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Home / Weed & Pest Directory / Species / Poison Hemlock (Conium maculatum)

Poison Hemlock (Conium maculatum)

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Poison hemlock flowers. Photo Credit: Eric Coombs, ODA, Bugwood.org
  • White poison hemlock flowers
    Poison hemlock flowers. Photo Credit: Eric Coombs, ODA, Bugwood.org
  • Tiny, white poison hemlock flowers.
    Poison hemlock flowers. Photo Credit: John Cardina, the Ohio State University, Bugwood.org
  • Fern-like poison hemlock leaves.
    Poison hemlock leaves. Photo Credit: Ansel Oommen, Bugwood.org
  • Fern-like poison hemlock leaves
    Poison hemlock leaves. Photo Credit: John Cardina, the Ohio State University, Bugwood.org
  • Leaf comparison of poison hemlock and water hemlock.
    Leaf comparison of poison hemlock (right) and water hemlock (left). Photo Credit: Steve Dewey, Utah State University, Bugwood.org

Also known as: poison parsley, spotted hemlock, carrot-fern

Poison hemlock is considered one of the most poisonous plants in North America. The entire plant is toxic to people and animals. Once established, it can spread quickly across pastures and along waterways.

Description:

Life Cycle:Biennial (life cycle lasts two years)
Early Detection and Rapid Response species:No
Height:Up to 8 feet (2.4 meters) tall
Leaf Description:The leaves are compoundCompound A leaf consisting of several distinct leaflets joined to a single stem. and grow alternate along the stem. Its fern-like leaves are light green. Its leaves emit an unpleasant smell when brushed against or crushed.
Fruit & Flower Description:Small, white flowers grow in umbrella shaped clusters at the end of the plant’s upright stems.

Small seeds are brown and barrel shaped. It exclusively spreads by seed and a single plant can produce 30,000 seeds annually.
Bloom Time:June to August

Habitat:

  • Prefers areas with full sun and moist soil but is adaptable to other conditions.
  • It is commonly found in grasslands, forests, pastures as well as along streams and freshwater wetlands.

Impact:

  • Poison hemlock is lethal when ingested. It’s roots and flowers are especially toxic.
  • It can quickly spread across pastures and riparian areasRiparian Areas Areas of land that occur along the edges of rivers, streams, lakes, and other bodies of water., reducing forage, poising livestock, degrading habitat, and outcompeting other plants.

What you can do about it:

  • Always remember to protect yourself from poison hemlock by wearing long sleeves, pants, and gloves.
  • Small patches can be controlled by hand pulling or digging. Place all plant material in a tied plastic bag and dispose of in the garbage – not yard debris or home compost.
  • Mowing and tilling are effective controls for larger infestations. Mowing in the spring and late summer before and after plants have produced flowers will provide better results. Several years of mowing will be needed.

Look-alikes:

Water hemlock leaves.
Water hemlock leaves. Photo Credit: Mary Ellen (Mel) Harte, Bugwood.org

Western water hemlock (Cicuta douglasii), another toxic plant, is often confused with poison hemlock. Both plants have small, white, umbrella shaped flowers, but water hemlock’s leaves are less incised and not as fern-like.

Noxious Weed Listing:

State of Oregon:Class B
State of Washington:Class B

More Resources:

Download the Poison Hemlock Best Management Practices factsheet

Download Factsheet
  • Pacific Northwest Weed Management Handbook: Poison hemlock
  • University of California, Weed Research & Information Center: Poison hemlock

« Emerald Ash Borer (Agrilus Planipennis)
Spotted Lanternfly (Lycorma delicatula) »

Posted In:

AgriculturalAquatic or SemiaquaticCommonForestPlantToxic
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