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Poison hemlock

(Conium maculatum)

Photo of poison hemlock
Photo credit: Wisconsin DNR

A biennial herb. The three to 10-foot tall stems are hollow, hairless and ridged with reddish-purple mottling on the lower stem and branched extensively.

Overview

Other names for this plant 
  • Common names: deadly hemlock, fool's parsley, spotted hemlock, spotted parsley
Ecological threat
  • It invades a range of habitats from roadsides, fields, stream banks, disturbed areas, riparian woodlands and floodplains.
  • An individual plant can produce over 30,000 seeds.
  • All parts of the plant are toxic to animals and humans.
Overview map of poison hemlock classification in WI
Prohibited (red) and restricted (orange) counties

Classification in Wisconsin: Prohibited/Restricted (Restricted in Buffalo, Crawford, Dane, Grant, Green, Iowa, Jefferson, Kenosha, La Crosse, Lafayette, Milwaukee, Monroe, Ozaukee, Racine, Richland, Rock, Sauk, Sheboygan, Trempealeau, Vernon, Walworth and Waukesha counties; Prohibited elsewhere)

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for poison hemlock was based upon this literature review [PDF] developed by the department.

Identification

Leaves: Pinnately compound, toothed, eight to 16 inches long and broadly triangular. Leaf veins end at the tips of the tooth margins. Leaves are shiny, green and emit an unpleasant odor when crushed.

Flowers: Numerous, small, five-petaled white flowers in umbels four to six inches across are at the end of an individual stem that extends from a common stalk. Blooms May-August.

Fruits & seeds: Seeds are ridged and flat. A single plant produces over 30,000 seeds.

Roots: Thick, white taproot.

Similar species: Water hemlock (Cicuta maculate; native) has leaf veins that end in the notches between the tooth margins of the leaflets. Giant hogweed (Heracleum mantegazzianum; invasive) has a hairy stem and larger, less divided leaves.

Control

Mechanical: Hand pulling, or mowing close to the ground multiple times for several seasons. Wear gloves when handling this plant.

Chemical: Foliar spray of 2, 4-D plus dicamba before buds are produced.

For more information on control techniques, visit the Poison hemlock factsheet [exit DNR] by University of Wisconsin-Extension.

Photos

View poison hemlock pictures in our photo gallery.

Resources

Sources for content
  • USDA Forest Service, Northeastern Area Forest Health Staff. Weed of the Week: Poison hemlock [exit DNR]
  • Center for Invasive Species and Ecosystem Health, Invasive.org. Poison hemlock
Links for more information