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Poison Hemlock (Conium maculatum)

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Summary

Poison hemlock, not closely related to the tree of the same name, is a biennial plant that is highly toxic to humans and most mammals. Native to Europe and the Middle East, poison hemlock has been introduced to North America where it has become widely distributed.

USDA Plants Profile for Conium maculatum

Illinois Wildflowers Page for Conium maculatum

Description & Identification

First-year plants consist of a basal rosette of leaves, whereas second-year plants grow upright, up to 7 feet in height, branching occasionally higher up. Leaves are alternately arranged on stems, and are doubly- or triply-pinnately compound, often resembling parsley but typically larger. The compound leaves of mature plants are quite large, up to 18" long and 12" wide, but leaflets are quite small, leading to a fernlike appearance.

Stems are hairless and are covered with dark purple splotches, with greater splotching concentrated lower down on the plant. On more heavily splotched plants, stems often appear purple from a distance.

Inflorescence is a compound umbel that superficially resembles Queen-anne's lace (wild carrot) but tends to have a round top.

The root is a white taproot, usually growing straight down but growing sideways when obstructed, and branching occasionally.

The entire plant has a foul odor, especially when bruised. Sometimes this odor can be evident even when approaching large patches of this plant.

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Life Cycle

Poison hemlock is a biennial with a life cycle typical of the Apiaceae family. First-year plants grow only as a basal rosette, often reaching a few feet in diameter. Second-year plants bolt, growing mostly straight, and only branching occasionally near the top. Flowering occurs late spring to mid-summer, after which the seeds mature and the foliage (and then whole plant) dies. Seeds fall close to the parent plant.

Cut or mowed second-year plants will resprout from roots, especially if cut earlier in season, but will die if mowed repeatedly.

The plant generally does not spread vegetatively, but can persist by reseeding where conditions are right, sometimes forming large colonies and even monocultures.

This plant is most common in disturbed areas, where it benefits from once-or-a few-times-a-season mowing schemes that remove competition early in winter or early spring before the plant grows upright, but leave enough time for it to flower and set seed before mowing again. Basal rosettes often survive occasional mowing, but the plant cannot reproduce in regularly mowed areas.

It occasionally invades more intact ecosystems.

Uses

This plant was used for executions in ancient Greece, and is well-known as the plant that killed Socrates.

This plant is the only Conium species found in North America, where it is introduced, although there are other species in this genus globally.

There are numerous members of the broader Apiaceae family in North America, overlapping with the range of this species. Relationships within this family are unclear, and it is thus not known which of these other plants is closest related to poison hemlock.

Notes

Poison hemlock is one of the most toxic plants to human; ingesting a small amount can kill a person. Poisonings are most common when people mistake this plant for other visually-similar members of the Apiaceae family.

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