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Poison Hemlock

Conium maculatum
Updated Apr 04, 2024
 
1

Make a Positive Identification

Species: Poison hemlock
Poison hemlock seedling

Joseph M. DiTomaso, University of California - Davis, Bugwood.org

Poison hemlock seedlings have two seed leaves that are three to five times longer than they are broad. The stalks may be colored purple. The first true leaves resemble parsley. They are smooth and deeply cut two to three times. They give off a musty odor if crushed.

Species: Poison hemlock
Poison hemlock leaves and stems 

Barry Rice, sarracenia.com, Bugwood.org

In the first year poison hemlock is a rosette of glossy-green leaves about 2-3 feet high.

Species: Poison hemlock
Poison hemlock leaves

Leaves are hairless and divided many times along the hollow, main leaf stem. They appear fern-like. Each leaf has a sheath at the base of the leaf stem.

Species: Poison hemlock
Poison hemlock stems with purple blotches

In spring of the second year of growth, established plants sprout hollow flowering stems. They display red-purple spots and have a sheath at the base of the side stems. They grow up to 12 feet tall. The plants have an unpleasant odor and toxic fluid when crushed.
 

Species: Poison hemlock
Poison hemlock plant blooming

"Photo 43033087" by johnnypeddler is licensed under CC BY-NC-4.0

Poison hemlock blooms from April through July. Each stalk carries loose, white, umbrella-like flowerheads (umbels) that are 4-8 inches across.

Species: Poison hemlock
Poison hemlock flowers

Eric Coombs, Oregon Department of Agriculture, Bugwood.org

Poison hemlock has white, umbrella-like flowerheads (umbels) that are 4-8 inches across.

Species: Poison hemlock
Dry seed heads of poison hemlock

"Conium maculatum" by Oskar Gran is licensed under CC BY-NC 2.0

Poison hemlock stems eventually die in summer. They remain standing and support seed heads.

Species: Poison hemlock
Poison hemlock seeds

Steve Hurst, USDA NRCS PLANTS Database, Bugwood.org

Each plant produces thousands of seeds. Most of the seeds fall to the ground and germinate whenever there is enough moisture. Some seeds remain dormant in the soil for as long as 6 years.

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LOOK-ALIKES: GIANT HOGWEED, WILD CARROT AND WATER HEMLOCK
Species: Giant hogweed
Blooming clump of giant hogweed

Terry English, USDA APHIS PPQ, Bugwood.org

Giant hogweed (Heracleum mantegazzianum) plants often reach 10–12 feet tall. The white flowers grow 8–24 inches across. Leaves are 3–5 feet wide, coarsely toothed, with deeply cut leaflets. Giant hogweed sap on your skin or eyes causes severe burns when exposed to sunlight. Report this invasive species if you think you’ve found it on your property. Giant hogweed is an Early Detection Rapid Response species.

How to Get Rid of Giant Hogweed
Species: Wild carrot or Queen Anne's lace
Wild carrot stems, leaves, and flowers

Chris Evans, University of Illinois, Bugwood.org

Wild carrot or Queen Anne’s lace (Daucus carota) is a common weed. It is in the same family as poison hemlock and looks similar. It needs management in fields and along roadsides to prevent spread into agricultural areas. Control is similar to poison hemlock. You can dig it out or use herbicides to kill it.

Species: Waterhemlock
Waterhemlock (left) and poison hemlock (right) leaf comparison

Steve Dewey, Utah State University, Bugwood.org

Waterhemlock (Cicuta douglasii) is a native plant found in wet areas. It is violently toxic to people and livestock.
The photos compare a water hemlock leaf (left) and poison hemlock leaf (right). Poison hemlock leaves are more finely divided.
Keep livestock away from water hemlock.

For more information about poison and water hemlock and livestock, see Poison hemlock and Western waterhemlock: deadly plants that may be growing in your pasture - OSU Extension Service

Free help Identifying Weeds, Insects & Pests
Get expert pest management info & advice online from OSU's Ask Extension.
Get Help
 
2

Poison Hemlock Benefits

  • The flowers attract pollinators and beneficial insects.
 

Poison Hemlock Risks

  • Poison hemlock quickly grows into large patches.
  • It crowds out desirable understory plants.
  • All parts of the plant are poisonous to humans, grazing animals and wild mammals.
Risk Card
Does it cause harm?
Adults & Children
High
Property
High
Pets
High
Annoyance
High
Environment
High
Action Highly Recommended
 
3

Take Action

If you have poison hemlock on your property, take action to control it.

Do I need to take action?
Yes. Remove individual plants and small patches. Established patches require several years to control. Natural resource professionals are available to help you create a control plan.

What if I do nothing?
Poison hemlock quickly grows into large patches and will take over large areas.

NEED HELP?

