an overlap between potential for poison hemlock exposure and existing environmental health impacts and inequalities.

Figure 1: Washington Poison Control Data on Poison Hemlock Exposure

Why is poison hemlock a health equity issue?

Poison hemlock grows more in communities with environmental health equity concerns, which means that individuals most affected by institutional racism are disproportionately vulnerable to the toxic plant growing in their sidewalks, drainage ditches, rights-of-way, parks, and other public spaces.

How are pollution and human health related?

Poison hemlock exposure is a form of pollution. Breathing, eating, drinking, and living near environmental hazards and pollution affects your health. Conditions like asthma, heart and lung disease, cancer, and other serious health abnormalities increase when living near pollutive sources.

Existing environmental inequity in King County

The growth of poison hemlock is associated with urbanization. Residents of southwest King County are disproportionally exposed to pollution and environmental hazards. In cities like Federal Way and Tukwila, neighborhoods connected by major roadways create significant soil disturbance. In our examination we found that people in southwest King County are also most likely to run into poison hemlock near their sidewalks, yards, and roads. Poison hemlock isn’t causing existing health problems in these neighborhoods, but it can add one more stress for communities already dealing with a lot of environmental issues.

Figure 2: Interactive Slider Map Overlapping Poison Hemlock Infestations & Environmental Disparities in King County

Drag the slider button in the center from left to right to compare the two maps.

How does the matrix tool consider environmental disparities?

Thankfully, we found that the matrix tool factors of public exposure risk and community density typically push us to prioritize sites in these areas. But as a safety measure it is expected that our team additionally looks at the Washington state environmental health disparities map to further prioritize sites in more at-risk areas.

Limitations of the matrix: occupational safety considerations

Using the matrix, site #2 in our example above scored as the lowest priority of the three test sites. This site was in a heavily industrialized area. Without permanent housing and neighborhoods around, community density scored low. Although people worked here and spent large amounts of time here, it scored an overall of 9, which indicates low priority.  

When considering poison hemlock infestations in industrial areas, modifying the rubric to consider occupational exposure risk would help reduce hazards that could cause harm. Creating guidance on how to safely work around poison hemlock and decontaminate oneself afterwards could improve worker health and safety. In the past decade, the volume of calls to the Washington Poison Control Center related to poison hemlock has increased by over 400% statewide (see figure 1). An increase in calls signals concern in the health outcomes and consequences of living and working around poison hemlock.

Moving forward – next steps & priority actions

Interagency Collaboration and Resource Sharing: Poison hemlock management is not collaborative across agencies. Transportation agencies face resource and accessibility challenges, public health agencies provide information but lack field staff, and occupational health agencies provide general worker-safety guidance but are lacking poison hemlock-specific resources. Greater collaboration and resource sharing could strengthen poison hemlock prevention and management across Washington.

Targeted Outreach: Improve communication on the risk of poison hemlock to the communities that are the most affected.

Worker Safety: develop poison hemlock-specific guidance for workers who may encounter the plant as part of their jobs.

Exposure Documentation: Encourage individuals to report reactions and exposures to poison hemlock to health surveillance agencies, such as the Washington State Department of Health and King County Public Health, to help build a better understanding of exposure patterns and health effects.

Controlling poison hemlock on properties you manage: Poison hemlock is manageable, and people can control the plant following KCNWCP poison hemlock best management practices.

The King County Noxious Weeds Program will continue to prioritize poison hemlock control, but it is up to land managers, both public and private, to prioritize the work on properties they manage. Everyone deserves to live and work in safe, healthy environments. Noxious weeds programs, public health agencies, and transportation departments can all work towards the same goal of fostering resilient community health.