Ohio State University Extension Bulletin 866-98

Identifying Noxious Weeds of Ohio

Bulletin 866-98


Poison Hemlock
(Conium maculatum)

Habitat: wet sites, gardens, roadsides, wastelands.

Life cycle: biennial

First Year Growth Habit: basal rosette of finely divided leaves with a pungent odor

Second-Year Growth Habit: 2-7 feet tall, branched plant with flowers

Leaves: alternate, pinnately compound, finely divided and toothed

Stems: branched, waxy with purple blotches; hollow between nodes

Inflorescence: June - August (second year). White flowers with five petals borne in large terminal compound umbels, 2-8 inches across.

Root: fleshy taproot

Similar plants: During the first year, poison hemlock resembles wild carrot, but has a strong, pungent odor; further, young leaves of wild carrot are more finely divided and its stem is grooved. At maturity, poison hemlock can be difficult to distinguish from water parsnip and water hemlock. Look for purple blotches on the stem to identify poison hemlock. Water hemlock (Cicuta maculata), which is also highly poisonous, has a magenta-streaked stem and lanceolate leaflets with sharply-toothed edges. Water parsnip (Sium suave), is not poisonous and has toothed lanceolate leaflets.

The problem is....this plant is highly poisonous to both humans and animals. Poison hemlock is a large and impressive plant which has been planted as an ornamental in some areas. It grows quickly in fertile soils.

Poison hemlock

Poison hemlock, allegedly the plant used to poison the Greek philosopher
Socrates, is a common sight along Ohio roadsides. All parts of this
plant are highly toxic.

stems of poison hemlock covered with purplish blotches

The stems of poison hemlock are typically covered with purplish
blotches. The stems are also hollow between the nodes.

poison hemlock


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