Consider a licensed pest control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon  and other states  can suggest other methods.

 
4
Solutions for Poison Hemlock

Early Detection and Rapid Response

  • Look for poison hemlock leaves in early spring and flowers throughout summer.
  • Act before its flowering period, while the plant is still small and before it sets seeds.
  • Remove it before it becomes a bigger problem. It spreads quickly.
  • Control poison hemlock as needed each year.

Physically Remove Plants and Non-Chemical Options

  • Wear gloves and mask when removing hemlock.
  • Manage small patches by removing plants. Dig out individual plants and root crowns.
  • Dispose of mature seed heads in the trash.

Control with Herbicides (Weed Killers)

  • Herbicides effectively control poison hemlock when used according to the label instructions.

Monitoring and Follow-Up

  • Return to the stand and look for regrowth and seedlings during early spring and cool, moist periods.

Get Help From Natural Resource Professionals

We recommend contacting your county Weed Control program to help you create a management plan. Find contact information here: https://www.oregon.gov/oda/programs/Weeds/Pages/CountyWeedPrograms.aspx. They can help determine whether you need to hire a licensed pest control company. They should have experience controlling large stands of poison hemlock.

Jump To

Method Does it work? Is it safe? Recommendation
A
Physically Remove Plants
Very effective
High risk
Recommended
B
Control with Herbicides
Effective
Moderate risk
Use if Necessary
C
If Using Herbicides, Protect Yourself & Minimize Risks
D
Prevent Poison Hemlock
 
A

Physically Remove Plants

Non-Chemical Method

"Hemlock or Poison Hemlock (Conium maculatum) 3 NNI" by John Brandauer is licensed under CC BY-NC-ND 2.0

Physically Remove Plants

Recommended

In early spring, cut individual plants or small patches below the root crown. Dig up seedlings and small plants. Remove root crowns from the site to keep them from re-growing.

Does it work?
Very effective
  • Managing large stands requires several years of followup monitoring and treatment.
  • Use preventive measures for best results.
How much effort?
Moderate effort
  • Dig up seedlings and small plants. Remove the root crown.
What's the risk?
High risk
  • All parts are poisonous for people, pets, and livestock.
  • Sensitive people may suffer burns from the sap. Do NOT allow sap to remain on your skin or in your eyes.
  • Don't burn poison hemlock. This may release toxins into the air.
  • Don’t use a string trimmer on poison hemlock. The sap may get onto you and you will be injured.
  • Wear closed shoes, pants, long-sleeved shirt, gloves, & eye protection when handling poison hemlock.

 
Possible risk of exposure or harm from chemicals
NONE

We recommend getting help from a licensed pest control company with experience controlling large stands of poison hemlock.

Minimize soil disturbance as much as possible. Take steps to prevent erosion as needed. Replant the area.

Sealable plastic bag with seed head

Thomas B. Denholm, New Jersey Department of Agriculture, Bugwood.org

Put mature seed heads in a sealable plastic bag. Dispose of the bags in the trash or landfill. Don’t put mature seed heads in the compost pile or green waste bin.
Revisit plants and remove additional seed heads that mature later in the year.

Poison Hemlock Removal Tips

  • In early spring, cut individual plants below the root crown. Dig up seedlings and small plants. Remove root crowns from the site to keep them from re-growing.
  • Leaves and stems can be left on the ground to decompose.
  • Remove flowers before seeds mature.
  • Place mature seed heads in a sturdy plastic bag. Put the bag in the trash.
  • Do not compost the seed heads.
 
B

Control with Herbicides

Chemical Method: Use with caution

"Conium maculatum habit2" by Macleay Grass Man is licensed under CC BY 2.0

Control with Herbicides

Use if Necessary

Herbicide products containing active ingredients glyphosate, or 2,4-D (or MCPA) are effective treatments for poison hemlock when label directions are followed. The site where the poison hemlock is growing will affect the herbicide product choice. Products are registered for use on different sites.

Consider using herbicides to remove large stands of poison hemlock.
Spray poison hemlock before it flowers.The photo shows it at a good stage for herbicide treatment.

Does it work?
Effective
  • Control of poison hemlock requires several years of monitoring and treatment. Seeds in the soil sprout and grow when mature plants are killed.
  • Use preventive measures for best results.
How much effort?
Moderate effort
  • Treat individual poison hemlock plants and patches.
  • It will take several weeks for the treated plants to die.
  • Monitor the area and repeat treatment each year, as needed.
What's the risk?
Moderate risk
  • Herbicides come with real risks. ALWAYS read the entire label front to back. Review instructions even for brands you know.
  • Poison hemlock often grows near water. Herbicide application permits may be required for riparian areas.
Possible risk of exposure or harm from chemicals
Using herbicides includes some amount of risk. The lowest risk comes with using alternative methods.

