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Integrated Crop Management
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Poison hemlock

Poison hemlock - Conium maculatum L.

Family: Apiaceae (Parsley family)
Life cycle: Biennial
Native status: Introduced
Habitat: Pastures, roadsides, no-till fields

General description: Basal rosette during the first year and then erect flower stalk 6 to 8 ft tall. Leaves are triangular, finely divided, and glabrous. Umbels up to 4 in. wide with white flowers. Rosettes resume growth in March/April.

Key ID traits: Leaves are triangular in shape and pinnately lobed up to 18" long. Hairless stem with purple spots, often covered with a waxy bloom.

Similar species: Wild carrot is found in similar habitats, but the leaves of wild carrot are pubescent, linear in shape, and more finely divided than poison hemlock. Poison hemlock is a much larger plant than wild carrot. Water hemlock has a spotted stem like poison hemlock, but it are oftenis a perennial that produces a cluster of fleshy tubers at the crown, and the leaflets are not finely divided like poison hemlock.

Miscellaneous: Poison hemlock contains the toxin coniine, which disrupts the central nervous system. Coniine has a structure similar to nicotine. Poison hemlock was commonly used to poison condemned prisoners in ancient Greece, and is believed to be the poison used to kill Socrates.

deeply lobed leaves of poison hemlock
The deeply lobed leaves of poison hemlock are glabrous and have a triangular shape.
basal rosettes
Basal rosettes are present in late summer and early spring. Poison hemlock rosettes are the biggest of the weedy Apiacea species in Iowa, exceeding two feet in diameter.
stems have purple spots and often are covered with a waxy bloom
Stems have purple spots and often are covered with a waxy bloom

 

Compound umbel with rounded top of individual umbels
Compound umbel with rounded top of individual umbels.
large poison hemlock
Poison hemlock is much larger than Iowa's other weedy Apiaceae.
wild carrot, poison hemlock, and wild parsnip seedlings
Seedlings: wild carrot (L), poison hemlock (C), and wild parsnip (R)

 

Authored by:
Bob Hartzler
Bob Hartzler
  • Retired Weed Scientist and Professor of Agronomy
Dr. Bob Hartzler is a former professor of agronomy and a former extension weed specialist with a Ph.D. in Crop Physiology from Iowa State University. He conducted research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also taught undergraduate classes in weed science and weed identification.
Meaghan Anderson
Meaghan Anderson
  • Field Agronomist in Central Iowa
Meaghan Anderson is a field agronomist serving nine counties in central Iowa and an extension specialist with Iowa State University Extension and Outreach. Areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and integrated pest management.

Meaghan holds a bachelor's degree in agronomy and a master's degree in crop production and physiology (focus: weed science) from Iowa State University.
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