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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, ridged with reddish-purple mottling on the lower limb and branched extensively.

Other names for this plant include:

  • Common names: deadly hemlock, fool's parsley, spotted hemlock, spotted parsley
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)

Ecological Threat
  • It invades 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.

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 more prominent, less divided leaves.


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 producing buds.

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

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, Poison hemlock
Links for more information