You may be exposed to an herbicide if you:

  • Get it on your skin
  • Breathe it in
  • Eat or smoke afterward without washing hands
  • Touch or eat plants that are wet with spray (you, pets, or children)
  • Bring it inside on your shoes or clothes

Follow directions closely to reduce risk.

If you choose to do it yourself, some products are available for home use. These products, used individually or in a mixture, are effective chemical treatments for poison hemlock. Look for these chemical names in the “Active Ingredients” section of product labels.

  • Glyphosate (example: Bonide Kleenup)
  • 2,4-D (or MCPA) (example: Home Front Lawn Weed Killer)

Suggested pesticides are from the Pesticide Information Center Online (PICOL) Database. They are permitted for HOME use in Oregon. Read the label for application directions and effectiveness information.

ONLY professional pesticide applicators can use Restricted-Use Pesticides (RUP) that may be more effective. For larger stands of poison hemlock contact a licensed pesticide applicator with experience controlling poison hemlock.

Photo of herbicide label highlighting active ingredient glyphosate
  • A white box on the example label highlights active ingredient glyphosate. Text on the label states “Kills grass and weeds around flower beds, trees, shrubs....”
  • Glyphosate will damage most plants and grasses. Don’t let the spray contact plants you want to keep.
Photo of herbicide label highlighting active ingredient 2,4-D
  • A white box on the example label highlights active ingredient 2,4-D. Text on the label states “Kills up to 140 weed types as listed"
  • 2,4-D will damage most plants and grasses. Don’t let the spray contact plants you want to keep.

Herbicide Application Tips

  • Apply herbicides in fall or early spring before poison hemlock starts to bolt and/or flower.
  • Best time to apply is during the rapid growth of leaves.
  • It will take several weeks for the treated plants to die.
  • Monitor the area and repeat treatment each year, as needed.
  • Poison hemlock often grows near water. Contact your county Weed Control program for herbicide application information for riparian areas. Find contact information here: County Weed Control Programs
 

If Using Herbicides, Protect Yourself & Minimize Risks

Chemical Method: Use with Caution
Great blue heron in marsh

Brian Lasenby, iStock

Why is it important to read herbicide labels?

  • The label is a legal, binding agreement between the applicator, the pesticide manufacturer, and regulatory agencies.
  • Labels have detailed information on how to use the product correctly and legally.
  • Not following label directions breaks state/federal law.
  • They contain information on potential hazards of the product.
  • They provide instructions you should follow for poisonings and spills.
  • Following label instructions helps you to minimize the risks and maximize the benefits.

Key Herbicide Safety Tips

  • Read the entire label front to back.
  • Follow the instructions.
  • Review the instructions even for brands you know.
  • Only apply the product where the label says it may be applied.
  • Be precise in your application. More is not better.

 

Minimize the possible impact of herbicides on bees and other pollinators. Treat poison hemlock plants in the seedling to rosette stage before they flower. Avoid spraying pollinators directly. For more information, see OSU’s How to reduce bee poisoning from pesticides.

The Label is the Law

ALWAYS read the label before using herbicide products. The label is a legal document that provides information on how to safely use the herbicide. This helps avoid harm to human health and the environment. Using an herbicide in off-label ways is illegal. It can result in legal enforcement actions.

READ THE LABEL & Follow Instructions
It has instructions to protect you and the environment.

  • Labels are different for every product and they often change over time.
  • Use a magnifying glass if needed.
  • Pay attention to CAUTION, WARNING, and DANGER statements.
  • Pay attention to the PRECAUTIONARY STATEMENTS.
  • The law states you must read and follow herbicide instructions.

Protect Yourself
Eye, skin & lung irritants

  • Wear the right protective gear. This often includes chemical-resistant gloves, safety glasses, a long-sleeve shirt, pants, socks, and shoes.
  • Mix outdoors or in a well-ventilated area.
  • Wash hands after mixing or applying, and before eating or smoking.
  • Take a shower immediately after handling herbicides.
  • Wash clothes worn while mixing or applying separately from other laundry.

Protect Children & Pets
Children and pets are at risk if they eat or touch the plants before it dries.

  • Keep them away during and after applying herbicides (read label for how long).
  • Remove toys and pet dishes from yard before applying.
  • Don’t track herbicide products into your home on shoes or clothes.

Protect Pollinators

  • Apply in the early morning or evening when bees are less active.
  • Kill weeds before they flower. Avoid spraying flowering plants.
  • Do not spray on bees or insects.

Protect Plants You Want to Keep

  • Glyphosate and similar herbicide ingredients damage both grass and broadleaf plants.
  • Minimize spraying of foliage, stems, exposed roots, or the trunks of desirable shrubs or trees to avoid harm.
  • Follow the label to avoid damaging the roots of trees and shrubs.

Avoid Wet, Windy, or Hot Weather
Use during favorable weather for best results.

