Poison Hemlock

Conium maculatum

WA B desig. Apiaceae
Data Maturity Baseline

Washington State Classification

Class B — Control Required (Designated)

The entire plant is toxic to animals and humans, containing the poisonous alkaloid coniine and other alkaloids.

Required control in Region 1 (all western Washington counties)

Quick Reference

Type
biennial herb
Origin
Europe, Asia and Northern Africa

Identification

Growth Habit

Poison hemlock is a very tall biennial plant that can grow up to 12 feet in height. It grows into a rosette the first year--a cluster of leaves growing on the ground and then flowering stems the next year.

Leaves

Leaves are fern-like, toothed, finely divided and have a strong odor when crushed.

Flowers

Flower are small and white and occur in 4 to 8 inch umbrella shaped clusters.

Fruit & Seeds

Seed hairless, egg-shaped and about 0.09 inches (2 mm) long with prominent ridges.

Impact

The entire plant is toxic to animals and humans, containing the poisonous alkaloid coniine and other alkaloids. Poison hemlock can quickly infest large areas of pasture as well as open waste places.

Ecology & Spread

Habitat

Poison hemlock prefers rich, moist soil, but is highly adaptable to other conditions. Please click here to see a county level distribution map of poison hemlock in Washington.

Spread Mechanisms

seed

Reproduction

Poison hemlock reproduces by seed.

Regional Notes — Puget Sound

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Control Methods

Mechanical

  • Digging up small infestations and removing the entire taproot is effective. Mowing is ineffective as plants will re-sprout, sending up new stalks in the same season mowing occurs. Toxins will remain potent in dried plant material. Never put pulled plants in the compost or leave them where children or livestock might eat them. Removed pulled plants from site, bag and put in the trash. Monitor sites for resprouts and seedlings as seeds will readily germinate on disturbed ground.

Cultural / Prevention

  • Cultivation (rototilling or hoeing) will effectively eliminate plants. Reduce weed infestation by handpulling weeds. Mowing to prevent seed production is a very effective means of management. In lawns, mowing regularly at the proper height for the grass species may help minimize weed growth and invasion. Careful digging is useful to manage weed populations. However, digging can carry undesirable w

Biological Control

  • There are no approved biological control agents for poison hemlock. See the WSU Integrated Weed Control Project for further information.