What's the Most Poisonous Plant in North America?

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The carrot (Apiaceae) family comprises 434 genera with about 3,700 species, and is characterized by a flat-topped flower cluster called an umbel. Water hemlock (Cicuta sp.), one of several toxic members of this family, is considered to be the most toxic plant in North America.1

There are four species of water hemlock in North America, all highly poisonous and native to North America: spotted (C. maculata), western (C. douglasii), bulblet-bearing (C. bulbifera), and Mackenzie’s (C. virosa).

Toxicity to Humans and Livestock

The four species of water hemlock contain cicutoxin, a highly poisonous, unsaturated alcohol with a strong, carrot-like odor. The highest concentration of toxin is found in the roots/tubers; however, it is also present in leaves and stems, especially early in the growing season. Leaves and stems lose most of their toxicity as they mature, but green seed heads remain poisonous.

Only a small amount of the plant’s toxic substance will poison livestock 2 or humans. The toxin acts on the central nervous system, resulting in grand mal seizures and death in humans. Convulsions must be controlled to preserve normal ventilation and cardiovascular function.

Distribution and Identification

Spotted water hemlock is the most widely distributed in North America; however, overlap in distribution occurs between all four species (Figure 1). Desirable habitat includes bogs and other wetlands, such as stream banks, creeks, riparian woodlands, and irrigated/sub-irrigated meadows and pastures. Bulblet-bearing water hemlock is listed as endangered in Maryland and sensitive in Washington.

[ 1 ] The Poisonous Plant Lab in Logan, UT, lists water hemlock as the most violently toxic plant to humans and livestock in North America.
[ 2 ] Toxic properties of water hemlock are complex. Feeding studies show that 3 ounces (fresh weight) of western water hemlock (C. douglasii) tuber will kill a 150-pound sheep. The difference between a fatal and non-fatal dose to sheep was extremely narrow in the studies. If this rate is extrapolated to cattle, a few bites—or about ½-pound of fresh, water hemlock tuber—may be fatal to a mature cow (Pfister personal communication).

FIGURE 1: Distribution of four water hemlock species in North America (USDA, NRCS Plants 2018).

  Spotted water hemlock

Spotted water hemlock

  Western water hemlock

Western water hemlock

  Bulblet-bearing water hemlock

Bulblet-bearing water hemlock

  MacKenzie's water hemlock

MacKenzie's water hemlock


 Compound-umbel inforescence

Compound-umbel inforescence

 Hollow stems

Hollow stems

The four species of water hemlock are perennial plants with similar characteristics, including a compound-umbel flower arrangement, hollow stems, and thick/fleshy tubers with slender individual roots growing from the bottom of the rootstalk. Small chambers in the tubers and rootstalk contain the highest concentration of toxin. Morphological differences between spotted and western water hemlock are mainly microscopic. These two species are grouped below for discussion of general characteristics.

 
  Photo: Steve Dewey, Utah State Univ., Bugwood.org

Photo: Steve Dewey, Utah State Univ., Bugwood.org

Spotted/Western Water Hemlock

  • Perennial
  • Height: up to 7 feet
  • Leaves: double compound over a foot long in the lower part of the plant; 1 to 3 times pinnately compound upper; leaf veins end at notches rather than toothed tip
  • Stem: stout, erect, hollow; may be purple-striped, or mottled
  • Reproduction: seed
  Photo: Jackie Donnelly, Saratoga Woods and Waterways

Photo: Jackie Donnelly, Saratoga Woods and Waterways

Bulblet-Bearing Water Hemlock

  • Perennial
  • Height: up to 3 feet
  • Leaves: alternate; lower leaves 2 to 3 times pinnately divided into narrow leaflets .7 to 2.3 in long, sparsely toothed; small bulblets present in upper leaf axils
  • Stem: slender, erect, hollow; light green to slightly reddish
  • Reproduction: seed
  • Key characteristic: smaller than spotted or western water hemlock; narrow leaflets; bulbils in leaf axis
  Photo: Courtesy of Kristian Peters. CC BY-SA 3.0 License

Photo: Courtesy of Kristian Peters. CC BY-SA 3.0 License

MacKenzie’s Water Hemlock
 

  • Perennial
  • Height: up to 4 feet
  • Leaves: alternate, 2 to 3 times pinnately compound; narrower and longer leaflet than spotted water hemlock; entire leaf is 12 to 16 inches long; lanceolate and deeply toothed leaflets
  • Stem: erect, hollow; slightly striated, light green
  • Reproduction: seed

Management Information on several poisonous plants in the carrot family is available here.

References

  • Cicuta virosa—Water hemlock or Cowbane. Dals Wildlife. Online: bit.ly/DALSHemlock
  • Moss, E.H. 1983. Flora of Alberta. University of Toronto Press. p. 428.
  • Pfister, Jim. Personal Communication. Research Rangeland Management Specialist. Poisonous Plant Lab, Logan UT.
  • USDA,ARS. Poisonous Plant Research Lab. Water Hemlock (Cicuta douglasii). Online: bit.ly/USDAWaterHemlock
  • USDA, NRCS. 2018. The PLANTS Database (plants.usda.gov, 11 August 2018). National Plant Data Team, Greensboro, NC 27401-4901 USA.