Conium maculatum, poison hemlock
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Herbaceous

Conium maculatum

poison hemlock

Apiaceae

Europe, western Asia, northern Africa

At a Glance

TypeBiennial
HabitUpright
FoliageDeciduous
Height36-96 inches (90-240 cm)
Width24-48 inches (60-120 cm)

Growing Zones

USDA Hardiness Zones

4 - 9
Zone 4
Zone 5
Zone 6
Zone 7
Zone 8
Zone 9
These zones indicate the coldest temperatures this plant can typically survive.
Frost Tolerancehardy

Key Features

Attracts Pollinators
Deer Resistant
Fragrant (moderate)
Maintenancehigh

Overview

Conium maculatum is a tall biennial in the carrot family (Apiaceae) reaching 36–96 inches (90–240 cm) tall with a 24–48 inch (60–120 cm) spread. First-year plants form a basal rosette of glossy, dark green, finely 3–4 times pinnately compound leaves up to 20 inches (50 cm) long, resembling fern fronds or large parsley. Second-year stems are erect, hollow, smooth, and distinctively marked with purple to reddish-purple blotches and streaks. Crushed foliage emits a pungent, musty odor described as mouse-like. Flowers are white, small, borne in compound umbels 2–4 inches (5–10 cm) across. Fruit is a pair of rounded, ribbed mericarps 0.1 inch (2.5–3 mm) long. All parts of the plant are highly toxic, containing piperidine alkaloids (coniine, γ-coniceine) that cause respiratory failure in mammals. Ingestion of as little as 0.2% of body weight of fresh foliage is lethal to cattle. The plant caused the death of Socrates in 399 BCE, one of the most documented poisonings in history. A single plant produces 35,000–40,000 seeds that remain viable in soil for 3–6 years. Naturalized widely across North America in ditches, riparian areas, roadsides, and waste ground. Populations are expanding rapidly in the Pacific Northwest.

Native Range

Native to Europe, western Asia, and northern Africa, occurring in moist meadows, streambanks, roadsides, and waste areas from sea level to approximately 5,000 feet (1,500 m). Introduced to North America as a garden plant in the 1800s. Now naturalized across most of the United States and southern Canada, with populations expanding in the Pacific Northwest, California, and the eastern United States.

Suggested Uses

Used in toxic plant identification training for veterinary students, livestock producers, land managers, and poison control professionals. Studied in toxicology and pharmacology as a source of piperidine alkaloids. Included in Apiaceae identification courses to teach critical differentiation from edible species such as Daucus carota (wild carrot) and Anthriscus cerefolium (garden chervil).

How to Identify

Distinguished from other large Apiaceae species by the smooth, hollow stems with distinctive purple to reddish-purple blotches and streaks. Crushed foliage has a pungent, musty odor, unlike the garlic odor of Alliaria petiolata or the carrot-like scent of Daucus carota. Leaves are 3–4 times pinnately compound, glossy dark green, more finely dissected than Anthriscus sylvestris (wild chervil). Distinguished from Angelica species by the purple-spotted stems (versus uniformly green or purple stems) and the smaller fruit. First-year rosettes are identified by the glossy, fern-like leaves and musty odor.

Appearance

Size & Dimensions

Height3' - 8'
Width/Spread2' - 4'

Colors

Flower Colors

white

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~4 weeks
J
F
M
A
M
J
J
A
S
O
N
D
SpringSummer
Flowers from May through July in the second year. Individual umbels bloom for 7–10 days. Compound umbels contain 8–16 umbellets, flowering from the outside inward. In the Pacific Northwest, peak bloom occurs in June. Seeds ripen by July through August, approximately 4–6 weeks after pollination. Pollinated by a wide range of insects including flies, beetles, and small bees.

Detailed Descriptions

Flower Description

White

Foliage Description

Dark green, glossy, finely 3-4 times pinnately compound with a fern-like appearance; leaflets deeply cut

Growing Conditions

Sun Requirements

Full Sun
Partial Shade
Requires 4-10 hours of direct sunlight daily
• Full Sun: 6+ hours of direct sunlight
• Partial Shade: 3-6 hours of direct sunlight
• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 8.0(Neutral)
357912
Soil Types
loamclaysilt
Drainage
moist

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

1-2 years

Care & Maintenance

Care Guide

All parts of the plant are toxic to humans and livestock; skin protection (gloves, long sleeves) is essential during removal, as alkaloids can be absorbed through skin on prolonged contact. First-year rosettes are hand-pulled or dug when soil is moist; the fleshy taproot extracts more completely from wet soil. Second-year plants are cut at ground level before seed set. Plants cut after flowering but before seed maturity may still produce viable seed on cut stems. Mowing reduces seed production when timed at early bloom but does not kill established plants that retain root crowns. Repeated management over 3–6 years is required to exhaust the soil seed bank. Dead stems retain alkaloid toxicity for several months; carcasses should not be composted in home systems. Populations are expanding along waterways and roadsides in the Pacific Northwest, often in dense, monoculture stands.

Pruning

No pruning applicable. Plants are removed by pulling, digging, or cutting before seed maturity. Cutting at ground level during early bloom prevents seed set. All plant material, including roots, should be bagged and disposed of in landfill rather than composted, due to alkaloid content. Repeated removal is necessary because seed bank viability extends 3–6 years.

Maintenance Level

high

⚠️ Toxicity Warning

Toxic to pets and humans