  • Don’t spray when it’s raining or when rain is expected in the next 24 hours.
  • Wind causes spray to drift that can get on you and desired plants.
  • Herbicides may be less effective in hot weather if the target plants are moisture-stressed.
  • Some herbicides can turn into a vapor in hot weather and damage nearby plants.

Don’t Spray into Water

  • It’s illegal to apply herbicides in a stream or slow moving/wetland pool.
  • You need a product registered for aquatic areas. This includes waterways, ditches, drains, and other places where water collects.

Storage & Disposal

  • Store in a secure area away from children.
  • Don’t put unused herbicide products in the trash.
  • Never pour down any drain or waterway.
  • Take unused herbicides to a hazardous waste facility.

Call  1-800-CLEANUP (1-800-253-2687) to find out where to dispose of herbicides.

For the Portland metro region in Oregon, contact Metro’s Recycling Information. Call  503-234-3000, email   or visit Metro’s website  

More about:

About Using Pesticides on School Grounds in Oregon

If using pesticides on school grounds, there are special rules in Oregon. See School Integrated Pest Management  (Oregon Department of Agriculture).

NEED HELP?

The National Pesticide Information Center (NPIC)  can answer questions about pest control chemicals.
 1-800-858-7378 or npic@ace.orst.edu  

Consider using a licensed pest or weed control company. Learn How to Hire a Pest Control Company.

Your local Extension Specialist in Oregon  and other states  can suggest other methods.

 

Prevent Poison Hemlock

Poison hemlock young leaves emerging from soil
Look for Poison Hemlock Seedlings and New Growth
  • Look for and control poison hemlock in the fall and early spring.
  • Prevent establishment by controlling or destroying newly germinated seedlings.
  • Remove plants before they form seeds. Use hand removal, cultivation, or mowing as site conditions allow.
  • Frequent removal will deplete root reserves and reduce seed production.
  • Managing large stands requires several years of followup monitoring and treatment.
  • Continue to monitor the area each year.
Gloved hand using metal brush to clean shovel

Weston Miller, Oregon State University

Remove Dirt from Shoes and Equipment
  • After working or traveling in an area with poison hemlock seeds in the soil, clean your boots and tools. Use a wire brush to remove all soil that may contain seeds.
  • If you drive into a poison hemlock stand, clean your vehicle.
Landscape area with native plants growing densely together

Weston Miller, Oregon State University

Install New Plants
  • Poison hemlock presence and removal activities may significantly disturb a site. As a result there may be few or no desirable plants remaining.
  • Replanting a site after poison hemlock removal stabilizes the soil surface, shades poison hemlock seedlings, and creates wildlife habitat.
  • If a site lacks desirable vegetation, then a multi-year re-vegetation plan is needed.
  • Once a site is re-planted, take proper care of the new plants. This will get them established quickly and suppress poison hemlock regrowth.
  • Your local extension specialist, soil and water conservation district, or a professional re-vegetation specialist can suggest strategies for your area.

Invasive Species Alert

  • Invasives are non-native species that spread aggressively and alter the environment.
  • Controlling unwanted poison hemlock is costly.
  • Please do your part to control poison hemlock on property you manage. It can spread beyond your property and impact your neighbors.
static invasive map
Invasive species data @ 2022, iMapInvasives (NatureServe)

The map shows poison hemlock distribution in Oregon. It is already widespread. There’s no need to report infestations of it.

SEE OTHER COMMMON PACIFIC NORTHWEST WEEDS

Content provided by editor Weston Miller and writer J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.

 Peer reviewed by OSU Department of Horticulture

Photo of Weston Miller

Weston Miller

Project Founder and Content Writer

Weston Miller served as Community and Urban Horticulture faculty for Oregon State University Extension Service for Clackamas, Multnomah, and Washington Counties. Weston is an author for content for this website. He developed funding partnerships with Portland area agencies to initiate and build out the Solve Pest Problems website focused on this goals:

J. Jeremiah Mann

J. Jeremiah Mann

J. Jeremiah Mann completed a Physical Science undergraduate degree at Humboldt State University, and M.S, Ph.D focusing on plant science topics at UC Davis. He went on to work for the Natural Resources Conservation Service and in a leadership position serving a private agricultural technology company. He currently lives in Sacramento California where he consults on pest and property management topics.

Photo of Kaci Buhl

Kaci Buhl

At the state level, I lead the Pesticide Safety Education Program (PSEP). The program hosts live recertification events around the state, serving over 1,000 licensed pesticide applicators each year. We also produce web-based training modules and license-preparation study manuals. Special training for unlicensed pesticide applicators is also available through a grant from the USDA Specialty Crop Block Grant Program. The PSEP at OSU works closely with the Oregon Department of Agriculture's Pesticides Division.

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